catena

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 Go to latest Published: Aug 31, 2026 License: Apache-2.0 Imports: 5 Imported by: 0

README

catena

Lazy, fully typed sequence pipelines for Go. Kotlin-stdlib and LINQ ergonomics on top of iter.Seq, built on Go 1.27 generic methods.

CI Release Go Reference 

top := catena.FromSlice(orders).
    Filter(func(o Order) bool { return o.Paid }).
    TopNBy(10, func(o Order) int { return o.Amount })

byUser := catena.FromSlice(orders).FoldBy(
    func(o Order) UserID { return o.User },
    func(UserID) int { return 0 },
    func(sum int, o Order) int { return sum + o.Amount },
)

go get github.com/NerdMeNot/catena

Requires Go 1.27+ for generic methods. No runtime dependencies — the one require line is a test-only property-testing library, and go list -deps on the package resolves to the standard library alone.

Catena is Latin for "chain".

Why

Go finally has the two pieces this needed — iter.Seq as a common iterator currency, and generic methods so a chain can change element type mid-flight. What was missing was a library willing to be precise about the things that make or break iterator code in practice.

Nothing is hidden. No reflect, no any in value position, no hidden buffering, and exactly one goroutine in the library — ToChan's, declared in the first line of its doc comment. An operator that buffers says so too, with its bound; a terminal that would hang on an infinite sequence is marked; an invalid argument panics at construction, naming the operator you actually called. The only panics during iteration are Once's and Must's, which is what those two are for.

The contracts are tested, not promised. Every exported operator is registered in a conformance harness, compared against a hand-written loop, and put through the invariants its class allows: re-iteration, early termination over an infinite source, the producer's cleanup running on an early break, and a bound on how much of the source it may consume to produce k elements. A completeness check parses the package and fails CI for any operator nothing registers. Library statement coverage is 100%, and the badge is not a snapshot: CI recomputes it on every push and fails the build below 100%, so a regression turns the build red rather than the badge stale.

Slow only where Go is slow. Measured against the same pipelines hand-built from raw iter.Seq closures, catena beats the raw mechanism on multi-stage pipelines and on terminals, and costs 4–13% more on single-stage shapes — where the baseline is inlined into its consumer and a call across a package boundary cannot be. What remains against a plain loop is the iterator protocol itself, and every number is published rather than waved at.

In practice

Pipelines stay lazy until a terminal consumes them, and early termination propagates through every stage — Take(2) here computes exactly two squares, out of an infinite sequence:

catena.Generate(1, func(n int) int { return n * 2 }). // 1, 2, 4, 8, ...
    Map(func(n int) int { return n * n }).
    Take(2).
    Collect() // [1 4]

When elements can fail, the pipeline carries the errors and the consumer picks the policy — abort, gather, or skip:

nums := catena.FromSlice(lines).MapErr(strconv.Atoi)

vals, err  := nums.Collect()          // stop at the first bad line
vals, errs := nums.CollectAll()       // keep everything, report everything
vals       := nums.Ignore().Collect() // skip bad lines

Producers that own a resource open it lazily inside the pipeline, so an unconsumed pipeline holds nothing and a downstream Take(3) still closes the rows — proven against the real database/sql stack in bake_test.go.

The operators worth switching for

Not syntax sugar — these change the memory class of the computation.

Use Instead of And it costs
FoldBy GroupBy + fold per bucket 2.97 MB → 1.2 KB (bounded by keys, not elements)
TopNBy(10, sel) SortedDesc().Take(10) 4.1 MB → 1 KB, 32× faster (bounded heap)
DedupeBy DistinctBy on sorted input unbounded seen-set → O(1)

Performance, before you choose

Measured three ways — a hand-written loop, the same pipeline hand-built from raw iter.Seq closures, and catena (Apple M5 Max, Go 1.27, 100k elements, bench_baseline_test.go):

Shape Hand loop Raw closures catena
4-stage pipeline (filter→map→filter→sum) 39µs 350µs 262µs
Sum 107µs 178µs 70µs
Contains — 116µs 70µs

Two facts fall out of that. catena adds essentially nothing on top of Go's iterator mechanism — on these three shapes it beats the raw-closure baseline outright, and across the full table its worst showing is 13% behind on a single-stage Map, where the baseline inlines into its caller and a library cannot. So what you give up is close to what iter.Seq itself costs. And that cost is real: against a plain loop doing trivial arithmetic a pipeline runs about 6.6× slower — roughly 2.2ns per element across the whole four-stage chain, in closure calls the compiler cannot flatten. If your loop body does anything beyond arithmetic — parses, allocates, touches a map or a socket — the per-element work buries it and the two converge.

The full benchmarks show the code both ways, so you can judge what the nanoseconds buy.

The shape of the library

Type What it is
Seq[T] lazy iter.Seq[T] with 84 methods — 37 chainable, 47 terminal; range works directly
Seq2[K, V] the stdlib pair currency; a bridge back to Seq, not a peer
Try[T] iter.Seq2[T, error]; error policy chosen by the consumer
List[T] eager []T with the mirrored operation set — generated, and conformance-checked to agree with AsSeq()

Interop is free in both directions: iter.Seq[T](s), s.Seq(), catena.From(anyPushIterator). Operations that constrain the element type (Distinct, Sorted, Sum, Max, …) are package functions, because a method on Seq[T any] may require nothing of T; the chain continues normally after them.

Documentation

Operator reference — all 182 operators across 14 pages, each with its signature, its memory and termination behaviour, and a worked example that go test runs and verifies.

Guides, in reading order:

  1. Getting started — the five-minute tour
  2. Concepts — the four types, eager vs lazy, and the contract system
  3. Error handling — Try and resource-owning producers
  4. Operator catalog — the index, with memory and drain classes
  5. Performance — the three-way benchmarks, honestly
  6. Releasing — how a release is cut and verified

Also: runnable examples · API reference · SPEC.md, the full design — every decision, and what was rejected.

Status

v1.1.0 is the first supported release, and it freezes the API: anything removed or changed incompatibly waits for a major version. Earlier tags were withdrawn and are retracted in go.mod — don't use them. Changes are tracked in CHANGELOG.md.

Licensed under Apache-2.0.

Documentation

Overview

Package catena is a lazy, fully-typed sequence library for Go 1.27+, built on iter.Seq and generic methods.

The four types:

  • Seq[T]: lazy, single-pass by contract, possibly infinite.
  • Seq2[K, V]: the stdlib pairing currency; a bridge back to Seq, not a peer.
  • Try[T]: Seq2[T, error]; error policy is chosen by the consumer (Collect stops at the first error, CollectAll drains, Ignore skips).
  • List[T]: eager []T with the mirrored operation set.

Contracts every caller should know:

  • Treat every Seq as single-pass. Re-iterability depends entirely on the producer (see the constructor table). Once() is a development guard.
  • Nil sequences are empty: every method and package function accepts a nil receiver or argument and treats it as an empty sequence.
  • Invalid construction arguments (negative counts, zero step) panic at construction time with a "catena: " prefixed message. Nil callbacks are not defended and panic at first use.
  • Operators marked as buffering state their bound; terminals marked as full-drain hang on infinite input.
  • Map-returning terminals return Go maps: iteration order is undefined.
  • When a Try element carries a non-nil error, do not read the value.
  • Not safe for concurrent use; ToChan is the fan-out mechanism.
  • comparable-constrained functions panic at runtime if T is an interface type holding a non-comparable value (Go 1.20 semantics).
Example (LazyAcquisition) 
package main

// Runnable examples: the resource-owning producer patterns the spec
// commits to documenting (§4.2 lazy acquisition, §7.7 reader adapter),
// plus the operators that justify the library.

import (
	"bufio"
	"database/sql"
	"fmt"
	"strings"

	"github.com/NerdMeNot/catena"
)

// Lines adapts an io.Reader into a Try of its lines — the §7.7 pattern.
// The reader is wrapped lazily inside the closure: a sequence that is
// never consumed never reads, and early termination stops the scan.
func Lines(open func() (*strings.Reader, error)) catena.Try[string] {
	return func(yield func(string, error) bool) {
		r, err := open()
		if err != nil {
			yield("", err)
			return
		}
		sc := bufio.NewScanner(r)
		for sc.Scan() {
			if !yield(sc.Text(), nil) {
				return
			}
		}
		if err := sc.Err(); err != nil {
			yield("", err)
		}
	}
}

func main() {
	opened := 0
	logs := Lines(func() (*strings.Reader, error) {
		opened++
		return strings.NewReader("GET /a\nPOST /b\nGET /c\nGET /d"), nil
	})

	// Building the pipeline opens nothing.
	gets := logs.Ignore().
		Filter(func(l string) bool { return strings.HasPrefix(l, "GET ") }).
		Take(2)
	fmt.Println("opened after building:", opened)

	fmt.Println(gets.Collect())
	fmt.Println("opened after consuming:", opened)
}

// Rows is the §4.2 producer pattern for *sql.Rows: lazy acquisition, so
// an unconsumed Try holds no resource, and defer runs on early
// termination through any number of stages. bake_test.go exercises it
// against a real database/sql driver.
func Rows[T any](open func() (*sql.Rows, error), scan func(*sql.Rows) (T, error)) catena.Try[T] {
	return func(yield func(T, error) bool) {
		var zero T
		rows, err := open()
		if err != nil {
			yield(zero, err)
			return
		}
		defer rows.Close()
		for rows.Next() {
			v, err := scan(rows)
			if err != nil {
				v = zero
			}
			if !yield(v, err) {
				return
			}
		}
		if err := rows.Err(); err != nil {
			yield(zero, err)
		}
	}
}

Output:
opened after building: 0
[GET /a GET /c]
opened after consuming: 1

Index

Examples

Constants

View Source 
const Version = "1.1.0"

Version is the library's version, kept in lockstep with release tags: the release workflow refuses to publish a tag that disagrees with it.

Variables

This section is empty.

Functions

func AssociateWith 

func AssociateWith[T comparable, V any](s Seq[T], f func(T) V) map[T]V

AssociateWith maps each element to f(element); on duplicate elements the last value wins. ⚠ Full drain; map iteration order is undefined.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	m := catena.AssociateWith(catena.Of("go", "rust"), func(s string) int { return len(s) })
	fmt.Println(m["go"], m["rust"])
}

Output:
2 4

func Average 

func Average[T Numeric](s Seq[T]) (float64, bool)

Average returns the mean, accumulating in float64; (0, false) on empty input. ⚠ Full drain.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	avg, ok := catena.Average(catena.Of(2.0, 4.0))
	fmt.Println(avg, ok)
}

Output:
3 true

func CollectMap 

func CollectMap[K comparable, V any](s Seq2[K, V]) map[K]V

CollectMap drains a pair sequence into a map; on duplicate keys the last value wins. ⚠ Full drain; map iteration order is undefined.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	// To a map. On a duplicate key the last value wins, as with plain
	// map assignment.
	fmt.Println(catena.CollectMap(catena.Of("a", "b").WithIndex()))
}

Output:
map[0:a 1:b]

func Contains 

func Contains[T comparable](s Seq[T], v T) bool

Contains reports whether v occurs in s; stops at the first match.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	fmt.Println(catena.Contains(catena.Of(1, 2, 3), 2))
}

Output:
true

func Equal 

func Equal[T comparable](a, b Seq[T]) bool

Equal reports whether a and b yield the same elements in the same order. Consumes both sequences up to and including the first difference — fully when they are equal. b is consumed through iter.Pull (its cleanup always runs).

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	fmt.Println(catena.Equal(catena.Of(1, 2), catena.Of(1, 2)))
	fmt.Println(catena.Equal(catena.Of(1, 2), catena.Of(1)))
}

Output:
true
false

func IndexOf 

func IndexOf[T comparable](s Seq[T], v T) int

IndexOf returns the index of the first occurrence of v; -1 if none.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	fmt.Println(catena.IndexOf(catena.Of("a", "b"), "b"))
	fmt.Println(catena.IndexOf(catena.Of("a"), "z"))
}

Output:
1
-1

func Join 

func Join(s Seq[string], sep string) string

Join concatenates a string sequence with sep between elements. ⚠ Full drain.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	fmt.Println(catena.Join(catena.Of("a", "b", "c"), "-"))
}

Output:
a-b-c

func Max 

func Max[T cmp.Ordered](s Seq[T]) (T, bool)

Max returns the largest element; NaN orders below everything (cmp.Compare). ⚠ Full drain.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	v, ok := catena.Max(catena.Of(3, 9, 1))
	fmt.Println(v, ok)
}

Output:
9 true

func Min 

func Min[T cmp.Ordered](s Seq[T]) (T, bool)

Min returns the smallest element; NaN orders below everything, so a NaN in the input is the minimum. ⚠ Full drain.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	v, ok := catena.Min(catena.Of(3, 9, 1))
	fmt.Println(v, ok)
}

Output:
1 true

func MinMax 

func MinMax[T cmp.Ordered](s Seq[T]) (min, max T, ok bool)

MinMax returns the smallest and largest elements in one pass. ⚠ Full drain.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	lo, hi, ok := catena.MinMax(catena.Of(3, 9, 1))
	fmt.Println(lo, hi, ok)
}

Output:
1 9 true

func Product 

func Product[T Numeric](s Seq[T]) T

Product multiplies the elements. Empty input yields 1, the multiplicative identity. ⚠ Full drain.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	fmt.Println(catena.Product(catena.Of(2, 3, 4)))
}

Output:
24

func Self 

func Self[T any](v T) T

Self is the identity selector: catena.Flatten(s) is s.FlatMap(Self).

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	// The identity selector, for the -By operators when the element is
	// already the key.
	fmt.Println(catena.Of(3, 1, 2).TallyBy(catena.Self[int])[3])
}

Output:
1

func Sum 

func Sum[T Numeric](s Seq[T]) T

Sum adds the elements; integer overflow wraps like +. Empty input sums to 0. ⚠ Full drain.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	// Integer overflow wraps, exactly as + does.
	fmt.Println(catena.Sum(catena.Of(1, 2, 3)))
}

Output:
6

func Tally 

func Tally[T comparable](s Seq[T]) map[T]int

Tally counts occurrences per value. ⚠ Full drain; map iteration order is undefined.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	fmt.Println(catena.Tally(catena.Of("a", "b", "a"))["a"])
}

Output:
2

func ToKeySet added in v1.1.0 

func ToKeySet[T comparable](s Seq[T]) map[T]struct{}

ToKeySet drains the sequence into a membership map, whose keys are the distinct elements. Named for what it returns rather than for the set it stands in for: ToSet is reserved for a real set type, should Go ever grow one. ⚠ Full drain; map iteration order is undefined.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	set := catena.ToKeySet(catena.Of(1, 2, 2))
	_, has := set[2]
	fmt.Println(len(set), has)
}

Output:
2 true

func TopN 

func TopN[T cmp.Ordered](s Seq[T], n int) []T

TopN returns the n largest elements, sorted descending; equal elements retain encounter order. Memory is O(n). ⚠ Full drain. Panics if n is negative.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	fmt.Println(catena.TopN(catena.Of(5, 1, 9, 3), 2))
}

Output:
[9 5]

func Unzip 

func Unzip[K, V any](s Seq2[K, V]) ([]K, []V)

Unzip drains a pair sequence into its two sides; nil slices for empty input. ⚠ Full drain; buffers both sides.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	// Both sides in one pass — the safe way to get keys and values from
	// a single-use Seq2.
	idx, vals := catena.Unzip(catena.Of("a", "b").WithIndex())
	fmt.Println(idx, vals)
}

Output:
[0 1] [a b]

Types

type Integer 

type Integer interface {
	~int | ~int8 | ~int16 | ~int32 | ~int64 |
		~uint | ~uint8 | ~uint16 | ~uint32 | ~uint64 | ~uintptr
}

Integer is the constraint for Range.

type List 

type List[T any] []T

List is an eager []T with the mirrored operation set. []T(l) unwraps it with no copy.

func (List[T]) All 

func (l List[T]) All(pred func(T) bool) bool

All mirrors Seq.All eagerly.

func (List[T]) Any 

func (l List[T]) Any(pred func(T) bool) bool

Any mirrors Seq.Any eagerly.

func (List[T]) Append 

func (l List[T]) Append(vals ...T) List[T]

Append returns a new List with vals appended. Unlike the built-in append, the result ALWAYS has a fresh backing array and never aliases l — consistent with every other List transform. Callers who want the built-in's amortized behavior can use it directly: List is a []T.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	// Unlike the builtin, the result never aliases the receiver — every
	// List transform returns fresh backing memory.
	base := make(catena.List[int], 2, 10)
	grown := base.Append(9)
	grown[0] = 99
	fmt.Println(base, grown)
}

Output:
[0 0] [99 0 9]

func (List[T]) AsSeq 

func (l List[T]) AsSeq() Seq[T]

AsSeq returns a lazy, re-iterable view of the list. The list is not copied; mutations are visible to later iterations.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	// Crossing to lazy is explicit, and free — it is a view, not a copy.
	l := catena.List[int]{1, 2, 3, 4}
	first, _ := l.AsSeq().Find(func(n int) bool { return n > 2 })
	fmt.Println(first)
}

Output:
3

func (List[T]) Associate 

func (l List[T]) Associate[K comparable, V any](f func(T) (K, V)) map[K]V

Associate mirrors Seq.Associate eagerly.

func (List[T]) At 

func (l List[T]) At(i int) T

At returns the element at index i, panicking exactly like l[i] on an out-of-range index. O(1).

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	// Panics exactly like l[i] — it IS the index expression.
	l := catena.List[string]{"a", "b"}
	fmt.Println(l.At(1))
}

Output:
b

func (List[T]) AverageOf 

func (l List[T]) AverageOf[N Numeric](sel func(T) N) (float64, bool)

AverageOf mirrors Seq.AverageOf eagerly.

func (List[T]) BottomNBy 

func (l List[T]) BottomNBy[K cmp.Ordered](n int, sel func(T) K) []T

BottomNBy mirrors Seq.BottomNBy eagerly.

func (List[T]) Clone 

func (l List[T]) Clone() List[T]

Clone returns a shallow copy with a fresh backing array.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	l := catena.List[int]{1, 2}
	c := l.Clone()
	c[0] = 99
	fmt.Println(l, c)
}

Output:
[1 2] [99 2]

func (List[T]) Collect 

func (l List[T]) Collect() []T

Collect mirrors Seq.Collect eagerly. O(1)/exact-allocation override.

func (List[T]) Concat 

func (l List[T]) Concat(others ...Seq[T]) List[T]

Concat mirrors Seq.Concat eagerly.

func (List[T]) Count 

func (l List[T]) Count() int

Count mirrors Seq.Count eagerly. O(1)/exact-allocation override.

func (List[T]) CountWhere 

func (l List[T]) CountWhere(pred func(T) bool) int

CountWhere mirrors Seq.CountWhere eagerly.

func (List[T]) DedupeBy 

func (l List[T]) DedupeBy[K comparable](sel func(T) K) List[T]

DedupeBy mirrors Seq.DedupeBy eagerly.

func (List[T]) DistinctBy 

func (l List[T]) DistinctBy[K comparable](sel func(T) K) List[T]

DistinctBy mirrors Seq.DistinctBy eagerly.

func (List[T]) DistinctWith 

func (l List[T]) DistinctWith(eq func(a, b T) bool) List[T]

DistinctWith mirrors Seq.DistinctWith eagerly.

func (List[T]) Drain 

func (l List[T]) Drain()

Drain mirrors Seq.Drain eagerly.

func (List[T]) Drop 

func (l List[T]) Drop(n int) List[T]

Drop mirrors Seq.Drop eagerly.

func (List[T]) DropLast 

func (l List[T]) DropLast(n int) List[T]

DropLast mirrors Seq.DropLast eagerly.

func (List[T]) DropWhile 

func (l List[T]) DropWhile(pred func(T) bool) List[T]

DropWhile mirrors Seq.DropWhile eagerly.

func (List[T]) ElementAt 

func (l List[T]) ElementAt(i int) (T, bool)

ElementAt mirrors Seq.ElementAt eagerly. O(1)/exact-allocation override.

func (List[T]) Filter 

func (l List[T]) Filter(pred func(T) bool) List[T]

Filter mirrors Seq.Filter eagerly.

func (List[T]) FilterErr 

func (l List[T]) FilterErr(pred func(T) (bool, error)) Try[T]

FilterErr mirrors Seq.FilterErr eagerly.

func (List[T]) FilterIndexed 

func (l List[T]) FilterIndexed(pred func(int, T) bool) List[T]

FilterIndexed mirrors Seq.FilterIndexed eagerly.

func (List[T]) FilterMap 

func (l List[T]) FilterMap[U any](f func(T) (U, bool)) List[U]

FilterMap mirrors Seq.FilterMap eagerly.

func (List[T]) FilterNot 

func (l List[T]) FilterNot(pred func(T) bool) List[T]

FilterNot mirrors Seq.FilterNot eagerly.

func (List[T]) Find 

func (l List[T]) Find(pred func(T) bool) (T, bool)

Find mirrors Seq.Find eagerly.

func (List[T]) FindIndex 

func (l List[T]) FindIndex(pred func(T) bool) int

FindIndex mirrors Seq.FindIndex eagerly.

func (List[T]) FindLast 

func (l List[T]) FindLast(pred func(T) bool) (T, bool)

FindLast mirrors Seq.FindLast eagerly.

func (List[T]) FindMap 

func (l List[T]) FindMap[U any](f func(T) (U, bool)) (U, bool)

FindMap mirrors Seq.FindMap eagerly.

func (List[T]) First 

func (l List[T]) First() (T, bool)

First mirrors Seq.First eagerly. O(1)/exact-allocation override.

func (List[T]) FlatMap 

func (l List[T]) FlatMap[U any](f func(T) Seq[U]) List[U]

FlatMap mirrors Seq.FlatMap eagerly.

func (List[T]) FlatMapSlice 

func (l List[T]) FlatMapSlice[U any](f func(T) []U) List[U]

FlatMapSlice mirrors Seq.FlatMapSlice eagerly.

func (List[T]) Fold 

func (l List[T]) Fold[A any](init A, f func(A, T) A) A

Fold mirrors Seq.Fold eagerly.

func (List[T]) FoldBy 

func (l List[T]) FoldBy[K comparable, A any](key func(T) K, init func(K) A, f func(A, T) A) map[K]A

FoldBy mirrors Seq.FoldBy eagerly.

func (List[T]) FoldErr 

func (l List[T]) FoldErr[A any](init A, f func(A, T) (A, error)) (A, error)

FoldErr mirrors Seq.FoldErr eagerly.

func (List[T]) FoldIndexed 

func (l List[T]) FoldIndexed[A any](init A, f func(int, A, T) A) A

FoldIndexed mirrors Seq.FoldIndexed eagerly.

func (List[T]) FoldRight 

func (l List[T]) FoldRight[A any](init A, f func(T, A) A) A

FoldRight reduces right to left. It exists only on List: a right fold needs the whole sequence in memory, which a List already is (§7.5).

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	// Exists only on List: a right fold needs the whole sequence, which
	// a List already is.
	l := catena.List[string]{"a", "b", "c"}
	fmt.Println(l.FoldRight("|", func(s, acc string) string { return "(" + s + acc + ")" }))
}

Output:
(a(b(c|)))

func (List[T]) FoldWhile 

func (l List[T]) FoldWhile[A any](init A, f func(A, T) (A, bool)) A

FoldWhile mirrors Seq.FoldWhile eagerly.

func (List[T]) ForEach 

func (l List[T]) ForEach(f func(T))

ForEach mirrors Seq.ForEach eagerly.

func (List[T]) ForEachErr 

func (l List[T]) ForEachErr(f func(T) error) error

ForEachErr mirrors Seq.ForEachErr eagerly.

func (List[T]) ForEachIndexed 

func (l List[T]) ForEachIndexed(f func(int, T))

ForEachIndexed mirrors Seq.ForEachIndexed eagerly.

func (List[T]) Get 

func (l List[T]) Get(i int) (T, bool)

Get returns the element at index i; (zero, false) for a negative or out-of-range index. O(1).

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	// The comma-ok form, for when the index may be out of range.
	l := catena.List[string]{"a"}
	v, ok := l.Get(0)
	_, bad := l.Get(9)
	fmt.Println(v, ok, bad)
}

Output:
a true false

func (List[T]) GroupBy 

func (l List[T]) GroupBy[K comparable](sel func(T) K) map[K][]T

GroupBy mirrors Seq.GroupBy eagerly.

func (List[T]) IfEmpty 

func (l List[T]) IfEmpty(defaults ...T) List[T]

IfEmpty mirrors Seq.IfEmpty eagerly.

func (List[T]) IndexBy 

func (l List[T]) IndexBy[K comparable](sel func(T) K) map[K]T

IndexBy mirrors Seq.IndexBy eagerly.

func (List[T]) Intersperse 

func (l List[T]) Intersperse(sep T) List[T]

Intersperse mirrors Seq.Intersperse eagerly.

func (List[T]) IsEmpty 

func (l List[T]) IsEmpty() bool

IsEmpty mirrors Seq.IsEmpty eagerly. O(1)/exact-allocation override.

func (List[T]) JoinBy 

func (l List[T]) JoinBy[U any, K comparable, R any](other Seq[U], leftKey func(T) K, rightKey func(U) K, combine func(T, U) R) List[R]

JoinBy mirrors Seq.JoinBy eagerly.

func (List[T]) JoinToString 

func (l List[T]) JoinToString(sep string, sel func(T) string) string

JoinToString mirrors Seq.JoinToString eagerly.

func (List[T]) Last 

func (l List[T]) Last() (T, bool)

Last mirrors Seq.Last eagerly. O(1)/exact-allocation override.

func (List[T]) Len 

func (l List[T]) Len() int

Len returns the number of elements. O(1).

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	l := catena.List[int]{3, 1, 2}
	fmt.Println(l.Len())
}

Output:
3

func (List[T]) Map 

func (l List[T]) Map[U any](f func(T) U) List[U]

Map mirrors Seq.Map eagerly. O(1)/exact-allocation override.

func (List[T]) MapErr 

func (l List[T]) MapErr[U any](f func(T) (U, error)) Try[U]

MapErr mirrors Seq.MapErr eagerly.

func (List[T]) MapIndexed 

func (l List[T]) MapIndexed[U any](f func(int, T) U) List[U]

MapIndexed mirrors Seq.MapIndexed eagerly. O(1)/exact-allocation override.

func (List[T]) MaxBy 

func (l List[T]) MaxBy[K cmp.Ordered](sel func(T) K) (T, bool)

MaxBy mirrors Seq.MaxBy eagerly.

func (List[T]) MaxOf 

func (l List[T]) MaxOf[K cmp.Ordered](sel func(T) K) (K, bool)

MaxOf mirrors Seq.MaxOf eagerly.

func (List[T]) MaxWith 

func (l List[T]) MaxWith(cmp func(a, b T) int) (T, bool)

MaxWith mirrors Seq.MaxWith eagerly.

func (List[T]) MinBy 

func (l List[T]) MinBy[K cmp.Ordered](sel func(T) K) (T, bool)

MinBy mirrors Seq.MinBy eagerly.

func (List[T]) MinMaxOf 

func (l List[T]) MinMaxOf[K cmp.Ordered](sel func(T) K) (min, max K, ok bool)

MinMaxOf mirrors Seq.MinMaxOf eagerly.

func (List[T]) MinOf 

func (l List[T]) MinOf[K cmp.Ordered](sel func(T) K) (K, bool)

MinOf mirrors Seq.MinOf eagerly.

func (List[T]) MinWith 

func (l List[T]) MinWith(cmp func(a, b T) int) (T, bool)

MinWith mirrors Seq.MinWith eagerly.

func (List[T]) None 

func (l List[T]) None(pred func(T) bool) bool

None mirrors Seq.None eagerly.

func (List[T]) OnEach 

func (l List[T]) OnEach(f func(T)) List[T]

OnEach mirrors Seq.OnEach eagerly.

func (List[T]) Partition 

func (l List[T]) Partition(pred func(T) bool) (yes, no []T)

Partition mirrors Seq.Partition eagerly.

func (List[T]) Prepend 

func (l List[T]) Prepend(vals ...T) List[T]

Prepend mirrors Seq.Prepend eagerly.

func (List[T]) ProductOf 

func (l List[T]) ProductOf[N Numeric](sel func(T) N) N

ProductOf mirrors Seq.ProductOf eagerly.

func (List[T]) Reduce 

func (l List[T]) Reduce(f func(T, T) T) (T, bool)

Reduce mirrors Seq.Reduce eagerly.

func (List[T]) Reversed 

func (l List[T]) Reversed() List[T]

Reversed mirrors Seq.Reversed eagerly. O(1)/exact-allocation override.

func (List[T]) Scan 

func (l List[T]) Scan[A any](init A, f func(A, T) A) List[A]

Scan mirrors Seq.Scan eagerly.

func (List[T]) Single 

func (l List[T]) Single() (T, bool)

Single mirrors Seq.Single eagerly.

func (List[T]) Slice 

func (l List[T]) Slice(i, j int) List[T]

Slice returns l[i:j] as a List, panicking exactly like the slice expression — and, exactly like it, sharing the backing array.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	// It is the slice expression, aliasing included.
	l := catena.List[int]{1, 2, 3, 4}
	fmt.Println(l.Slice(1, 3))
}

Output:
[2 3]

func (List[T]) SortedBy 

func (l List[T]) SortedBy[K cmp.Ordered](sel func(T) K) List[T]

SortedBy mirrors Seq.SortedBy eagerly.

func (List[T]) SortedByDesc 

func (l List[T]) SortedByDesc[K cmp.Ordered](sel func(T) K) List[T]

SortedByDesc mirrors Seq.SortedByDesc eagerly.

func (List[T]) SortedWith 

func (l List[T]) SortedWith(cmp func(a, b T) int) List[T]

SortedWith mirrors Seq.SortedWith eagerly.

func (List[T]) Step 

func (l List[T]) Step(n int) List[T]

Step mirrors Seq.Step eagerly.

func (List[T]) SumOf 

func (l List[T]) SumOf[N Numeric](sel func(T) N) N

SumOf mirrors Seq.SumOf eagerly.

func (List[T]) Take 

func (l List[T]) Take(n int) List[T]

Take mirrors Seq.Take eagerly.

func (List[T]) TakeLast 

func (l List[T]) TakeLast(n int) List[T]

TakeLast mirrors Seq.TakeLast eagerly.

func (List[T]) TakeWhile 

func (l List[T]) TakeWhile(pred func(T) bool) List[T]

TakeWhile mirrors Seq.TakeWhile eagerly.

func (List[T]) TallyBy 

func (l List[T]) TallyBy[K comparable](sel func(T) K) map[K]int

TallyBy mirrors Seq.TallyBy eagerly.

func (List[T]) ToList 

func (l List[T]) ToList() List[T]

ToList mirrors Seq.ToList eagerly. O(1)/exact-allocation override.

func (List[T]) TopNBy 

func (l List[T]) TopNBy[K cmp.Ordered](n int, sel func(T) K) []T

TopNBy mirrors Seq.TopNBy eagerly.

func (List[T]) WithIndex 

func (l List[T]) WithIndex() Seq2[int, T]

WithIndex mirrors Seq.WithIndex eagerly.

func (List[T]) ZipWithNext 

func (l List[T]) ZipWithNext() Seq2[T, T]

ZipWithNext mirrors Seq.ZipWithNext eagerly.

type Numeric 

type Numeric interface {
	~int | ~int8 | ~int16 | ~int32 | ~int64 |
		~uint | ~uint8 | ~uint16 | ~uint32 | ~uint64 | ~uintptr |
		~float32 | ~float64
}

Numeric is the constraint for arithmetic aggregations (Sum, Product, Average). Complex types are deliberately excluded: they are unordered and half the aggregation surface would be meaningless for them.

type Seq 

type Seq[T any] iter.Seq[T]

Seq is a lazy sequence: iter.Seq with methods. Range over it directly.

func Chain 

func Chain[T any](seqs ...Seq[T]) Seq[T]

Chain yields each sequence's elements in order.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	// The variadic form, for when the sequences are in a slice already.
	fmt.Println(catena.Chain(catena.Of(1), catena.Of(2), catena.Of(3)).Collect())
}

Output:
[1 2 3]

func Chunked 

func Chunked[T any](s Seq[T], n int) Seq[[]T]

Chunked yields consecutive chunks of n elements; the last chunk may be partial. Every chunk is a fresh slice (safe to retain). Panics if n <= 0.

Chunked is a package function, not a method: a method on Seq[T] returning Seq[[]T] is an instantiation cycle (each Seq[T] would require Seq[[]T], which would require Seq[[][]T], forever).

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	// Fixed-size batches; the last one may be short. Every chunk is a
	// fresh slice, so keeping one is safe.
	for c := range catena.Chunked(catena.Range(1, 8, 1), 3).Seq() {
		fmt.Println(c)
	}
}

Output:
[1 2 3]
[4 5 6]
[7]

func ChunkedBy 

func ChunkedBy[T any, K comparable](s Seq[T], sel func(T) K) Seq[[]T]

ChunkedBy yields runs of consecutive elements sharing a key: a chunk closes when the key changes. Memory is bounded by the longest run. Every chunk is a fresh slice. A package function for the same instantiation- cycle reason as Chunked.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	// A chunk per run of equal keys — the streaming answer to grouping
	// already-sorted input, in memory bounded by the longest run.
	for c := range catena.ChunkedBy(catena.Of(1, 1, 2, 3, 3), catena.Self[int]).Seq() {
		fmt.Println(c)
	}
}

Output:
[1 1]
[2]
[3 3]

func Cycle 

func Cycle[T any](s Seq[T]) Seq[T]

Cycle yields s over and over, forever. The first pass is buffered and replayed (⚠ unbounded memory in len(s)). An empty s yields an empty Cycle — it terminates rather than spinning.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	// Infinite — except over an empty source, which terminates rather
	// than spinning.
	fmt.Println(catena.Cycle(catena.Of("a", "b")).Take(5).Collect())
	fmt.Println(catena.Cycle(catena.Empty[string]()).Collect())
}

Output:
[a b a b a]
[]

func Dedupe 

func Dedupe[T comparable](s Seq[T]) Seq[T]

Dedupe yields elements that differ from their predecessor — consecutive duplicates only, O(1) memory.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	// Consecutive duplicates only. On sorted input it equals Distinct at
	// a fraction of the cost; on unsorted input the two differ.
	fmt.Println(catena.Dedupe(catena.Of(3, 3, 1, 1, 3)).Collect())
}

Output:
[3 1 3]

func Distinct 

func Distinct[T comparable](s Seq[T]) Seq[T]

Distinct yields elements not seen before; the first occurrence wins. ⚠ Retains one entry per distinct value — unbounded.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	// Distinct constrains the element type, so it is a package function
	// rather than a method — a method on Seq[T any] may require nothing
	// of T. First occurrence wins, and encounter order is preserved.
	fmt.Println(catena.Distinct(catena.Of(3, 1, 3, 2, 1)).Collect())
}

Output:
[3 1 2]

func Empty 

func Empty[T any]() Seq[T]

Empty returns the empty Seq.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	fmt.Println(catena.Empty[int]().Collect(), catena.Empty[int]().Count())
}

Output:
[] 0

func Except 

func Except[T comparable](a, b Seq[T]) Seq[T]

Except yields the distinct elements of a that do not occur in b, in a's encounter order. ⚠ Buffers all of b before a is consumed, plus a seen- set of a's distinct values.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	fmt.Println(catena.Except(catena.Of(1, 2, 3), catena.Of(2)).Collect())
}

Output:
[1 3]

func Flatten 

func Flatten[T any](s Seq[Seq[T]]) Seq[T]

Flatten yields every element of every inner sequence, in order.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	inner := catena.Of(catena.Of(1, 2), catena.Of(3))
	fmt.Println(catena.Flatten(inner).Collect())
}

Output:
[1 2 3]

func FlattenSlices 

func FlattenSlices[T any](s Seq[[]T]) Seq[T]

FlattenSlices yields every element of every slice, in order.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	fmt.Println(catena.FlattenSlices(catena.Of([]int{1, 2}, []int{3})).Collect())
}

Output:
[1 2 3]

func From 

func From[T any](seq func(func(T) bool)) Seq[T]

From adapts any push-function sequence — iter.Seq, catena.Seq, or a third-party alias — with no conversion at the call site. Re-iterability depends on the source.

Example 
package main

import (
	"fmt"
	"slices"

	"github.com/NerdMeNot/catena"
)

func main() {
	// Takes the literal function type, so any iterator adapts without a
	// conversion at the call site — including the standard library's.
	fmt.Println(catena.From(slices.Values([]string{"a", "b"})).Collect())
}

Output:
[a b]

func FromChan 

func FromChan[T any](ctx context.Context, ch <-chan T) Seq[T]

FromChan yields values received from ch until ch is closed or ctx is done. Single-use. No goroutine is started; a sequence that is never consumed never receives.

Example 
package main

import (
	"context"
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	ch := make(chan int, 3)
	for i := 1; i <= 3; i++ {
		ch <- i
	}
	close(ch)

	// Single-use, and it starts no goroutine: a sequence that is never
	// consumed never receives.
	fmt.Println(catena.FromChan(context.Background(), ch).Collect())
}

Output:
[1 2 3]

func FromSlice 

func FromSlice[T any](s []T) Seq[T]

FromSlice returns a re-iterable Seq over s. The slice is not copied; mutations to it are visible to later iterations.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	// The slice is not copied: later mutations are visible to later
	// iterations, which is what makes this free.
	xs := []int{1, 2, 3}
	s := catena.FromSlice(xs)
	xs[0] = 99
	fmt.Println(s.Collect())
}

Output:
[99 2 3]

func Generate 

func Generate[T any](seed T, next func(T) T) Seq[T]

Generate yields seed, then next(seed), then next(next(seed)), forever. Infinite. Re-iterable iff next is pure.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	// The seed is yielded first, then next applied repeatedly. Infinite.
	fmt.Println(catena.Generate(1, func(n int) int { return n * 3 }).
		Take(4).
		Collect())
}

Output:
[1 3 9 27]

func GenerateWhile 

func GenerateWhile[T any](seed T, next func(T) (T, bool)) Seq[T]

GenerateWhile yields seed unconditionally, then successive next values until next reports false.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	// The seed is yielded unconditionally; a value produced alongside
	// ok=false is not.
	fmt.Println(catena.GenerateWhile(1, func(n int) (int, bool) {
		return n * 3, n < 9
	}).Collect())
}

Output:
[1 3 9]

func Intersect 

func Intersect[T comparable](a, b Seq[T]) Seq[T]

Intersect yields the distinct elements of a that occur in b, in a's encounter order. ⚠ Buffers all of b before a is consumed, plus a seen- set of a's distinct values.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	fmt.Println(catena.Intersect(catena.Of(1, 2, 3), catena.Of(3, 1)).Collect())
}

Output:
[1 3]

func NonZero 

func NonZero[T comparable](s Seq[T]) Seq[T]

NonZero yields the elements that are not the zero value of T.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	// Drops the zero value of T — empty strings here, but equally 0,
	// nil pointers, or a zero struct.
	fmt.Println(catena.NonZero(catena.Of("go", "", "rust", "")).Collect())
}

Output:
[go rust]

func Of 

func Of[T any](vals ...T) Seq[T]

Of returns a re-iterable Seq over the given values.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	fmt.Println(catena.Of("go", "rust", "zig").Collect())
}

Output:
[go rust zig]

func Once1 

func Once1[T any](v T) Seq[T]

Once1 returns a re-iterable Seq of exactly one value. (Once, without the suffix, is the single-use guard method on Seq.)

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	// Once1, not Once: Once is the single-use guard method on Seq.
	fmt.Println(catena.Once1("only").Collect())
}

Output:
[only]

func Range 

func Range[I Integer](start, stop, step I) Seq[I]

Range yields start, start+step, ... while the value is before stop (exclusive). step == 0 panics at construction; a sign mismatch between step and the start→stop direction yields an empty sequence. Termination is overflow-guarded: a step past the type's edge stops rather than wrapping. Unsigned types cannot step downward.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	// Half-open, like a slice expression. A sign mismatch between step
	// and direction yields nothing rather than panicking, so a computed
	// step is safe.
	fmt.Println(catena.Range(0, 10, 3).Collect())
	fmt.Println(catena.Range(3, 0, -1).Collect())
	fmt.Println(catena.Range(0, 10, -1).Collect())
}

Output:
[0 3 6 9]
[3 2 1]
[]

func Repeat 

func Repeat[T any](v T) Seq[T]

Repeat yields v forever. Infinite: pair with Take or a conditional terminal.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	// Infinite, so it must be bounded by something downstream.
	fmt.Println(catena.Repeat("ha").Take(3).Collect())
}

Output:
[ha ha ha]

func RepeatN 

func RepeatN[T any](v T, n int) Seq[T]

RepeatN yields v exactly n times. Panics if n is negative.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	fmt.Println(catena.RepeatN(0, 4).Collect())
}

Output:
[0 0 0 0]

func Sorted 

func Sorted[T cmp.Ordered](s Seq[T]) Seq[T]

Sorted yields the elements in ascending order, stably. NaN sorts first. ⚠ Buffers the entire input.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	fmt.Println(catena.Sorted(catena.Of(3, 1, 2)).Collect())
}

Output:
[1 2 3]

func SortedDesc 

func SortedDesc[T cmp.Ordered](s Seq[T]) Seq[T]

SortedDesc yields the elements in descending order, stably. ⚠ Buffers the entire input.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	fmt.Println(catena.SortedDesc(catena.Of(3, 1, 2)).Collect())
}

Output:
[3 2 1]

func Union 

func Union[T comparable](a, b Seq[T]) Seq[T]

Union yields the distinct elements of a, then the distinct elements of b not in a — set semantics, encounter order. ⚠ Retains one entry per distinct value — unbounded.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	// Set semantics: the result is deduplicated, in encounter order,
	// left operand first.
	fmt.Println(catena.Union(catena.Of(1, 2, 2), catena.Of(3, 1)).Collect())
}

Output:
[1 2 3]

func Windowed 

func Windowed[T any](s Seq[T], size, step int) Seq[[]T]

Windowed yields sliding windows of exactly size elements, advancing by step; trailing elements that do not fill a window are dropped. step > size is valid and samples with gaps. ⚠ Buffers size elements. Every window is a fresh slice. Panics if size or step is <= 0. A package function for the same instantiation-cycle reason as Chunked.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	// Overlapping windows: size 3 advancing by 1. Trailing elements that
	// cannot fill a window are dropped.
	for w := range catena.Windowed(catena.Range(1, 6, 1), 3, 1).Seq() {
		fmt.Println(w)
	}
}

Output:
[1 2 3]
[2 3 4]
[3 4 5]

func (Seq[T]) All 

func (s Seq[T]) All(pred func(T) bool) bool

All reports whether pred admits every element; stops at the first counterexample. Vacuously true on empty input.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	// Vacuously true on an empty sequence.
	fmt.Println(catena.Of(2, 4).All(func(n int) bool { return n%2 == 0 }))
	fmt.Println(catena.Empty[int]().All(func(n int) bool { return false }))
}

Output:
true
true

func (Seq[T]) Any 

func (s Seq[T]) Any(pred func(T) bool) bool

Any reports whether pred admits any element; stops at the first match.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	// Stops at the first match, so it terminates on an infinite source.
	fmt.Println(catena.Generate(1, func(n int) int { return n + 1 }).
		Any(func(n int) bool { return n > 100 }))
}

Output:
true

func (Seq[T]) Append 

func (s Seq[T]) Append(vals ...T) Seq[T]

Append yields s, then the given values.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	fmt.Println(catena.Of("a").Append("b", "c").Collect())
}

Output:
[a b c]

func (Seq[T]) Associate 

func (s Seq[T]) Associate[K comparable, V any](f func(T) (K, V)) map[K]V

Associate builds a map from f's key/value pairs; on duplicate keys the last pair wins. ⚠ Full drain; map iteration order is undefined.

Example 
package main

import (
	"fmt"
	"strings"

	"github.com/NerdMeNot/catena"
)

func main() {
	m := catena.Of(1, 2).Associate(func(n int) (int, string) {
		return n, strings.Repeat("*", n)
	})
	fmt.Println(m[1], m[2])
}

Output:
* **

func (Seq[T]) AverageOf 

func (s Seq[T]) AverageOf[N Numeric](sel func(T) N) (float64, bool)

AverageOf returns the mean of the selected values, accumulating in float64 (naive summation — precision for large integer inputs is not guaranteed); (0, false) on empty input. ⚠ Full drain.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	// Accumulates in float64, and reports false on empty rather than
	// dividing by zero.
	avg, ok := catena.Of(1, 2, 4).AverageOf(catena.Self[int])
	_, empty := catena.Empty[int]().AverageOf(catena.Self[int])
	fmt.Printf("%.2f %v %v\n", avg, ok, empty)
}

Output:
2.33 true false

func (Seq[T]) BottomNBy 

func (s Seq[T]) BottomNBy[K cmp.Ordered](n int, sel func(T) K) []T

BottomNBy returns the n elements with the smallest keys, sorted ascending by key; equal keys retain encounter order. Memory is O(n). ⚠ Full drain. Panics if n is negative.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	fmt.Println(catena.Of(5, 1, 9, 3, 7).BottomNBy(2, catena.Self[int]))
}

Output:
[1 3]

func (Seq[T]) Collect 

func (s Seq[T]) Collect() []T

Collect drains the sequence into a slice; nil for empty. ⚠ Full drain.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	// nil for an empty sequence, matching slices.Collect.
	fmt.Println(catena.Of(1, 2).Collect(), catena.Empty[int]().Collect() == nil)
}

Output:
[1 2] true

func (Seq[T]) Concat 

func (s Seq[T]) Concat(others ...Seq[T]) Seq[T]

Concat yields s, then each of the others in order.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	fmt.Println(catena.Of(1, 2).Concat(catena.Of(3), catena.Of(4, 5)).Collect())
}

Output:
[1 2 3 4 5]

func (Seq[T]) Count 

func (s Seq[T]) Count() int

Count returns the number of elements. ⚠ Full drain.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	fmt.Println(catena.Of("a", "b", "c").Count())
}

Output:
3

func (Seq[T]) CountWhere 

func (s Seq[T]) CountWhere(pred func(T) bool) int

CountWhere returns the number of elements pred admits. ⚠ Full drain.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	// Fused filter and count: one stage rather than two.
	fmt.Println(catena.Range(1, 11, 1).CountWhere(func(n int) bool { return n%3 == 0 }))
}

Output:
3

func (Seq[T]) DedupeBy 

func (s Seq[T]) DedupeBy[K comparable](sel func(T) K) Seq[T]

DedupeBy yields elements whose key differs from the previous element's key — consecutive duplicates only, O(1) memory. On key-sorted input it equals DistinctBy at a fraction of the cost.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	// Collapses CONSECUTIVE runs only, in O(1) memory — the streaming
	// alternative to DistinctBy, and the right choice on an unbounded
	// source where a seen-set would grow forever.
	type reading struct {
		Tick int
		Zone string
	}
	readings := catena.Of(
		reading{1, "cold"}, reading{2, "cold"},
		reading{3, "warm"}, reading{4, "cold"},
	)

	fmt.Println(readings.
		DedupeBy(func(r reading) string { return r.Zone }).
		Collect())
}

Output:
[{1 cold} {3 warm} {4 cold}]

func (Seq[T]) DistinctBy 

func (s Seq[T]) DistinctBy[K comparable](sel func(T) K) Seq[T]

DistinctBy yields elements whose key has not been seen before; the first occurrence wins. ⚠ Retains one key per distinct value — unbounded.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	type user struct {
		Org, Name string
	}
	users := catena.Of(
		user{"acme", "ada"},
		user{"globex", "bob"},
		user{"acme", "eve"},
	)

	// One user per org; the first occurrence wins.
	fmt.Println(users.
		DistinctBy(func(u user) string { return u.Org }).
		Collect())
}

Output:
[{acme ada} {globex bob}]

func (Seq[T]) DistinctWith 

func (s Seq[T]) DistinctWith(eq func(a, b T) bool) Seq[T]

DistinctWith yields elements no earlier element equals under eq. First occurrence wins. ⚠ Retains all distinct elements and compares in O(n²) — small inputs only.

Example 
package main

import (
	"fmt"
	"strings"

	"github.com/NerdMeNot/catena"
)

func main() {
	// For keys that are not comparable, or an equality of your own.
	// Retains every distinct element and compares against all of them,
	// so this is for small inputs.
	fmt.Println(catena.Of("Go", "GO", "rust", "go").
		DistinctWith(strings.EqualFold).
		Collect())
}

Output:
[Go rust]

func (Seq[T]) Drain 

func (s Seq[T]) Drain()

Drain consumes the sequence for its side effects. ⚠ Full drain.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	// Consume for side effects alone, discarding the elements.
	count := 0
	catena.Of(1, 2, 3).OnEach(func(int) { count++ }).Drain()
	fmt.Println(count)
}

Output:
3

func (Seq[T]) Drop 

func (s Seq[T]) Drop(n int) Seq[T]

Drop skips the first n elements. Panics if n is negative.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	fmt.Println(catena.Of("a", "b", "c", "d").Drop(2).Collect())
}

Output:
[c d]

func (Seq[T]) DropLast 

func (s Seq[T]) DropLast(n int) Seq[T]

DropLast yields all but the final n elements, emitting with an n-element lag. Panics if n is negative. ⚠ Buffers n elements.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	fmt.Println(catena.Range(1, 6, 1).DropLast(2).Collect())
}

Output:
[1 2 3]

func (Seq[T]) DropWhile 

func (s Seq[T]) DropWhile(pred func(T) bool) Seq[T]

DropWhile skips elements until pred first returns false, then yields the rest.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	// Drops only the leading run; once the predicate fails, everything
	// after is kept.
	fmt.Println(catena.Of(0, 0, 3, 0, 5).
		DropWhile(func(n int) bool { return n == 0 }).
		Collect())
}

Output:
[3 0 5]

func (Seq[T]) ElementAt 

func (s Seq[T]) ElementAt(i int) (T, bool)

ElementAt returns the element at index i; (zero, false) for a negative or out-of-range index.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	v, ok := catena.Of("a", "b", "c").ElementAt(1)
	_, neg := catena.Of("a").ElementAt(-1)
	fmt.Println(v, ok, neg)
}

Output:
b true false

func (Seq[T]) Filter 

func (s Seq[T]) Filter(pred func(T) bool) Seq[T]

Filter yields the elements for which pred returns true.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	fmt.Println(catena.Of(1, 2, 3, 4, 5).
		Filter(func(n int) bool { return n%2 == 0 }).
		Collect())
}

Output:
[2 4]

func (Seq[T]) FilterErr 

func (s Seq[T]) FilterErr(pred func(T) (bool, error)) Try[T]

FilterErr yields elements pred admits, as a Try; a failed pred call yields (zero, err).

Example 
package main

import (
	"fmt"
	"strings"

	"github.com/NerdMeNot/catena"
)

func main() {
	// A predicate that can fail produces a Try, so the caller chooses
	// what a failure means rather than the pipeline deciding.
	ports := catena.Of("80", "443", "https", "8080")
	valid := ports.FilterErr(func(s string) (bool, error) {
		if strings.ContainsAny(s, "abcdefghijklmnopqrstuvwxyz") {
			return false, fmt.Errorf("not a port: %q", s)
		}
		return len(s) > 2, nil
	})

	kept, errs := valid.CollectAll()
	fmt.Println(kept, errs)
}

Output:
[443 8080] [not a port: "https"]

func (Seq[T]) FilterIndexed 

func (s Seq[T]) FilterIndexed(pred func(int, T) bool) Seq[T]

FilterIndexed yields the elements for which pred(index, element) returns true. The index counts source elements from 0.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	// The index counts source elements, not surviving ones.
	fmt.Println(catena.Of("a", "b", "c", "d", "e").
		FilterIndexed(func(i int, _ string) bool { return i%2 == 0 }).
		Collect())
}

Output:
[a c e]

func (Seq[T]) FilterMap 

func (s Seq[T]) FilterMap[U any](f func(T) (U, bool)) Seq[U]

FilterMap yields the mapped value for each element f reports true for — a fused Map + Filter.

Example 
package main

import (
	"fmt"
	"strconv"

	"github.com/NerdMeNot/catena"
)

func main() {
	// Fused filter and map: one stage, and the comma-ok shape means the
	// mapped value is discarded rather than computed twice.
	fmt.Println(catena.Of("1", "x", "3").
		FilterMap(func(s string) (int, bool) {
			n, err := strconv.Atoi(s)
			return n, err == nil
		}).
		Collect())
}

Output:
[1 3]

func (Seq[T]) FilterNot 

func (s Seq[T]) FilterNot(pred func(T) bool) Seq[T]

FilterNot yields the elements for which pred returns false.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	// The negated form, for when the predicate reads better positively.
	fmt.Println(catena.Of("go", "", "rust", "", "zig").
		FilterNot(func(s string) bool { return s == "" }).
		Collect())
}

Output:
[go rust zig]

func (Seq[T]) Find 

func (s Seq[T]) Find(pred func(T) bool) (T, bool)

Find returns the first element pred admits.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	v, ok := catena.Of(1, 4, 9).Find(func(n int) bool { return n > 3 })
	fmt.Println(v, ok)
}

Output:
4 true

func (Seq[T]) FindIndex 

func (s Seq[T]) FindIndex(pred func(T) bool) int

FindIndex returns the index of the first element pred admits; -1 if none.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	fmt.Println(catena.Of("a", "b").FindIndex(func(s string) bool { return s == "b" }))
	fmt.Println(catena.Of("a").FindIndex(func(s string) bool { return s == "z" }))
}

Output:
1
-1

func (Seq[T]) FindLast 

func (s Seq[T]) FindLast(pred func(T) bool) (T, bool)

FindLast returns the final element pred admits. ⚠ Full drain.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	v, ok := catena.Of(1, 4, 9).FindLast(func(n int) bool { return n > 3 })
	fmt.Println(v, ok)
}

Output:
9 true

func (Seq[T]) FindMap 

func (s Seq[T]) FindMap[U any](f func(T) (U, bool)) (U, bool)

FindMap returns the first mapped value f reports true for — a fused Find + Map.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	// Fused find and map: the mapped value is returned, not the element.
	v, ok := catena.Of("x", "12", "y").FindMap(func(s string) (int, bool) {
		n := 0
		_, err := fmt.Sscanf(s, "%d", &n)
		return n, err == nil
	})
	fmt.Println(v, ok)
}

Output:
12 true

func (Seq[T]) First 

func (s Seq[T]) First() (T, bool)

First returns the first element.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	v, ok := catena.Of(3, 1).First()
	_, empty := catena.Empty[int]().First()
	fmt.Println(v, ok, empty)
}

Output:
3 true false

func (Seq[T]) FlatMap 

func (s Seq[T]) FlatMap[U any](f func(T) Seq[U]) Seq[U]

FlatMap yields all elements of f(v) for each element v, in order.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	fmt.Println(catena.Of(1, 2).
		FlatMap(func(n int) catena.Seq[int] { return catena.Of(n, -n) }).
		Collect())
}

Output:
[1 -1 2 -2]

func (Seq[T]) FlatMapSlice 

func (s Seq[T]) FlatMapSlice[U any](f func(T) []U) Seq[U]

FlatMapSlice yields all elements of the slice f(v) for each element v.

Example 
package main

import (
	"fmt"
	"strings"

	"github.com/NerdMeNot/catena"
)

func main() {
	// The same, when the callback already has a slice in hand.
	fmt.Println(catena.Of("a b", "c d").
		FlatMapSlice(func(s string) []string { return strings.Fields(s) }).
		Collect())
}

Output:
[a b c d]

func (Seq[T]) Fold 

func (s Seq[T]) Fold[A any](init A, f func(A, T) A) A

Fold reduces the sequence into an accumulator, left to right. ⚠ Full drain.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	fmt.Println(catena.Of(1, 2, 3).Fold(100, func(acc, n int) int { return acc + n }))
}

Output:
106

func (Seq[T]) FoldBy 

func (s Seq[T]) FoldBy[K comparable, A any](
	key func(T) K, init func(K) A, f func(A, T) A,
) map[K]A

FoldBy folds each element into a per-key accumulator: streaming grouped aggregation with no intermediate per-key slices. init is called once per distinct key. Memory is bounded by the number of distinct keys, not elements. ⚠ Full drain; map iteration order is undefined.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	// Streaming aggregation per key. GroupBy would retain every element;
	// this retains one accumulator per distinct key.
	type sale struct {
		Region string
		Amount int
	}
	sales := catena.Of(
		sale{"west", 100}, sale{"east", 40}, sale{"west", 20},
	)
	totals := sales.FoldBy(
		func(s sale) string { return s.Region },
		func(string) int { return 0 },
		func(sum int, s sale) int { return sum + s.Amount },
	)
	fmt.Println(totals["west"], totals["east"])
}

Output:
120 40

func (Seq[T]) FoldErr 

func (s Seq[T]) FoldErr[A any](init A, f func(A, T) (A, error)) (A, error)

FoldErr folds until f fails, returning the accumulator so far and the first error.

Example 
package main

import (
	"errors"
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	// Stops at the first error and returns the accumulator so far.
	acc, err := catena.Of(1, 2, 3).FoldErr(0, func(acc, n int) (int, error) {
		if n == 3 {
			return 0, errors.New("three is too many")
		}
		return acc + n, nil
	})
	fmt.Println(acc, err)
}

Output:
3 three is too many

func (Seq[T]) FoldIndexed 

func (s Seq[T]) FoldIndexed[A any](init A, f func(int, A, T) A) A

FoldIndexed is Fold with the element index. ⚠ Full drain.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	fmt.Println(catena.Of(10, 20).FoldIndexed(0, func(i, acc, n int) int { return acc + i*n }))
}

Output:
20

func (Seq[T]) FoldWhile 

func (s Seq[T]) FoldWhile[A any](init A, f func(A, T) (A, bool)) A

FoldWhile folds until f reports false; the accumulator from the stopping call is included in the result.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	// Stops when the callback says so; the accumulator from the stopping
	// call is included.
	fmt.Println(catena.Of(1, 2, 3, 4).FoldWhile(0, func(acc, n int) (int, bool) {
		acc += n
		return acc, acc < 5
	}))
}

Output:
6

func (Seq[T]) ForEach 

func (s Seq[T]) ForEach(f func(T))

ForEach calls f on every element. ⚠ Full drain.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	catena.Of("a", "b").ForEach(func(s string) { fmt.Print(s) })
}

Output:
ab

func (Seq[T]) ForEachErr 

func (s Seq[T]) ForEachErr(f func(T) error) error

ForEachErr calls f on every element, stopping at and returning the first non-nil error; nil if the sequence drains clean.

Example 
package main

import (
	"errors"
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	// Stops at the first error the callback returns, and returns it.
	err := catena.Of(1, 2, 3).ForEachErr(func(n int) error {
		if n == 2 {
			return errors.New("stopped at 2")
		}
		fmt.Println("handled", n)
		return nil
	})
	fmt.Println("err:", err)
}

Output:
handled 1
err: stopped at 2

func (Seq[T]) ForEachIndexed 

func (s Seq[T]) ForEachIndexed(f func(int, T))

ForEachIndexed calls f(index, element) on every element. ⚠ Full drain.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	catena.Of("a", "b").ForEachIndexed(func(i int, s string) { fmt.Printf("%d=%s ", i, s) })
}

Output:
0=a 1=b

func (Seq[T]) GroupBy 

func (s Seq[T]) GroupBy[K comparable](sel func(T) K) map[K][]T

GroupBy collects elements into per-key buckets, each in encounter order. ⚠ Full drain; retains every element; map iteration order is undefined.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	// Retains every element. For an aggregate, FoldBy is bounded by keys.
	byParity := catena.Range(1, 6, 1).GroupBy(func(n int) string {
		if n%2 == 0 {
			return "even"
		}
		return "odd"
	})
	fmt.Println(byParity["odd"], byParity["even"])
}

Output:
[1 3 5] [2 4]

func (Seq[T]) IfEmpty 

func (s Seq[T]) IfEmpty(defaults ...T) Seq[T]

IfEmpty yields s, or the given defaults if s yields nothing.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	// A fallback for the whole sequence, not per element.
	fmt.Println(catena.Of(1, 2).IfEmpty(0).Collect())
	fmt.Println(catena.Empty[int]().IfEmpty(0).Collect())
}

Output:
[1 2]
[0]

func (Seq[T]) IndexBy 

func (s Seq[T]) IndexBy[K comparable](sel func(T) K) map[K]T

IndexBy maps each key to its element; on duplicate keys the last element wins. ⚠ Full drain; map iteration order is undefined.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	// A lookup table; on a duplicate key the last element wins.
	m := catena.Of("apple", "avocado", "blueberry").
		IndexBy(func(s string) byte { return s[0] })
	fmt.Println(string(m['a']), string(m['b']))
}

Output:
avocado blueberry

func (Seq[T]) Intersperse 

func (s Seq[T]) Intersperse(sep T) Seq[T]

Intersperse yields sep between consecutive elements.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	fmt.Println(catena.Of("a", "b", "c").Intersperse("-").Collect())
}

Output:
[a - b - c]

func (Seq[T]) IsEmpty 

func (s Seq[T]) IsEmpty() bool

IsEmpty reports whether the sequence yields nothing. ⚠ It does so by consuming one element — on a single-pass source that element is lost.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	// Answers by consuming one element — on a single-pass source that
	// element is gone.
	fmt.Println(catena.Empty[int]().IsEmpty(), catena.Of(1).IsEmpty())
}

Output:
true false

func (Seq[T]) JoinBy 

func (s Seq[T]) JoinBy[U any, K comparable, R any](
	other Seq[U],
	leftKey func(T) K, rightKey func(U) K,
	combine func(T, U) R,
) Seq[R]

JoinBy is a relational inner join: it pairs each element of s with every element of other sharing the same key and yields combine for each pair. Unmatched elements on either side are dropped; duplicate keys produce the cross product per key. Output order is left encounter order, then right encounter order within a key. ⚠ Buffers all of other before the first emission.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	// A relational inner join: unmatched rows on either side are dropped,
	// and duplicate keys produce the cross product.
	type order struct {
		Customer int
		Amount   int
	}
	type customer struct {
		ID   int
		Name string
	}
	orders := catena.Of(order{1, 30}, order{2, 10}, order{9, 99})
	customers := catena.Of(customer{1, "ada"}, customer{2, "bob"})

	fmt.Println(orders.JoinBy(customers,
		func(o order) int { return o.Customer },
		func(c customer) int { return c.ID },
		func(o order, c customer) string { return fmt.Sprintf("%s:%d", c.Name, o.Amount) },
	).Collect())
}

Output:
[ada:30 bob:10]

func (Seq[T]) JoinToString 

func (s Seq[T]) JoinToString(sep string, sel func(T) string) string

JoinToString concatenates the selected strings with sep between elements. ⚠ Full drain.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	type user struct{ Name string }
	users := catena.Of(user{"ada"}, user{"bob"})
	fmt.Println(users.JoinToString(", ", func(u user) string { return u.Name }))
}

Output:
ada, bob

func (Seq[T]) Last 

func (s Seq[T]) Last() (T, bool)

Last returns the final element. ⚠ Full drain.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	v, ok := catena.Of(3, 1).Last()
	fmt.Println(v, ok)
}

Output:
1 true

func (Seq[T]) Map 

func (s Seq[T]) Map[U any](f func(T) U) Seq[U]

Map yields f applied to each element.

Example 
package main

import (
	"fmt"
	"strings"

	"github.com/NerdMeNot/catena"
)

func main() {
	// The element type changes mid-chain, which is what generic methods
	// made possible.
	fmt.Println(catena.Of(1, 2, 3).
		Map(func(n int) string { return strings.Repeat("*", n) }).
		Collect())
}

Output:
[* ** ***]

func (Seq[T]) MapErr 

func (s Seq[T]) MapErr[U any](f func(T) (U, error)) Try[U]

MapErr yields f applied to each element as a Try; a failed call yields (zero, err).

Example 
package main

import (
	"fmt"
	"strconv"

	"github.com/NerdMeNot/catena"
)

func main() {
	// A mapping that can fail produces a Try; a failed call yields the
	// zero value alongside the error, never a half-built one.
	parsed := catena.Of("1", "two", "3").MapErr(strconv.Atoi)
	vals, errs := parsed.CollectAll()
	fmt.Println(vals, len(errs))
}

Output:
[1 3] 1

func (Seq[T]) MapIndexed 

func (s Seq[T]) MapIndexed[U any](f func(int, T) U) Seq[U]

MapIndexed yields f(index, element), counting from 0.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	fmt.Println(catena.Of("a", "b", "c").
		MapIndexed(func(i int, s string) string { return fmt.Sprintf("%d:%s", i, s) }).
		Collect())
}

Output:
[0:a 1:b 2:c]

func (Seq[T]) MaxBy 

func (s Seq[T]) MaxBy[K cmp.Ordered](sel func(T) K) (T, bool)

MaxBy returns the element with the largest key; the earliest maximal element wins ties. NaN keys order below everything (cmp.Compare). ⚠ Full drain.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	// Returns the ELEMENT with the largest key; ties go to the first.
	type run struct {
		Name string
		Secs int
	}
	runs := catena.Of(run{"ada", 42}, run{"bob", 51}, run{"eve", 51})
	slowest, _ := runs.MaxBy(func(r run) int { return r.Secs })
	fmt.Println(slowest.Name)
}

Output:
bob

func (Seq[T]) MaxOf 

func (s Seq[T]) MaxOf[K cmp.Ordered](sel func(T) K) (K, bool)

MaxOf returns the largest key. ⚠ Full drain.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	// Returns the KEY, where MaxBy returns the element — the -By/-Of
	// distinction, which holds across the whole library.
	longest, _ := catena.Of("go", "rust", "c").MaxOf(func(s string) int { return len(s) })
	fmt.Println(longest)
}

Output:
4

func (Seq[T]) MaxWith 

func (s Seq[T]) MaxWith(cmp func(a, b T) int) (T, bool)

MaxWith returns the largest element under cmp; the earliest maximal element wins ties. ⚠ Full drain.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	// A comparator, for orderings no single key expresses.
	v, _ := catena.Of("bb", "a", "cccc").
		MaxWith(func(x, y string) int { return len(x) - len(y) })
	fmt.Println(v)
}

Output:
cccc

func (Seq[T]) MinBy 

func (s Seq[T]) MinBy[K cmp.Ordered](sel func(T) K) (T, bool)

MinBy returns the element with the smallest key; the earliest minimal element wins ties. ⚠ Full drain.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	type run struct {
		Name string
		Secs int
	}
	runs := catena.Of(run{"ada", 42}, run{"bob", 51})
	fastest, _ := runs.MinBy(func(r run) int { return r.Secs })
	fmt.Println(fastest.Name)
}

Output:
ada

func (Seq[T]) MinMaxOf 

func (s Seq[T]) MinMaxOf[K cmp.Ordered](sel func(T) K) (min, max K, ok bool)

MinMaxOf returns the smallest and largest keys in one pass. ⚠ Full drain.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	// Both ends in a single pass.
	lo, hi, ok := catena.Of("go", "rust", "c").MinMaxOf(func(s string) int { return len(s) })
	fmt.Println(lo, hi, ok)
}

Output:
1 4 true

func (Seq[T]) MinOf 

func (s Seq[T]) MinOf[K cmp.Ordered](sel func(T) K) (K, bool)

MinOf returns the smallest key. ⚠ Full drain.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	shortest, _ := catena.Of("go", "rust", "c").MinOf(func(s string) int { return len(s) })
	fmt.Println(shortest)
}

Output:
1

func (Seq[T]) MinWith 

func (s Seq[T]) MinWith(cmp func(a, b T) int) (T, bool)

MinWith returns the smallest element under cmp; the earliest minimal element wins ties. ⚠ Full drain.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	v, _ := catena.Of("bb", "a", "cccc").
		MinWith(func(x, y string) int { return len(x) - len(y) })
	fmt.Println(v)
}

Output:
a

func (Seq[T]) None 

func (s Seq[T]) None(pred func(T) bool) bool

None reports whether pred admits no element; stops at the first match.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	fmt.Println(catena.Of(1, 3).None(func(n int) bool { return n%2 == 0 }))
}

Output:
true

func (Seq[T]) OnEach 

func (s Seq[T]) OnEach(f func(T)) Seq[T]

OnEach calls f on every element and passes it through unchanged.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	// Side effects without changing the stream — logging, metrics,
	// progress. Elements pass through untouched.
	seen := 0
	total := catena.Of(1, 2, 3).
		OnEach(func(int) { seen++ }).
		SumOf(catena.Self[int])
	fmt.Println(total, seen)
}

Output:
6 3

func (Seq[T]) Once 

func (s Seq[T]) Once() Seq[T]

Once returns a sequence that panics if iterated more than once — a development guard for the single-pass contract, not a synchronization mechanism. This is the one operator whose state deliberately lives outside the iteration closure.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	// A development guard for the single-pass contract: the second
	// consumption panics instead of silently re-running the producer.
	s := catena.Of(1, 2).Once()
	fmt.Println(s.Collect())

	defer func() { fmt.Println("recovered:", recover()) }()
	s.Collect()
}

Output:
[1 2]
recovered: catena: Once: sequence consumed more than once

func (Seq[T]) Partition 

func (s Seq[T]) Partition(pred func(T) bool) (yes, no []T)

Partition splits elements by pred, preserving encounter order on both sides; nil slices for empty sides. ⚠ Full drain.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	// Both sides in one pass, each in encounter order.
	even, odd := catena.Range(1, 6, 1).Partition(func(n int) bool { return n%2 == 0 })
	fmt.Println(even, odd)
}

Output:
[2 4] [1 3 5]

func (Seq[T]) Prepend 

func (s Seq[T]) Prepend(vals ...T) Seq[T]

Prepend yields the given values, then s.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	fmt.Println(catena.Of("c").Prepend("a", "b").Collect())
}

Output:
[a b c]

func (Seq[T]) ProductOf 

func (s Seq[T]) ProductOf[N Numeric](sel func(T) N) N

ProductOf multiplies the selected values. Empty input yields 1, the multiplicative identity. ⚠ Full drain.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	// The empty product is 1, the multiplicative identity.
	fmt.Println(catena.Of(2, 3, 4).ProductOf(catena.Self[int]))
	fmt.Println(catena.Empty[int]().ProductOf(catena.Self[int]))
}

Output:
24
1

func (Seq[T]) Pull 

func (s Seq[T]) Pull() (next func() (T, bool), stop func())

Pull converts s to a pull-based iterator. THE CALLER MUST CALL stop, even if next has returned false, or resources held by s will leak.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	// Inverts control for hand-written loops. THE CALLER MUST CALL stop,
	// or a producer holding a resource never releases it.
	next, stop := catena.Of("a", "b").Pull()
	defer stop()

	v, ok := next()
	fmt.Println(v, ok)
}

Output:
a true

func (Seq[T]) Reduce 

func (s Seq[T]) Reduce(f func(T, T) T) (T, bool)

Reduce folds the sequence using its first element as the initial accumulator; (zero, false) on empty input. ⚠ Full drain.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	// Uses the first element as the seed, so it reports false on empty
	// rather than inventing a zero.
	v, ok := catena.Of(3, 1, 2).Reduce(func(a, b int) int { return a * b })
	_, empty := catena.Empty[int]().Reduce(func(a, b int) int { return a })
	fmt.Println(v, ok, empty)
}

Output:
6 true false

func (Seq[T]) Reversed 

func (s Seq[T]) Reversed() Seq[T]

Reversed yields the elements in reverse order. ⚠ Buffers the entire input — hangs on infinite input.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	fmt.Println(catena.Of(1, 2, 3).Reversed().Collect())
}

Output:
[3 2 1]

func (Seq[T]) Scan 

func (s Seq[T]) Scan[A any](init A, f func(A, T) A) Seq[A]

Scan yields the running accumulator: f(init, e0), f(that, e1), ... The initial value itself is not yielded.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	// A running fold: the accumulator is emitted at each step. The
	// initial value itself is not emitted, so the output is as long as
	// the input.
	fmt.Println(catena.Of(1, 2, 3, 4).
		Scan(0, func(sum, n int) int { return sum + n }).
		Collect())
}

Output:
[1 3 6 10]

func (Seq[T]) Seq 

func (s Seq[T]) Seq() iter.Seq[T]

Seq converts to the stdlib iterator type. Free.

Example 
package main

import (
	"fmt"
	"slices"

	"github.com/NerdMeNot/catena"
)

func main() {
	// A free conversion to the standard iterator type, in both
	// directions — Seq IS iter.Seq underneath.
	fmt.Println(slices.Collect(catena.Of(1, 2, 3).Seq()))
}

Output:
[1 2 3]

func (Seq[T]) Single 

func (s Seq[T]) Single() (T, bool)

Single returns the element iff the sequence has exactly one; it stops consuming upon seeing a second.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	// True only for exactly one element; it stops as soon as a second
	// arrives rather than counting the rest.
	a, ok1 := catena.Of(7).Single()
	_, ok2 := catena.Of(7, 8).Single()
	fmt.Println(a, ok1, ok2)
}

Output:
7 true false

func (Seq[T]) SortedBy 

func (s Seq[T]) SortedBy[K cmp.Ordered](sel func(T) K) Seq[T]

SortedBy yields the elements sorted ascending by key, stably. sel is called exactly once per element (decorate-sort-undecorate). ⚠ Buffers the entire input — hangs on infinite input.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	// Stable, and the selector runs exactly once per element rather than
	// once per comparison — so an expensive key is affordable. Stability
	// shows here: kiwi and date are both 4 long, and kiwi came first.
	words := catena.Of("kiwi", "fig", "banana", "date")
	fmt.Println(words.SortedBy(func(s string) int { return len(s) }).Collect())
}

Output:
[fig kiwi date banana]

func (Seq[T]) SortedByDesc 

func (s Seq[T]) SortedByDesc[K cmp.Ordered](sel func(T) K) Seq[T]

SortedByDesc yields the elements sorted descending by key, stably. sel is called exactly once per element. ⚠ Buffers the entire input — hangs on infinite input.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	words := catena.Of("kiwi", "fig", "banana")
	fmt.Println(words.SortedByDesc(func(s string) int { return len(s) }).Collect())
}

Output:
[banana kiwi fig]

func (Seq[T]) SortedWith 

func (s Seq[T]) SortedWith(cmp func(a, b T) int) Seq[T]

SortedWith yields the elements sorted by cmp, stably. ⚠ Buffers the entire input — hangs on infinite input.

Example 
package main

import (
	"fmt"
	"strings"

	"github.com/NerdMeNot/catena"
)

func main() {
	// A comparator, for orderings a single key cannot express.
	fmt.Println(catena.Of("b", "A", "c").
		SortedWith(func(x, y string) int { return strings.Compare(strings.ToLower(x), strings.ToLower(y)) }).
		Collect())
}

Output:
[A b c]

func (Seq[T]) Step 

func (s Seq[T]) Step(n int) Seq[T]

Step yields the first element and every nth element after it. Panics if n <= 0.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	// The first element always survives, then every nth after it.
	fmt.Println(catena.Range(0, 10, 1).Step(3).Collect())
}

Output:
[0 3 6 9]

func (Seq[T]) SumOf 

func (s Seq[T]) SumOf[N Numeric](sel func(T) N) N

SumOf sums the selected values; integer overflow wraps like +. Empty input sums to 0. ⚠ Full drain.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	type item struct{ Qty int }
	items := catena.Of(item{2}, item{3})
	fmt.Println(items.SumOf(func(i item) int { return i.Qty }))
}

Output:
5

func (Seq[T]) Take 

func (s Seq[T]) Take(n int) Seq[T]

Take yields at most the first n elements, consuming exactly as many as it yields. Panics if n is negative.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	// Consumes exactly what it emits, so it bounds an infinite source.
	fmt.Println(catena.Generate(1, func(n int) int { return n + 1 }).
		Take(3).
		Collect())
}

Output:
[1 2 3]

func (Seq[T]) TakeLast 

func (s Seq[T]) TakeLast(n int) Seq[T]

TakeLast yields the final n elements. ⚠ Buffers n elements and fully drains the source before emitting — hangs on infinite input. Panics if n is negative.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	fmt.Println(catena.Range(1, 8, 1).TakeLast(3).Collect())
}

Output:
[5 6 7]

func (Seq[T]) TakeWhile 

func (s Seq[T]) TakeWhile(pred func(T) bool) Seq[T]

TakeWhile yields elements until pred first returns false.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	// Stops at the first element that fails — unlike Filter, which would
	// keep testing the rest.
	fmt.Println(catena.Of(1, 2, 9, 3).
		TakeWhile(func(n int) bool { return n < 5 }).
		Collect())
}

Output:
[1 2]

func (Seq[T]) TallyBy 

func (s Seq[T]) TallyBy[K comparable](sel func(T) K) map[K]int

TallyBy counts elements per key. ⚠ Full drain; map iteration order is undefined.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	counts := catena.Of("apple", "avocado", "blueberry").
		TallyBy(func(s string) byte { return s[0] })
	fmt.Println(counts['a'], counts['b'])
}

Output:
2 1

func (Seq[T]) ToChan 

func (s Seq[T]) ToChan(ctx context.Context) <-chan T

ToChan starts a goroutine that sends every element on the returned unbuffered channel. The channel is closed when the sequence ends or ctx is done — the consumer must drain or cancel, or the goroutine leaks.

Example 
package main

import (
	"context"
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	// The fan-out mechanism: a Seq is not safe to consume from two
	// goroutines, a channel is. Cancelling ctx closes the channel.
	for v := range catena.Of(1, 2, 3).ToChan(context.Background()) {
		fmt.Print(v, " ")
	}
}

Output:
1 2 3

func (Seq[T]) ToList 

func (s Seq[T]) ToList() List[T]

ToList drains the sequence into a List; nil for empty. ⚠ Full drain.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	// The eager twin: a List has the same operations, evaluated at once.
	l := catena.Of(3, 1, 2).ToList()
	fmt.Println(l.Len(), l.At(0))
}

Output:
3 3

func (Seq[T]) TopNBy 

func (s Seq[T]) TopNBy[K cmp.Ordered](n int, sel func(T) K) []T

TopNBy returns the n elements with the largest keys, sorted descending by key; equal keys retain encounter order and the earliest elements win at the cut. Memory is O(n) — the streaming alternative to SortedByDesc().Take(n). ⚠ Full drain. Panics if n is negative.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	// A bounded heap of n, not a sort: memory is O(n) rather than O(all),
	// which is the difference between a working pipeline and an OOM on a
	// large scan. Output is sorted descending, ties in encounter order.
	fmt.Println(catena.Of(5, 1, 9, 3, 7).TopNBy(3, catena.Self[int]))
}

Output:
[9 7 5]

func (Seq[T]) UntilDone 

func (s Seq[T]) UntilDone(ctx context.Context) Try[T]

UntilDone passes elements through until ctx is done, then yields (zero, ctx.Err()) and stops.

Example 
package main

import (
	"context"
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	// Cancellation enters at the edge rather than threading a context
	// through every stage. The context's error arrives as an element.
	ctx, cancel := context.WithCancel(context.Background())
	cancel()

	_, err := catena.Of(1, 2, 3).UntilDone(ctx).Collect()
	fmt.Println(err)
}

Output:
context canceled

func (Seq[T]) WithIndex 

func (s Seq[T]) WithIndex() Seq2[int, T]

WithIndex pairs each element with its index, counting from 0.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	fmt.Println(catena.Of("a", "b").WithIndex().
		MapTo(func(i int, s string) string { return fmt.Sprintf("%d%s", i, s) }).
		Collect())
}

Output:
[0a 1b]

func (Seq[T]) Zip 

func (s Seq[T]) Zip[U any](other Seq[U]) Seq2[T, U]

Zip pairs elements of s with elements of other, stopping at the shorter side. The receiver drives; other is consumed through iter.Pull (its cleanup always runs). other is pulled once per emitted pair; the receiver is consumed one element past the pair count when other is shorter.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	// Pairs elements positionally and stops at the shorter side.
	names := catena.Of("ada", "bob", "eve")
	scores := catena.Of(90, 85)
	fmt.Println(names.Zip(scores).
		MapTo(func(n string, s int) string { return fmt.Sprintf("%s=%d", n, s) }).
		Collect())
}

Output:
[ada=90 bob=85]

func (Seq[T]) ZipWithNext 

func (s Seq[T]) ZipWithNext() Seq2[T, T]

ZipWithNext yields each adjacent pair (element, next element). Empty and single-element input yield nothing.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	// Each element paired with its successor — deltas, gaps, transitions.
	fmt.Println(catena.Of(3, 7, 12).ZipWithNext().
		MapTo(func(a, b int) int { return b - a }).
		Collect())
}

Output:
[4 5]

type Seq2 

type Seq2[K, V any] iter.Seq2[K, V]

Seq2 is a lazy pair sequence: iter.Seq2 with methods. It is a bridge back to Seq (via Keys, Values, MapTo), deliberately not a full peer surface.

func Empty2 

func Empty2[K, V any]() Seq2[K, V]

Empty2 returns the empty Seq2.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	fmt.Println(catena.Empty2[string, int]().Count())
}

Output:
0

func From2 

func From2[K, V any](seq func(func(K, V) bool)) Seq2[K, V]

From2 adapts any push-function pair sequence.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	pairs := catena.From2(func(yield func(string, int) bool) {
		yield("a", 1)
		yield("b", 2)
	})
	fmt.Println(pairs.MapTo(func(k string, v int) string {
		return fmt.Sprintf("%s=%d", k, v)
	}).Collect())
}

Output:
[a=1 b=2]

func FromMap 

func FromMap[K comparable, V any](m map[K]V) Seq2[K, V]

FromMap returns a re-iterable Seq2 over m, in undefined (map) order.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	ages := map[string]int{"ada": 36}
	fmt.Println(catena.CollectMap(catena.FromMap(ages)))
}

Output:
map[ada:36]

func (Seq2[K, V]) All 

func (s Seq2[K, V]) All(pred func(K, V) bool) bool

All reports whether pred admits every pair; stops at the first counterexample. Vacuously true on empty input.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	fmt.Println(catena.Of(2, 4).WithIndex().All(func(i, v int) bool { return v%2 == 0 }))
}

Output:
true

func (Seq2[K, V]) Any 

func (s Seq2[K, V]) Any(pred func(K, V) bool) bool

Any reports whether pred admits any pair; stops at the first match.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	fmt.Println(catena.Of(1, 2).WithIndex().Any(func(i, v int) bool { return v == 2 }))
}

Output:
true

func (Seq2[K, V]) Count 

func (s Seq2[K, V]) Count() int

Count returns the number of pairs. ⚠ Full drain.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	fmt.Println(catena.Of("a", "b", "c").WithIndex().Count())
}

Output:
3

func (Seq2[K, V]) Drop 

func (s Seq2[K, V]) Drop(n int) Seq2[K, V]

Drop skips the first n pairs. Panics if n is negative.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	fmt.Println(catena.Range(1, 5, 1).WithIndex().Drop(2).Values().Collect())
}

Output:
[3 4]

func (Seq2[K, V]) Filter 

func (s Seq2[K, V]) Filter(pred func(K, V) bool) Seq2[K, V]

Filter yields the pairs pred admits.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	pairs := catena.Of(1, 2, 3, 4).WithIndex().
		Filter(func(i, v int) bool { return v%2 == 0 })
	fmt.Println(pairs.Values().Collect())
}

Output:
[2 4]

func (Seq2[K, V]) FilterNot 

func (s Seq2[K, V]) FilterNot(pred func(K, V) bool) Seq2[K, V]

FilterNot yields the pairs pred rejects.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	pairs := catena.Of(1, 2, 3).WithIndex().
		FilterNot(func(i, v int) bool { return v == 2 })
	fmt.Println(pairs.Values().Collect())
}

Output:
[1 3]

func (Seq2[K, V]) First 

func (s Seq2[K, V]) First() (K, V, bool)

First returns the first pair.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	i, v, ok := catena.Of("a", "b").WithIndex().First()
	fmt.Println(i, v, ok)
}

Output:
0 a true

func (Seq2[K, V]) Fold 

func (s Seq2[K, V]) Fold[A any](init A, f func(A, K, V) A) A

Fold reduces the pairs into an accumulator, left to right. ⚠ Full drain.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	total := catena.Of(10, 20, 30).WithIndex().
		Fold(0, func(acc, i, v int) int { return acc + i*v })
	fmt.Println(total)
}

Output:
80

func (Seq2[K, V]) ForEach 

func (s Seq2[K, V]) ForEach(f func(K, V))

ForEach calls f on every pair. ⚠ Full drain.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	catena.Of("a", "b").WithIndex().ForEach(func(i int, s string) {
		fmt.Printf("%d=%s ", i, s)
	})
}

Output:
0=a 1=b

func (Seq2[K, V]) Keys 

func (s Seq2[K, V]) Keys() Seq[K]

Keys yields the first element of each pair. Calling Keys and Values on the same single-pass Seq2 is a double consume — use Unzip.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	// Keys and Values on the SAME single-pass Seq2 is a double consume;
	// Unzip does both in one pass.
	fmt.Println(catena.Of("a", "b").WithIndex().Keys().Collect())
}

Output:
[0 1]

func (Seq2[K, V]) Map 

func (s Seq2[K, V]) Map[K2, V2 any](f func(K, V) (K2, V2)) Seq2[K2, V2]

Map yields f applied to each pair.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	pairs := catena.Of("a", "b").WithIndex().
		Map(func(i int, s string) (string, int) { return s, i * 10 })
	fmt.Println(catena.CollectMap(pairs))
}

Output:
map[a:0 b:10]

func (Seq2[K, V]) MapTo 

func (s Seq2[K, V]) MapTo[U any](f func(K, V) U) Seq[U]

MapTo collapses each pair into one value — the intended exit back to Seq and its full API.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	// The intended exit: collapse each pair into one value and continue
	// in Seq, where the full API lives.
	fmt.Println(catena.Of("a", "b").WithIndex().
		MapTo(func(i int, s string) string { return fmt.Sprintf("%d%s", i, s) }).
		Collect())
}

Output:
[0a 1b]

func (Seq2[K, V]) MapValues 

func (s Seq2[K, V]) MapValues[V2 any](f func(K, V) V2) Seq2[K, V2]

MapValues yields each pair with its value replaced by f(k, v). f receives the key too (Kotlin-consistent).

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	// The callback receives the key as well as the value.
	pairs := catena.Of("a", "b").WithIndex().
		MapValues(func(i int, s string) string { return fmt.Sprintf("%d%s", i, s) })
	fmt.Println(pairs.Values().Collect())
}

Output:
[0a 1b]

func (Seq2[K, V]) Pull 

func (s Seq2[K, V]) Pull() (next func() (K, V, bool), stop func())

Pull converts s to a pull-based iterator. THE CALLER MUST CALL stop, even if next has returned false, or resources held by s will leak.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	next, stop := catena.Of("a", "b").WithIndex().Pull()
	defer stop()
	i, v, ok := next()
	fmt.Println(i, v, ok)
}

Output:
0 a true

func (Seq2[K, V]) Seq2 

func (s Seq2[K, V]) Seq2() iter.Seq2[K, V]

Seq2 converts to the stdlib iterator type. Free.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	for i, v := range catena.Of("a", "b").WithIndex().Seq2() {
		fmt.Printf("%d=%s ", i, v)
	}
}

Output:
0=a 1=b

func (Seq2[K, V]) Swap 

func (s Seq2[K, V]) Swap() Seq2[V, K]

Swap yields each pair with its sides exchanged.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	pairs := catena.Of("a", "b").WithIndex().Swap()
	fmt.Println(pairs.Keys().Collect())
}

Output:
[a b]

func (Seq2[K, V]) Take 

func (s Seq2[K, V]) Take(n int) Seq2[K, V]

Take yields at most the first n pairs. Panics if n is negative.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	fmt.Println(catena.Range(1, 9, 1).WithIndex().Take(3).Values().Collect())
}

Output:
[1 2 3]

func (Seq2[K, V]) Values 

func (s Seq2[K, V]) Values() Seq[V]

Values yields the second element of each pair.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	fmt.Println(catena.Of("a", "b").WithIndex().Values().Collect())
}

Output:
[a b]

type Try 

type Try[T any] iter.Seq2[T, error]

Try is a lazy sequence of fallible elements: iter.Seq2[T, error] with methods. When err != nil the value must not be read.

func EmptyTry 

func EmptyTry[T any]() Try[T]

EmptyTry returns the empty Try.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	vals, err := catena.EmptyTry[int]().Collect()
	fmt.Println(vals, err)
}

Output:
[] <nil>

func FromErrs 

func FromErrs[T any](seq func(func(T, error) bool)) Try[T]

FromErrs adapts any push-function fallible sequence.

Example 
package main

import (
	"fmt"

	"github.com/NerdMeNot/catena"
)

func main() {
	rows := catena.FromErrs(func(yield func(int, error) bool) {
		yield(1, nil)
		yield(0, fmt.Errorf("row 2: corrupt"))
	})
	vals, err := rows.Collect()
	fmt.Println(vals, err)
}

Output:
[1] row 2: corrupt

func (Try[T]) Collect 

func (t Try[T]) Collect() ([]T, error)

Collect gathers successful elements until the first error, returning the partial slice and that error (R5); (all elements, nil) on a clean drain. Nil slice for empty.

Example 
package main

import (
	"fmt"
	"strconv"

	"github.com/NerdMeNot/catena"
)

// parseAges is the fixture the Try examples share: two good values and one
// that fails, so every operator's treatment of an error is visible.
func parseAges() catena.Try[int] {
	return catena.Of("36", "unknown", "41").MapErr(strconv.Atoi)
}

func main() {
	// Abort: the values gathered before the failure, plus the error.
	vals, err := parseAges().Collect()
	fmt.Println(vals, err != nil)
}

Output:
[36] true

func (Try[T]) CollectAll 

func (t Try[T]) CollectAll() ([]T, []error)

CollectAll drains everything, gathering all successes and all errors. Positional correspondence between the two slices is lost. Nil slices when empty. ⚠ Full drain.

Example 
package main

import (
	"fmt"
	"strconv"

	"github.com/NerdMeNot/catena"
)

// parseAges is the fixture the Try examples share: two good values and one
// that fails, so every operator's treatment of an error is visible.
func parseAges() catena.Try[int] {
	return catena.Of("36", "unknown", "41").MapErr(strconv.Atoi)
}

func main() {
	// Gather: everything that worked and everything that did not, in one
	// pass. The two slices do not correspond positionally.
	vals, errs := parseAges().CollectAll()
	fmt.Println(vals, len(errs))
}

Output:
[36 41] 1

func (Try[T]) Count 

func (t Try[T]) Count() (int, error)

Count counts successful elements up to the first error, which is returned alongside the count so far (R5).

Example 
package main

import (
	"fmt"
	"strconv"

	"github.com/NerdMeNot/catena"
)

// parseAges is the fixture the Try examples share: two good values and one
// that fails, so every operator's treatment of an error is visible.
func parseAges() catena.Try[int] {
	return catena.Of("36", "unknown", "41").MapErr(strconv.Atoi)
}

func main() {
	// Successes counted up to the first error, which is returned with it.
	n, err := parseAges().Count()
	fmt.Println(n, err != nil)
}

Output:
1 true

func (Try[T]) Drop 

func (t Try[T]) Drop(n int) Try[T]

Drop skips the first n elements, errored or not (R2). Panics if n is negative.

Example 
package main

import (
	"fmt"
	"strconv"

	"github.com/NerdMeNot/catena"
)

// parseAges is the fixture the Try examples share: two good values and one
// that fails, so every operator's treatment of an error is visible.
func parseAges() catena.Try[int] {
	return catena.Of("36", "unknown", "41").MapErr(strconv.Atoi)
}

func main() {
	rest, _ := parseAges().Drop(1).CollectAll()
	fmt.Println(rest)
}

Output:
[41]

func (Try[T]) Err 

func (t Try[T]) Err() error

Err consumes until the first error and returns it; nil on a clean drain (R5).

Example 
package main

import (
	"fmt"
	"strconv"

	"github.com/NerdMeNot/catena"
)

// parseAges is the fixture the Try examples share: two good values and one
// that fails, so every operator's treatment of an error is visible.
func parseAges() catena.Try[int] {
	return catena.Of("36", "unknown", "41").MapErr(strconv.Atoi)
}

func main() {
	// Just the first error, if any — for pipelines run entirely for
	// their side effects.
	fmt.Println(parseAges().Err() != nil)
}

Output:
true

func (Try[T]) Errs 

func (t Try[T]) Errs() Seq[error]

Errs yields the errors, dropping successful elements — the dual of Ignore. Ignore and Errs on the same single-pass Try is a double consume; use CollectAll.

Example 
package main

import (
	"fmt"
	"strconv"

	"github.com/NerdMeNot/catena"
)

// parseAges is the fixture the Try examples share: two good values and one
// that fails, so every operator's treatment of an error is visible.
func parseAges() catena.Try[int] {
	return catena.Of("36", "unknown", "41").MapErr(strconv.Atoi)
}

func main() {
	// The dual of Ignore. Consuming both on one single-pass Try is a
	// double consume — use CollectAll instead.
	fmt.Println(parseAges().Errs().Count())
}

Output:
1

func (Try[T]) Filter 

func (t Try[T]) Filter(pred func(T) bool) Try[T]

Filter yields the successful elements pred admits; errored elements pass through unexamined.

Example 
package main

import (
	"fmt"
	"strconv"

	"github.com/NerdMeNot/catena"
)

// parseAges is the fixture the Try examples share: two good values and one
// that fails, so every operator's treatment of an error is visible.
func parseAges() catena.Try[int] {
	return catena.Of("36", "unknown", "41").MapErr(strconv.Atoi)
}

func main() {
	// The predicate is not called on errored elements, and they are not
	// filtered out — dropping them would silently discard failures.
	old := parseAges().Filter(func(n int) bool { return n > 40 })
	vals, errs := old.CollectAll()
	fmt.Println(vals, len(errs))
}

Output:
[41] 1

func (Try[T]) FilterErr 

func (t Try[T]) FilterErr(pred func(T) (bool, error)) Try[T]

FilterErr yields the successful elements pred admits; a failed pred call yields (zero, err); errored elements pass through unexamined.

Example 
package main

import (
	"errors"
	"fmt"
	"strconv"

	"github.com/NerdMeNot/catena"
)

// parseAges is the fixture the Try examples share: two good values and one
// that fails, so every operator's treatment of an error is visible.
func parseAges() catena.Try[int] {
	return catena.Of("36", "unknown", "41").MapErr(strconv.Atoi)
}

func main() {
	valid := parseAges().FilterErr(func(n int) (bool, error) {
		if n < 0 {
			return false, errors.New("negative")
		}
		return n > 40, nil
	})
	vals, errs := valid.CollectAll()
	fmt.Println(vals, len(errs))
}

Output:
[41] 1

func (Try[T]) FlatMap 

func (t Try[T]) FlatMap[U any](f func(T) Try[U]) Try[U]

FlatMap yields every element of f(v) for each successful element v, in order; an errored input element passes through un-mapped.

Example 
package main

import (
	"fmt"
	"strconv"

	"github.com/NerdMeNot/catena"
)

// parseAges is the fixture the Try examples share: two good values and one
// that fails, so every operator's treatment of an error is visible.
func parseAges() catena.Try[int] {
	return catena.Of("36", "unknown", "41").MapErr(strconv.Atoi)
}

func main() {
	// An errored input passes through un-mapped; inner errors flow in
	// order alongside the outer ones.
	pairs := parseAges().FlatMap(func(n int) catena.Try[int] {
		return catena.Of(n, n+1).MapErr(func(v int) (int, error) { return v, nil })
	})
	vals, errs := pairs.CollectAll()
	fmt.Println(vals, len(errs))
}

Output:
[36 37 41 42] 1

func (Try[T]) Fold 

func (t Try[T]) Fold[A any](init A, f func(A, T) A) (A, error)

Fold reduces successful elements until the first error, returning the accumulator so far and that error (R5).

Example 
package main

import (
	"fmt"
	"strconv"

	"github.com/NerdMeNot/catena"
)

// parseAges is the fixture the Try examples share: two good values and one
// that fails, so every operator's treatment of an error is visible.
func parseAges() catena.Try[int] {
	return catena.Of("36", "unknown", "41").MapErr(strconv.Atoi)
}

func main() {
	// Stops at the first error, returning the accumulator so far.
	sum, err := parseAges().Fold(0, func(acc, n int) int { return acc + n })
	fmt.Println(sum, err != nil)
}

Output:
36 true

func (Try[T]) ForEach 

func (t Try[T]) ForEach(f func(T) error) error

ForEach calls f on each successful element, stopping at and returning the first of an element error or a non-nil f return (R5).

Example 
package main

import (
	"fmt"
	"strconv"

	"github.com/NerdMeNot/catena"
)

// parseAges is the fixture the Try examples share: two good values and one
// that fails, so every operator's treatment of an error is visible.
func parseAges() catena.Try[int] {
	return catena.Of("36", "unknown", "41").MapErr(strconv.Atoi)
}

func main() {
	// Stops at the first of an element error or a callback error.
	err := parseAges().ForEach(func(n int) error {
		fmt.Println("handled", n)
		return nil
	})
	fmt.Println(err != nil)
}

Output:
handled 36
true

func (Try[T]) Ignore 

func (t Try[T]) Ignore() Seq[T]

Ignore yields the successful elements, dropping errored ones.

Example 
package main

import (
	"fmt"
	"strconv"

	"github.com/NerdMeNot/catena"
)

// parseAges is the fixture the Try examples share: two good values and one
// that fails, so every operator's treatment of an error is visible.
func parseAges() catena.Try[int] {
	return catena.Of("36", "unknown", "41").MapErr(strconv.Atoi)
}

func main() {
	// Skip: drop the failures and carry on as a plain Seq.
	fmt.Println(parseAges().Ignore().Collect())
}

Output:
[36 41]

func (Try[T]) Map 

func (t Try[T]) Map[U any](f func(T) U) Try[U]

Map yields f applied to each successful element; errored elements pass through.

Example 
package main

import (
	"fmt"
	"strconv"

	"github.com/NerdMeNot/catena"
)

// parseAges is the fixture the Try examples share: two good values and one
// that fails, so every operator's treatment of an error is visible.
func parseAges() catena.Try[int] {
	return catena.Of("36", "unknown", "41").MapErr(strconv.Atoi)
}

func main() {
	// The callback never sees an errored element; it passes through
	// untouched, so a failure is not silently mapped into a valid value.
	doubled := parseAges().Map(func(n int) int { return n * 2 })
	vals, errs := doubled.CollectAll()
	fmt.Println(vals, len(errs))
}

Output:
[72 82] 1

func (Try[T]) MapErr 

func (t Try[T]) MapErr[U any](f func(T) (U, error)) Try[U]

MapErr yields f applied to each successful element; a failed call yields (zero, err); errored elements pass through.

Example 
package main

import (
	"errors"
	"fmt"
	"strconv"

	"github.com/NerdMeNot/catena"
)

// parseAges is the fixture the Try examples share: two good values and one
// that fails, so every operator's treatment of an error is visible.
func parseAges() catena.Try[int] {
	return catena.Of("36", "unknown", "41").MapErr(strconv.Atoi)
}

func main() {
	// A mapping that can itself fail. Errors from either stage flow on.
	tenths := parseAges().MapErr(func(n int) (int, error) {
		if n > 40 {
			return 0, errors.New("too old")
		}
		return n * 10, nil
	})
	vals, errs := tenths.CollectAll()
	fmt.Println(vals, len(errs))
}

Output:
[360] 2

func (Try[T]) Must 

func (t Try[T]) Must() Seq[T]

Must yields the successful elements and panics with the error value on the first error — recover() receives the error itself.

Example 
package main

import (
	"fmt"
	"strconv"

	"github.com/NerdMeNot/catena"
)

// parseAges is the fixture the Try examples share: two good values and one
// that fails, so every operator's treatment of an error is visible.
func parseAges() catena.Try[int] {
	return catena.Of("36", "unknown", "41").MapErr(strconv.Atoi)
}

func main() {
	// For pipelines where a failure is a programming bug. The panic
	// value is the error itself, so recover() can inspect it.
	defer func() { fmt.Println("recovered:", recover()) }()
	parseAges().Must().Drain()
}

Output:
recovered: strconv.Atoi: parsing "unknown": invalid syntax

func (Try[T]) OnEach 

func (t Try[T]) OnEach(f func(T)) Try[T]

OnEach calls f on every successful element and passes everything through.

Example 
package main

import (
	"fmt"
	"strconv"

	"github.com/NerdMeNot/catena"
)

// parseAges is the fixture the Try examples share: two good values and one
// that fails, so every operator's treatment of an error is visible.
func parseAges() catena.Try[int] {
	return catena.Of("36", "unknown", "41").MapErr(strconv.Atoi)
}

func main() {
	// Runs only on successes; errors pass by untouched.
	seen := 0
	parseAges().OnEach(func(int) { seen++ }).CollectAll()
	fmt.Println(seen)
}

Output:
2

func (Try[T]) OnError 

func (t Try[T]) OnError(f func(error)) Try[T]

OnError calls f on every error and passes everything through — a logging hook.

Example 
package main

import (
	"fmt"
	"strconv"

	"github.com/NerdMeNot/catena"
)

// parseAges is the fixture the Try examples share: two good values and one
// that fails, so every operator's treatment of an error is visible.
func parseAges() catena.Try[int] {
	return catena.Of("36", "unknown", "41").MapErr(strconv.Atoi)
}

func main() {
	// The logging tap, the mirror of OnEach.
	logged := 0
	parseAges().OnError(func(error) { logged++ }).CollectAll()
	fmt.Println(logged)
}

Output:
1

func (Try[T]) Pull 

func (t Try[T]) Pull() (next func() (T, error, bool), stop func())

Pull converts t to a pull-based iterator. THE CALLER MUST CALL stop, even if next has returned false, or resources held by t will leak.

Example 
package main

import (
	"fmt"
	"strconv"

	"github.com/NerdMeNot/catena"
)

// parseAges is the fixture the Try examples share: two good values and one
// that fails, so every operator's treatment of an error is visible.
func parseAges() catena.Try[int] {
	return catena.Of("36", "unknown", "41").MapErr(strconv.Atoi)
}

func main() {
	next, stop := parseAges().Pull()
	defer stop()
	v, err, ok := next()
	fmt.Println(v, err, ok)
}

Output:
36 <nil> true

func (Try[T]) Recover 

func (t Try[T]) Recover(f func(error) (T, bool)) Try[T]

Recover offers each error to f: reporting true replaces the element with (v, nil); reporting false passes the error through unchanged.

Example 
package main

import (
	"fmt"
	"strconv"
	"strings"

	"github.com/NerdMeNot/catena"
)

// parseAges is the fixture the Try examples share: two good values and one
// that fails, so every operator's treatment of an error is visible.
func parseAges() catena.Try[int] {
	return catena.Of("36", "unknown", "41").MapErr(strconv.Atoi)
}

func main() {
	// Repair chosen errors mid-stream: reporting true replaces the
	// element, false lets the error continue.
	fixed := parseAges().Recover(func(err error) (int, bool) {
		return 0, strings.Contains(err.Error(), "unknown")
	})
	vals, errs := fixed.CollectAll()
	fmt.Println(vals, len(errs))
}

Output:
[36 0 41] 0

func (Try[T]) Seq2 

func (t Try[T]) Seq2() iter.Seq2[T, error]

Seq2 converts to the stdlib iterator type. Free.

Example 
package main

import (
	"fmt"
	"strconv"

	"github.com/NerdMeNot/catena"
)

// parseAges is the fixture the Try examples share: two good values and one
// that fails, so every operator's treatment of an error is visible.
func parseAges() catena.Try[int] {
	return catena.Of("36", "unknown", "41").MapErr(strconv.Atoi)
}

func main() {
	// A free conversion to the standard pair iterator.
	for v, err := range parseAges().Seq2() {
		if err != nil {
			fmt.Println("error at", v)
			break
		}
		fmt.Println("ok", v)
	}
}

Output:
ok 36
error at 0

func (Try[T]) Take 

func (t Try[T]) Take(n int) Try[T]

Take yields at most the first n elements, errored or not (R2). Panics if n is negative.

Example 
package main

import (
	"fmt"
	"strconv"

	"github.com/NerdMeNot/catena"
)

// parseAges is the fixture the Try examples share: two good values and one
// that fails, so every operator's treatment of an error is visible.
func parseAges() catena.Try[int] {
	return catena.Of("36", "unknown", "41").MapErr(strconv.Atoi)
}

func main() {
	// Counts elements, errored or not — so it consumes at most n. For
	// "n successes", use Ignore().Take(n).
	first, _ := parseAges().Take(2).CollectAll()
	successes := parseAges().Ignore().Take(2).Collect()
	fmt.Println(first, successes)
}

Output:
[36] [36 41]

func (Try[T]) TakeWhile 

func (t Try[T]) TakeWhile(pred func(T) bool) Try[T]

TakeWhile yields elements until pred rejects a successful element; errored elements pass through and do not terminate (R3).

Example 
package main

import (
	"fmt"
	"strconv"

	"github.com/NerdMeNot/catena"
)

// parseAges is the fixture the Try examples share: two good values and one
// that fails, so every operator's treatment of an error is visible.
func parseAges() catena.Try[int] {
	return catena.Of("36", "unknown", "41").MapErr(strconv.Atoi)
}

func main() {
	// An errored element passes through without ending the sequence;
	// only a successful element failing the predicate stops it.
	kept, errs := parseAges().TakeWhile(func(n int) bool { return n < 40 }).CollectAll()
	fmt.Println(kept, len(errs))
}

Output:
[36] 1

func (Try[T]) UntilDone 

func (t Try[T]) UntilDone(ctx context.Context) Try[T]

UntilDone passes elements through until ctx is done, then yields (zero, ctx.Err()) and stops.

Example 
package main

import (
	"context"
	"fmt"
	"strconv"

	"github.com/NerdMeNot/catena"
)

// parseAges is the fixture the Try examples share: two good values and one
// that fails, so every operator's treatment of an error is visible.
func parseAges() catena.Try[int] {
	return catena.Of("36", "unknown", "41").MapErr(strconv.Atoi)
}

func main() {
	ctx, cancel := context.WithCancel(context.Background())
	cancel()
	_, err := parseAges().UntilDone(ctx).Collect()
	fmt.Println(err)
}

Output:
context canceled

func (Try[T]) WrapErr 

func (t Try[T]) WrapErr(f func(error) error) Try[T]

WrapErr replaces each error with f(err) — the place to add positional context. If f returns nil (a caller bug), the original error is kept: an error is never converted into a zero-value success.

Example 
package main

import (
	"fmt"
	"strconv"
	"strings"

	"github.com/NerdMeNot/catena"
)

// parseAges is the fixture the Try examples share: two good values and one
// that fails, so every operator's treatment of an error is visible.
func parseAges() catena.Try[int] {
	return catena.Of("36", "unknown", "41").MapErr(strconv.Atoi)
}

func main() {
	// Add the context that only this stage has. Returning nil keeps the
	// original error rather than turning a failure into a zero value.
	wrapped := parseAges().WrapErr(func(err error) error {
		return fmt.Errorf("parsing ages: %w", err)
	})
	_, err := wrapped.Collect()
	fmt.Println(strings.HasPrefix(err.Error(), "parsing ages:"))
}

Output:
true

Source Files

View all Source files

Directories

Path Synopsis
examples
01-basics command
Basics: building a pipeline, what laziness means in practice, and how catena sequences interoperate with plain range loops and iter.Seq.
 Basics: building a pipeline, what laziness means in practice, and how catena sequences interoperate with plain range loops and iter.Seq.
02-grouping command
Grouping: FoldBy is the library's centerpiece — streaming aggregation bounded by distinct keys, not elements.
 Grouping: FoldBy is the library's centerpiece — streaming aggregation bounded by distinct keys, not elements.
03-selection command
Selection: finding extremes and top-k without sorting the world.
 Selection: finding extremes and top-k without sorting the world.
04-errors command
Errors: one parse pipeline, three policies.
 Errors: one parse pipeline, three policies.
05-resources command
Resources: a producer that owns something — here a real temp file — opens it lazily inside the iteration closure.
 Resources: a producer that owns something — here a real temp file — opens it lazily inside the iteration closure.
06-streaming command
Streaming: sequences with no end, and the operators that stay safe on them — running state with Scan, moving averages with Windowed, batching with Chunked, and change detection with DedupeBy.
 Streaming: sequences with no end, and the operators that stay safe on them — running state with Scan, moving averages with Windowed, batching with Chunked, and change detection with DedupeBy.
07-join command
Join: JoinBy is a relational inner join between two streams of structs — the right side is indexed by key, the left side streams past it, and the joined stream is a plain Seq you keep chaining on.
 Join: JoinBy is a relational inner join between two streams of structs — the right side is indexed by key, the left side streams past it, and the joined stream is a plain Seq you keep chaining on.
08-list command
List and the Seq2 bridge: when the data is small and already in memory, eager evaluation with exact preallocation beats a lazy chain — and the two worlds convert explicitly, in both directions.
 List and the Seq2 bridge: when the data is small and already in memory, eager evaluation with exact preallocation beats a lazy chain — and the two worlds convert explicitly, in both directions.
internal
gen/assets command
Command assets generates the catena mark as SVG.
 Command assets generates the catena mark as SVG.
gen/listgen command
Command listgen generates list_gen.go: the eager List[T] mirror of the Seq[T] operation set (§10 of the spec).
 Command listgen generates list_gen.go: the eager List[T] mirror of the Seq[T] operation set (§10 of the spec).
gen/opdocs command
Command opdocs generates the operator reference in docs/operators/.
 Command opdocs generates the operator reference in docs/operators/.

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