expr

README

Expr

Expr package provides an engine that can compile and evaluate expressions. An expression is a one-liner that returns a value (mostly, but not limited to, booleans). It is designed for simplicity, speed and safety.

The purpose of the package is to allow users to use expressions inside configuration for more complex logic. It is a perfect candidate for the foundation of a business rule engine. The idea is to let configure things in a dynamic way without recompile of a program:

# Get the special price if
user.Group in ["good_customers", "collaborator"]

# Promote article to the homepage when
len(article.Comments) > 100 and article.Category not in ["misc"]

# Send an alert when
product.Stock < 15

Features

• Seamless integration with Go (no need to redefine types)
• Static typing (example).

  out, err := expr.Compile(`name + age`)
  // err: invalid operation + (mismatched types string and int)
  // | name + age
  // | .....^
  

• User-friendly error messages.
• Reasonable set of basic operators.
• Builtins all, none, any, one, filter, map.

  all(Tweets, {.Size <= 280})
  

• Fast (benchmarks): uses bytecode virtual machine and optimizing compiler.

Install

go get github.com/ffenix113/expr

Documentation

• See Getting Started page for developer documentation.
• See Language Definition page to learn the syntax.

Expr Code Editor

Also, I have an embeddable code editor written in JavaScript which allows editing expressions with syntax highlighting and autocomplete based on your types declaration.

Learn more →

Examples

Play Online

package main

import (
	"fmt"
	"github.com/ffenix113/expr"
)

func main() {
	env := map[string]interface{}{
		"greet":   "Hello, %v!",
		"names":   []string{"world", "you"},
		"sprintf": fmt.Sprintf,
	}

	code := `sprintf(greet, names[0])`

	program, err := expr.Compile(code, expr.Env(env))
	if err != nil {
		panic(err)
	}

	output, err := expr.Run(program, env)
	if err != nil {
		panic(err)
	}

	fmt.Println(output)
}

Play Online

package main

import (
	"fmt"
	"github.com/ffenix113/expr"
)

type Tweet struct {
	Len int
}

type Env struct {
	Tweets []Tweet
}

func main() {
	code := `all(Tweets, {.Len <= 240})`

	program, err := expr.Compile(code, expr.Env(Env{}))
	if err != nil {
		panic(err)
	}

	env := Env{
		Tweets: []Tweet{{42}, {98}, {69}},
	}
	output, err := expr.Run(program, env)
	if err != nil {
		panic(err)
	}

	fmt.Println(output)
}

Contributing

Expr consist of a few packages for parsing source code to AST, type checking AST, compiling to bytecode and VM for running bytecode program.

Also expr provides powerful tool exe for debugging. It has interactive terminal debugger for our bytecode virtual machine.

Who is using Expr?

• Aviasales are actively using Expr for different parts of the search engine.
• Argo Rollouts - Progressive Delivery for Kubernetes.
• Argo Workflows - The workflow engine for KubernetesOverview.
• Crowdsec - A security automation tool.
• Mystery Minds uses Expr to allow easy yet powerful customization of its matching algorithm.

Add your company too

License

MIT

Documentation

Index

• func Compile(input string, ops ...Option) (*vm.Program, error)
• func Eval(input string, env interface{}) (interface{}, error)
• func Run(program *vm.Program, env interface{}) (interface{}, error)
• type Option
  • func AllowUndefinedVariables() Option
  • func AsBool() Option
  • func AsFloat64() Option
  • func AsInt64() Option
  • func ConstExpr(fn string) Option
  • func Env(env interface{}) Option
  • func Operator(operator string, fn ...string) Option
  • func Optimize(b bool) Option
  • func Patch(visitor ast.Visitor) Option

Examples

• AllowUndefinedVariables
• AllowUndefinedVariables (Zero_value)
• AllowUndefinedVariables (Zero_value_functions)
• AsBool
• AsBool (Error)
• AsFloat64
• AsFloat64 (Error)
• AsInt64
• Compile
• ConstExpr
• Env
• Eval
• Eval (Runtime_error)
• Operator
• Patch

Functions

func Compile

func Compile(input string, ops ...Option) (*vm.Program, error)

Compile parses and compiles given input expression to bytecode program.

Example

package main

import (
	"fmt"

	"github.com/ffenix113/expr"
)

func main() {
	env := map[string]interface{}{
		"foo": 1,
		"bar": 99,
	}

	program, err := expr.Compile("foo in 1..99 and bar in 1..99", expr.Env(env))
	if err != nil {
		fmt.Printf("%v", err)
		return
	}

	output, err := expr.Run(program, env)
	if err != nil {
		fmt.Printf("%v", err)
		return
	}

	fmt.Printf("%v", output)

}

Output:

true

func Eval

func Eval(input string, env interface{}) (interface{}, error)

Eval parses, compiles and runs given input.

Example

package main

import (
	"fmt"

	"github.com/ffenix113/expr"
)

func main() {
	output, err := expr.Eval("greet + name", map[string]interface{}{
		"greet": "Hello, ",
		"name":  "world!",
	})
	if err != nil {
		fmt.Printf("err: %v", err)
		return
	}

	fmt.Printf("%v", output)

}

Output:

Hello, world!

Example (Runtime_error)

package main

import (
	"fmt"

	"github.com/ffenix113/expr"
)

func main() {
	_, err := expr.Eval(`map(1..3, {1 / (# - 3)})`, nil)
	fmt.Print(err)

}

Output:

runtime error: integer divide by zero (1:14)
 | map(1..3, {1 / (# - 3)})
 | .............^

func Run

func Run(program *vm.Program, env interface{}) (interface{}, error)

Run evaluates given bytecode program.

Types

type Option

type Option func(c *conf.Config)

Option for configuring config.

func AllowUndefinedVariables

func AllowUndefinedVariables() Option

AllowUndefinedVariables allows to use undefined variables inside expressions. This can be used with expr.Env option to partially define a few variables. Note what this option is only works in map environment are used, otherwise runtime.fetch will panic as there is no way to get missing field zero value.

Example

package main

import (
	"fmt"

	"github.com/ffenix113/expr"
)

func main() {
	code := `name == nil ? "Hello, world!" : sprintf("Hello, %v!", name)`

	env := map[string]interface{}{
		"sprintf": fmt.Sprintf,
	}

	options := []expr.Option{
		expr.Env(env),
		expr.AllowUndefinedVariables(), // Allow to use undefined variables.
	}

	program, err := expr.Compile(code, options...)
	if err != nil {
		fmt.Printf("%v", err)
		return
	}

	output, err := expr.Run(program, env)
	if err != nil {
		fmt.Printf("%v", err)
		return
	}
	fmt.Printf("%v\n", output)

	env["name"] = "you" // Define variables later on.

	output, err = expr.Run(program, env)
	if err != nil {
		fmt.Printf("%v", err)
		return
	}
	fmt.Printf("%v\n", output)

}

Output:

Hello, world!
Hello, you!

Example (Zero_value)

package main

import (
	"fmt"

	"github.com/ffenix113/expr"
)

func main() {
	code := `name == "" ? foo + bar : foo + name`

	// If environment has different zero values, then undefined variables
	// will have it as default value.
	env := map[string]string{}

	options := []expr.Option{
		expr.Env(env),
		expr.AllowUndefinedVariables(), // Allow to use undefined variables.
	}

	program, err := expr.Compile(code, options...)
	if err != nil {
		fmt.Printf("%v", err)
		return
	}

	env = map[string]string{
		"foo": "Hello, ",
		"bar": "world!",
	}

	output, err := expr.Run(program, env)
	if err != nil {
		fmt.Printf("%v", err)
		return
	}
	fmt.Printf("%v", output)

}

Output:

Hello, world!

Example (Zero_value_functions)

package main

import (
	"fmt"
	"strings"

	"github.com/ffenix113/expr"
)

func main() {
	code := `words == "" ? Split("foo,bar", ",") : Split(words, ",")`

	// Env is map[string]string type on which methods are defined.
	env := mockMapStringStringEnv{}

	options := []expr.Option{
		expr.Env(env),
		expr.AllowUndefinedVariables(), // Allow to use undefined variables.
	}

	program, err := expr.Compile(code, options...)
	if err != nil {
		fmt.Printf("%v", err)
		return
	}

	output, err := expr.Run(program, env)
	if err != nil {
		fmt.Printf("%v", err)
		return
	}
	fmt.Printf("%v", output)

}

type mockMapStringStringEnv map[string]string

func (m mockMapStringStringEnv) Split(s, sep string) []string {
	return strings.Split(s, sep)
}

Output:

[foo bar]

func AsBool

func AsBool() Option

AsBool tells the compiler to expect boolean result.

Example

package main

import (
	"fmt"

	"github.com/ffenix113/expr"
)

func main() {
	env := map[string]int{
		"foo": 0,
	}

	program, err := expr.Compile("foo >= 0", expr.Env(env), expr.AsBool())
	if err != nil {
		fmt.Printf("%v", err)
		return
	}

	output, err := expr.Run(program, env)
	if err != nil {
		fmt.Printf("%v", err)
		return
	}

	fmt.Printf("%v", output.(bool))

}

Output:

true

Example (Error)

package main

import (
	"fmt"

	"github.com/ffenix113/expr"
)

func main() {
	env := map[string]interface{}{
		"foo": 0,
	}

	_, err := expr.Compile("foo + 42", expr.Env(env), expr.AsBool())

	fmt.Printf("%v", err)

}

Output:

expected bool, but got int

func AsFloat64

func AsFloat64() Option

AsFloat64 tells the compiler to expect float64 result.

Example

package main

import (
	"fmt"

	"github.com/ffenix113/expr"
)

func main() {
	program, err := expr.Compile("42", expr.AsFloat64())
	if err != nil {
		fmt.Printf("%v", err)
		return
	}

	output, err := expr.Run(program, nil)
	if err != nil {
		fmt.Printf("%v", err)
		return
	}

	fmt.Printf("%v", output.(float64))

}

Output:

42

Example (Error)

package main

import (
	"fmt"

	"github.com/ffenix113/expr"
)

func main() {
	_, err := expr.Compile(`!!true`, expr.AsFloat64())

	fmt.Printf("%v", err)

}

Output:

expected float64, but got bool

func AsInt64

func AsInt64() Option

AsInt64 tells the compiler to expect int64 result.

Example

package main

import (
	"fmt"

	"github.com/ffenix113/expr"
)

func main() {
	env := map[string]interface{}{
		"rating": 5.5,
	}

	program, err := expr.Compile("rating", expr.Env(env), expr.AsInt64())
	if err != nil {
		fmt.Printf("%v", err)
		return
	}

	output, err := expr.Run(program, env)
	if err != nil {
		fmt.Printf("%v", err)
		return
	}

	fmt.Printf("%v", output.(int64))

}

Output:

5

func ConstExpr

func ConstExpr(fn string) Option

ConstExpr defines func expression as constant. If all argument to this function is constants, then it can be replaced by result of this func call on compile step.

Example

package main

import (
	"fmt"

	"github.com/ffenix113/expr"
)

func fib(n int) int {
	if n <= 1 {
		return n
	}
	return fib(n-1) + fib(n-2)
}

func main() {
	code := `[fib(5), fib(3+3), fib(dyn)]`

	env := map[string]interface{}{
		"fib": fib,
		"dyn": 0,
	}

	options := []expr.Option{
		expr.Env(env),
		expr.ConstExpr("fib"), // Mark fib func as constant expression.
	}

	program, err := expr.Compile(code, options...)
	if err != nil {
		fmt.Printf("%v", err)
		return
	}

	// Only fib(5) and fib(6) calculated on Compile, fib(dyn) can be called at runtime.
	env["dyn"] = 7

	output, err := expr.Run(program, env)
	if err != nil {
		fmt.Printf("%v", err)
		return
	}

	fmt.Printf("%v\n", output)

}

Output:

[5 8 13]

func Env

func Env(env interface{}) Option

Env specifies expected input of env for type checks. If struct is passed, all fields will be treated as variables, as well as all fields of embedded structs and struct itself. If map is passed, all items will be treated as variables. Methods defined on this type will be available as functions.

Example

package main

import (
	"fmt"

	"github.com/ffenix113/expr"
)

func main() {
	type Segment struct {
		Origin string
	}
	type Passengers struct {
		Adults int
	}
	type Meta struct {
		Tags map[string]string
	}
	type Env struct {
		Meta
		Segments   []*Segment
		Passengers *Passengers
		Marker     string
	}

	code := `all(Segments, {.Origin == "MOW"}) && Passengers.Adults > 0 && Tags["foo"] startsWith "bar"`

	program, err := expr.Compile(code, expr.Env(Env{}))
	if err != nil {
		fmt.Printf("%v", err)
		return
	}

	env := Env{
		Meta: Meta{
			Tags: map[string]string{
				"foo": "bar",
			},
		},
		Segments: []*Segment{
			{Origin: "MOW"},
		},
		Passengers: &Passengers{
			Adults: 2,
		},
		Marker: "test",
	}

	output, err := expr.Run(program, env)
	if err != nil {
		fmt.Printf("%v", err)
		return
	}

	fmt.Printf("%v", output)

}

Output:

true

func Operator

func Operator(operator string, fn ...string) Option

Operator allows to override binary operator with function.

Example

package main

import (
	"fmt"
	"time"

	"github.com/ffenix113/expr"
)

func main() {
	code := `
		Now() > CreatedAt &&
		(Now() - CreatedAt).Hours() > 24
	`

	type Env struct {
		CreatedAt time.Time
		Now       func() time.Time
		Sub       func(a, b time.Time) time.Duration
		After     func(a, b time.Time) bool
	}

	options := []expr.Option{
		expr.Env(Env{}),
		expr.Operator(">", "After"),
		expr.Operator("-", "Sub"),
	}

	program, err := expr.Compile(code, options...)
	if err != nil {
		fmt.Printf("%v", err)
		return
	}

	env := Env{
		CreatedAt: time.Date(2018, 7, 14, 0, 0, 0, 0, time.UTC),
		Now:       func() time.Time { return time.Now() },
		Sub:       func(a, b time.Time) time.Duration { return a.Sub(b) },
		After:     func(a, b time.Time) bool { return a.After(b) },
	}

	output, err := expr.Run(program, env)
	if err != nil {
		fmt.Printf("%v", err)
		return
	}

	fmt.Printf("%v", output)

}

Output:

true

func Optimize

func Optimize(b bool) Option

Optimize turns optimizations on or off.

func Patch

func Patch(visitor ast.Visitor) Option

Patch adds visitor to list of visitors what will be applied before compiling AST to bytecode.

Example

package main

import (
	"fmt"

	"github.com/ffenix113/expr/ast"

	"github.com/ffenix113/expr"
)

func main() {
	/*
		type patcher struct{}

		func (p *patcher) Enter(_ *ast.Node) {}
		func (p *patcher) Exit(node *ast.Node) {
			switch n := (*node).(type) {
			case *ast.PropertyNode:
				ast.Patch(node, &ast.FunctionNode{
					Name:      "get",
					Arguments: []ast.Node{n.Node, &ast.StringNode{Value: n.Property}},
				})
			}
		}
	*/

	program, err := expr.Compile(
		`greet.you.world + "!"`,
		expr.Patch(&patcher{}),
	)
	if err != nil {
		fmt.Printf("%v", err)
		return
	}

	env := map[string]interface{}{
		"greet": "Hello",
		"get": func(a, b string) string {
			return a + ", " + b
		},
	}

	output, err := expr.Run(program, env)
	if err != nil {
		fmt.Printf("%v", err)
		return
	}
	fmt.Printf("%v", output)

	// Output : Hello, you, world!
}

type patcher struct{}

func (p *patcher) Enter(_ *ast.Node) {}
func (p *patcher) Exit(node *ast.Node) {
	switch n := (*node).(type) {
	case *ast.PropertyNode:
		ast.Patch(node, &ast.FunctionNode{
			Name:      "get",
			Arguments: []ast.Node{n.Node, &ast.StringNode{Value: n.Property}},
		})
	}
}