constant

package standard library
Version: go1.16.6 Latest Latest
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Published: Jul 12, 2021 License: BSD-3-Clause Imports: 8 Imported by: 1586

Documentation

Overview

Package constant implements Values representing untyped Go constants and their corresponding operations.

A special Unknown value may be used when a value is unknown due to an error. Operations on unknown values produce unknown values unless specified otherwise.

Example (ComplexNumbers)
package main

import (
	"fmt"
	"go/constant"
	"go/token"
)

func main() {
	// Create the complex number 2.3 + 5i.
	ar := constant.MakeFloat64(2.3)
	ai := constant.MakeImag(constant.MakeInt64(5))
	a := constant.BinaryOp(ar, token.ADD, ai)

	// Compute (2.3 + 5i) * 11.
	b := constant.MakeUint64(11)
	c := constant.BinaryOp(a, token.MUL, b)

	// Convert c into a complex128.
	Ar, exact := constant.Float64Val(constant.Real(c))
	if !exact {
		fmt.Printf("Could not represent real part %s exactly as float64\n", constant.Real(c))
	}
	Ai, exact := constant.Float64Val(constant.Imag(c))
	if !exact {
		fmt.Printf("Could not represent imaginary part %s as exactly as float64\n", constant.Imag(c))
	}
	C := complex(Ar, Ai)

	fmt.Println("literal", 25.3+55i)
	fmt.Println("go/constant", c)
	fmt.Println("complex128", C)

}
Output:


Could not represent real part 25.3 exactly as float64
literal (25.3+55i)
go/constant (25.3 + 55i)
complex128 (25.299999999999997+55i)

Index

Examples

Constants

This section is empty.

Variables

This section is empty.

Functions

func BitLen

func BitLen(x Value) int

BitLen returns the number of bits required to represent the absolute value x in binary representation; x must be an Int or an Unknown. If x is Unknown, the result is 0.

func BoolVal

func BoolVal(x Value) bool

BoolVal returns the Go boolean value of x, which must be a Bool or an Unknown. If x is Unknown, the result is false.

func Bytes

func Bytes(x Value) []byte

Bytes returns the bytes for the absolute value of x in little- endian binary representation; x must be an Int.

func Compare

func Compare(x_ Value, op token.Token, y_ Value) bool

Compare returns the result of the comparison x op y. The comparison must be defined for the operands. If one of the operands is Unknown, the result is false.

Example
package main

import (
	"fmt"
	"go/constant"
	"go/token"
	"sort"
)

func main() {
	vs := []constant.Value{
		constant.MakeString("Z"),
		constant.MakeString("bacon"),
		constant.MakeString("go"),
		constant.MakeString("Frame"),
		constant.MakeString("defer"),
		constant.MakeFromLiteral(`"a"`, token.STRING, 0),
	}

	sort.Slice(vs, func(i, j int) bool {
		// Equivalent to vs[i] <= vs[j].
		return constant.Compare(vs[i], token.LEQ, vs[j])
	})

	for _, v := range vs {
		fmt.Println(constant.StringVal(v))
	}

}
Output:


Frame
Z
a
bacon
defer
go

func Float32Val

func Float32Val(x Value) (float32, bool)

Float32Val is like Float64Val but for float32 instead of float64.

func Float64Val

func Float64Val(x Value) (float64, bool)

Float64Val returns the nearest Go float64 value of x and whether the result is exact; x must be numeric or an Unknown, but not Complex. For values too small (too close to 0) to represent as float64, Float64Val silently underflows to 0. The result sign always matches the sign of x, even for 0. If x is Unknown, the result is (0, false).

func Int64Val

func Int64Val(x Value) (int64, bool)

Int64Val returns the Go int64 value of x and whether the result is exact; x must be an Int or an Unknown. If the result is not exact, its value is undefined. If x is Unknown, the result is (0, false).

func Sign

func Sign(x Value) int

Sign returns -1, 0, or 1 depending on whether x < 0, x == 0, or x > 0; x must be numeric or Unknown. For complex values x, the sign is 0 if x == 0, otherwise it is != 0. If x is Unknown, the result is 1.

Example
package main

import (
	"fmt"
	"go/constant"
	"go/token"
)

func main() {
	zero := constant.MakeInt64(0)
	one := constant.MakeInt64(1)
	negOne := constant.MakeInt64(-1)

	mkComplex := func(a, b constant.Value) constant.Value {
		b = constant.MakeImag(b)
		return constant.BinaryOp(a, token.ADD, b)
	}

	vs := []constant.Value{
		negOne,
		mkComplex(zero, negOne),
		mkComplex(one, negOne),
		mkComplex(negOne, one),
		mkComplex(negOne, negOne),
		zero,
		mkComplex(zero, zero),
		one,
		mkComplex(zero, one),
		mkComplex(one, one),
	}

	for _, v := range vs {
		fmt.Printf("% d %s\n", constant.Sign(v), v)
	}

}
Output:


-1 -1
-1 (0 + -1i)
-1 (1 + -1i)
-1 (-1 + 1i)
-1 (-1 + -1i)
 0 0
 0 (0 + 0i)
 1 1
 1 (0 + 1i)
 1 (1 + 1i)

func StringVal

func StringVal(x Value) string

StringVal returns the Go string value of x, which must be a String or an Unknown. If x is Unknown, the result is "".

func Uint64Val

func Uint64Val(x Value) (uint64, bool)

Uint64Val returns the Go uint64 value of x and whether the result is exact; x must be an Int or an Unknown. If the result is not exact, its value is undefined. If x is Unknown, the result is (0, false).

func Val added in go1.13

func Val(x Value) interface{}

Val returns the underlying value for a given constant. Since it returns an interface, it is up to the caller to type assert the result to the expected type. The possible dynamic return types are:

x Kind             type of result
-----------------------------------------
Bool               bool
String             string
Int                int64 or *big.Int
Float              *big.Float or *big.Rat
everything else    nil
Example
package main

import (
	"fmt"
	"go/constant"
	"math"
)

func main() {
	maxint := constant.MakeInt64(math.MaxInt64)
	fmt.Printf("%v\n", constant.Val(maxint))

	e := constant.MakeFloat64(math.E)
	fmt.Printf("%v\n", constant.Val(e))

	b := constant.MakeBool(true)
	fmt.Printf("%v\n", constant.Val(b))

	b = constant.Make(false)
	fmt.Printf("%v\n", constant.Val(b))

}
Output:


9223372036854775807
6121026514868073/2251799813685248
true
false

Types

type Kind

type Kind int

Kind specifies the kind of value represented by a Value.

const (
	// unknown values
	Unknown Kind = iota

	// non-numeric values
	Bool
	String

	// numeric values
	Int
	Float
	Complex
)

type Value

type Value interface {
	// Kind returns the value kind.
	Kind() Kind

	// String returns a short, quoted (human-readable) form of the value.
	// For numeric values, the result may be an approximation;
	// for String values the result may be a shortened string.
	// Use ExactString for a string representing a value exactly.
	String() string

	// ExactString returns an exact, quoted (human-readable) form of the value.
	// If the Value is of Kind String, use StringVal to obtain the unquoted string.
	ExactString() string
	// contains filtered or unexported methods
}

A Value represents the value of a Go constant.

func BinaryOp

func BinaryOp(x_ Value, op token.Token, y_ Value) Value

BinaryOp returns the result of the binary expression x op y. The operation must be defined for the operands. If one of the operands is Unknown, the result is Unknown. BinaryOp doesn't handle comparisons or shifts; use Compare or Shift instead.

To force integer division of Int operands, use op == token.QUO_ASSIGN instead of token.QUO; the result is guaranteed to be Int in this case. Division by zero leads to a run-time panic.

Example
package main

import (
	"fmt"
	"go/constant"
	"go/token"
)

func main() {
	// 11 / 0.5
	a := constant.MakeUint64(11)
	b := constant.MakeFloat64(0.5)
	c := constant.BinaryOp(a, token.QUO, b)
	fmt.Println(c)

}
Output:

22

func Denom

func Denom(x Value) Value

Denom returns the denominator of x; x must be Int, Float, or Unknown. If x is Unknown, or if it is too large or small to represent as a fraction, the result is Unknown. Otherwise the result is an Int >= 1.

func Imag

func Imag(x Value) Value

Imag returns the imaginary part of x, which must be a numeric or unknown value. If x is Unknown, the result is Unknown.

func Make added in go1.13

func Make(x interface{}) Value

Make returns the Value for x.

type of x        result Kind
----------------------------
bool             Bool
string           String
int64            Int
*big.Int         Int
*big.Float       Float
*big.Rat         Float
anything else    Unknown

func MakeBool

func MakeBool(b bool) Value

MakeBool returns the Bool value for b.

func MakeFloat64

func MakeFloat64(x float64) Value

MakeFloat64 returns the Float value for x. If x is -0.0, the result is 0.0. If x is not finite, the result is an Unknown.

func MakeFromBytes

func MakeFromBytes(bytes []byte) Value

MakeFromBytes returns the Int value given the bytes of its little-endian binary representation. An empty byte slice argument represents 0.

func MakeFromLiteral

func MakeFromLiteral(lit string, tok token.Token, zero uint) Value

MakeFromLiteral returns the corresponding integer, floating-point, imaginary, character, or string value for a Go literal string. The tok value must be one of token.INT, token.FLOAT, token.IMAG, token.CHAR, or token.STRING. The final argument must be zero. If the literal string syntax is invalid, the result is an Unknown.

func MakeImag

func MakeImag(x Value) Value

MakeImag returns the Complex value x*i; x must be Int, Float, or Unknown. If x is Unknown, the result is Unknown.

func MakeInt64

func MakeInt64(x int64) Value

MakeInt64 returns the Int value for x.

func MakeString

func MakeString(s string) Value

MakeString returns the String value for s.

func MakeUint64

func MakeUint64(x uint64) Value

MakeUint64 returns the Int value for x.

func MakeUnknown

func MakeUnknown() Value

MakeUnknown returns the Unknown value.

func Num

func Num(x Value) Value

Num returns the numerator of x; x must be Int, Float, or Unknown. If x is Unknown, or if it is too large or small to represent as a fraction, the result is Unknown. Otherwise the result is an Int with the same sign as x.

func Real

func Real(x Value) Value

Real returns the real part of x, which must be a numeric or unknown value. If x is Unknown, the result is Unknown.

func Shift

func Shift(x Value, op token.Token, s uint) Value

Shift returns the result of the shift expression x op s with op == token.SHL or token.SHR (<< or >>). x must be an Int or an Unknown. If x is Unknown, the result is x.

func ToComplex added in go1.6

func ToComplex(x Value) Value

ToComplex converts x to a Complex value if x is representable as a Complex. Otherwise it returns an Unknown.

func ToFloat added in go1.6

func ToFloat(x Value) Value

ToFloat converts x to a Float value if x is representable as a Float. Otherwise it returns an Unknown.

func ToInt added in go1.6

func ToInt(x Value) Value

ToInt converts x to an Int value if x is representable as an Int. Otherwise it returns an Unknown.

func UnaryOp

func UnaryOp(op token.Token, y Value, prec uint) Value

UnaryOp returns the result of the unary expression op y. The operation must be defined for the operand. If prec > 0 it specifies the ^ (xor) result size in bits. If y is Unknown, the result is Unknown.

Example
package main

import (
	"fmt"
	"go/constant"
	"go/token"
)

func main() {
	vs := []constant.Value{
		constant.MakeBool(true),
		constant.MakeFloat64(2.7),
		constant.MakeUint64(42),
	}

	for i, v := range vs {
		switch v.Kind() {
		case constant.Bool:
			vs[i] = constant.UnaryOp(token.NOT, v, 0)

		case constant.Float:
			vs[i] = constant.UnaryOp(token.SUB, v, 0)

		case constant.Int:
			// Use 16-bit precision.
			// This would be equivalent to ^uint16(v).
			vs[i] = constant.UnaryOp(token.XOR, v, 16)
		}
	}

	for _, v := range vs {
		fmt.Println(v)
	}

}
Output:


false
-2.7
65493

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