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.
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)
}
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(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(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(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.
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))
}
}
func Float32Val(x Value) (float32, bool)
Float32Val is like Float64Val but for float32 instead of float64.
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(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(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.
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)
}
}
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(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(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
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))
}
Kind specifies the kind of value represented by a Value.
type Kind int
const (
// unknown values
Unknown Kind = iota
// non-numeric values
Bool
String
// numeric values
Int
Float
Complex
) A Value represents the value of a Go constant.
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
} 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.
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)
}
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(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(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(b bool) Value
MakeBool returns the Bool value for b.
func MakeFloat64(x float64) Value
MakeFloat64 returns the Float value for x. If x is not finite, the result is an Unknown.
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(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(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(x int64) Value
MakeInt64 returns the Int value for x.
func MakeString(s string) Value
MakeString returns the String value for s.
func MakeUint64(x uint64) Value
MakeUint64 returns the Int value for x.
func MakeUnknown() Value
MakeUnknown returns the Unknown value.
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(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(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(x Value) Value
ToComplex converts x to a Complex value if x is representable as a Complex. Otherwise it returns an Unknown.
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(x Value) Value
ToInt converts x to an Int value if x is representable as an Int. Otherwise it returns an Unknown.
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.
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)
}
}
© Google, Inc.
Licensed under the Creative Commons Attribution License 3.0.
https://golang.org/pkg/go/constant/