Involved Source Fileskind_string.go 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.
Code Examples
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)
}
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))
}
}
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)
}
}
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)
}
}
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))
}
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)
}
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.
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.
BoolVal returns the Go boolean value of x, which must be a Bool or an Unknown.
If x is Unknown, the result is false.
Bytes returns the bytes for the absolute value of x in little-
endian binary representation; x must be an Int.
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.
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.
Float32Val is like Float64Val but for float32 instead of float64.
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).
Imag returns the imaginary part of x, which must be a numeric or unknown value.
If x is Unknown, the result is Unknown.
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).
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
MakeBool returns the Bool value for b.
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.
MakeFromBytes returns the Int value given the bytes of its little-endian
binary representation. An empty byte slice argument represents 0.
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.
MakeImag returns the Complex value x*i;
x must be Int, Float, or Unknown.
If x is Unknown, the result is Unknown.
MakeInt64 returns the Int value for x.
MakeString returns the String value for s.
MakeUint64 returns the Int value for x.
MakeUnknown returns the Unknown 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.
Real returns the real part of x, which must be a numeric or unknown value.
If x is Unknown, the result is Unknown.
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.
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.
StringVal returns the Go string value of x, which must be a String or an Unknown.
If x is Unknown, the result is "".
ToComplex converts x to a Complex value if x is representable as a Complex.
Otherwise it returns an Unknown.
ToFloat converts x to a Float value if x is representable as a Float.
Otherwise it returns an Unknown.
ToInt converts x to an Int value if x is representable as an Int.
Otherwise it returns an Unknown.
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).
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.
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
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