go/constant: add examples for working with constant.Value

Updates #24352.

Change-Id: Ie7c1de905154a483b7f7748de28e35b484cce6ea
Reviewed-on: https://go-review.googlesource.com/101279Reviewed-by: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/go/constant/example_test.go

+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package constant_test
+
+import (
+	"fmt"
+	"go/constant"
+	"go/token"
+	"sort"
+)
+
+func Example_complexNumbers() {
+	// 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)
+}
+
+func ExampleBinaryOp() {
+	// 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 ExampleUnaryOp() {
+	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
+}
+
+func ExampleCompare() {
+	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 ExampleSign() {
+	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)
+}