go/constant: add Val accessor and Make constructor to handle varied types

This change adds a Val accessor that returns the underlying type for a
given constant.Value. This change also adds a Make constructor that builds a
constant.Value given a value of a specific type.

Fixes #29820

Change-Id: I4fc3f5221408e24af42ffecd21ce4099ee75b47a
Reviewed-on: https://go-review.googlesource.com/c/go/+/164538Reviewed-by: Robert Griesemer <gri@golang.org>

src/go/constant/example_test.go

@@ -8,6 +8,7 @@ import (
 	"fmt"
 	"go/constant"
 	"go/token"
+	"math"
 	"sort"
 )
 
@@ -156,3 +157,24 @@ func ExampleSign() {
 	//  1 (0 + 1i)
 	//  1 (1 + 1i)
 }
+
+func ExampleVal() {
+	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
+}

src/go/constant/value.go

@@ -562,6 +562,68 @@ func Float64Val(x Value) (float64, bool) {
 	}
 }
 
+// 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
+//
+func Val(x Value) interface{} {
+	switch x := x.(type) {
+	case boolVal:
+		return bool(x)
+	case *stringVal:
+		return x.string()
+	case int64Val:
+		return int64(x)
+	case intVal:
+		return x.val
+	case ratVal:
+		return x.val
+	case floatVal:
+		return x.val
+	default:
+		return nil
+	}
+}
+
+// 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 Make(x interface{}) Value {
+	switch x := x.(type) {
+	case bool:
+		return boolVal(x)
+	case string:
+		return &stringVal{s: x}
+	case int64:
+		return int64Val(x)
+	case *big.Int:
+		return intVal{x}
+	case *big.Rat:
+		return ratVal{x}
+	case *big.Float:
+		return floatVal{x}
+	default:
+		return unknownVal{}
+	}
+}
+
 // 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.

src/go/constant/value_test.go

@@ -7,6 +7,7 @@ package constant
 import (
 	"fmt"
 	"go/token"
+	"math/big"
 	"strings"
 	"testing"
 )
@@ -596,6 +597,22 @@ func TestUnknown(t *testing.T) {
 	}
 }
 
+func TestMake(t *testing.T) {
+	for _, want := range []interface{}{
+		false,
+		"hello",
+		int64(1),
+		big.NewInt(10),
+		big.NewFloat(2.0),
+		big.NewRat(1, 3),
+	} {
+		got := Val(Make(want))
+		if got != want {
+			t.Errorf("got %v; want %v", got, want)
+		}
+	}
+}
+
 func BenchmarkStringAdd(b *testing.B) {
 	for size := 1; size <= 65536; size *= 4 {
 		b.Run(fmt.Sprint(size), func(b *testing.B) {