Update REPL to use ext.Strings formatter when possible.

ext.Strings provides a string formatter that formats builtin values as
CEL literals in most cases.

ext/strings.go

@@ -681,7 +681,7 @@ func clauseForType(argType ref.Type) (clauseImpl, error) {
 	case types.IntType, types.UintType:
 		return formatDecimal, nil
 	case types.StringType, types.BytesType, types.BoolType, types.NullType, types.TypeType:
-		return formatString, nil
+		return FormatString, nil
 	case types.TimestampType, types.DurationType:
 		// special case to ensure timestamps/durations get printed as CEL literals
 		return func(arg ref.Val, locale string) (string, error) {
@@ -770,7 +770,7 @@ func formatMap(arg ref.Val, locale string) (string, error) {
 		var keyFormat clauseImpl
 		switch key.Type() {
 		case types.StringType, types.BoolType:
-			keyFormat = formatString
+			keyFormat = FormatString
 		case types.IntType, types.UintType:
 			keyFormat = formatDecimal
 		default:
@@ -844,7 +844,10 @@ func quoteForCEL(refVal ref.Val, unquotedValue string) string {
 	}
 }
 
-func formatString(arg ref.Val, locale string) (string, error) {
+// FormatString returns the string representation of a CEL value.
+// It is used to implement the %s specifier in the (string).format() extension
+// function.
+func FormatString(arg ref.Val, locale string) (string, error) {
 	switch arg.Type() {
 	case types.ListType:
 		return formatList(arg, locale)
@@ -1018,7 +1021,7 @@ func parseFormattingClause(lastStrIndex int, formatStr string) (int, clauseImpl,
 	i++
 	switch r {
 	case 's':
-		return i, formatString, nil
+		return i, FormatString, nil
 	case 'd':
 		return i, formatDecimal, nil
 	case 'f':

repl/evaluator.go

@@ -25,6 +25,7 @@ import (
 	"github.com/google/cel-go/checker/decls"
 	"github.com/google/cel-go/common/types"
 	"github.com/google/cel-go/common/types/ref"
+	"github.com/google/cel-go/ext"
 	"github.com/google/cel-go/interpreter"
 	"github.com/google/cel-go/interpreter/functions"
 
@@ -758,7 +759,13 @@ func (e *Evaluator) Process(cmd Cmder) (string, bool, error) {
 			return "", false, fmt.Errorf("expr failed:\n%v", err)
 		}
 		if val != nil {
-			return fmt.Sprintf("%v : %s", val.Value(), UnparseType(resultT)), false, nil
+			t := UnparseType(resultT)
+			v, err := ext.FormatString(val, "")
+			if err != nil {
+				// Default format if type is unsupported by ext.Strings formatter.
+				return fmt.Sprintf("%v : %s", val.Value(), t), false, nil
+			}
+			return fmt.Sprintf("%s : %s", v, t), false, nil
 		}
 	case *letVarCmd:
 		var err error

repl/evaluator_test.go

@@ -494,6 +494,50 @@ func TestProcess(t *testing.T) {
 		wantExit  bool
 		wantError bool
 	}{
+		{
+			name: "FormatNumberResult",
+			commands: []Cmder{
+				&evalCmd{
+					expr: "1u + 2u",
+				},
+			},
+			wantText:  "3 : uint",
+			wantExit:  false,
+			wantError: false,
+		},
+		{
+			name: "FormatStringResult",
+			commands: []Cmder{
+				&evalCmd{
+					expr: `'a' + r'b\1'`,
+				},
+			},
+			wantText:  `ab\1 : string`,
+			wantExit:  false,
+			wantError: false,
+		},
+		{
+			name: "FormatListResult",
+			commands: []Cmder{
+				&evalCmd{
+					expr: `['abc', 123, 3.14, duration('2m')]`,
+				},
+			},
+			wantText:  `["abc", 123, 3.140000, duration("120s")] : list(dyn)`,
+			wantExit:  false,
+			wantError: false,
+		},
+		{
+			name: "FormatMapResult",
+			commands: []Cmder{
+				&evalCmd{
+					expr: `{1: 123, 2: 3.14, 3: duration('2m'), 4: b'123'}`,
+				},
+			},
+			wantText:  `{1:123, 2:3.140000, 3:duration("120s"), 4:b"123"} : map(int, dyn)`,
+			wantExit:  false,
+			wantError: false,
+		},
 		{
 			name: "OptionBasic",
 			commands: []Cmder{