Add string.format.

cel/decls.go

@@ -564,6 +564,32 @@ func FunctionBinding(binding functions.FunctionOp) OverloadOpt {
 	}
 }
 
+// LocaleOptionBinding conditionally adds a new binding based on locale, passing newOp an additional final
+// argument that represents the configured locale as a string. If no locale was configured, LocaleOptionBinding
+// will do nothing. If no name/overloadID match is made, then LocaleOptionBinding will return an error.
+func LocaleOptionBinding(name, overloadID string, newOp functions.FunctionOp) EnvOption {
+	return func(e *Env) (*Env, error) {
+		if e.locale == "" {
+			return e, nil
+		}
+		if function, found := e.functions[name]; found {
+			for _, overload := range function.overloads {
+				if overload.id == overloadID {
+					overload.functionOp = func(values ...ref.Val) ref.Val {
+						values = append(values, types.String(e.locale))
+						return newOp(values...)
+					}
+					return e, nil
+				}
+				return nil, fmt.Errorf("could not find overloadID: %s", overloadID)
+			}
+		} else {
+			return nil, fmt.Errorf("could not find function: %s", name)
+		}
+		return nil, nil
+	}
+}
+
 // OverloadIsNonStrict enables the function to be called with error and unknown argument values.
 //
 // Note: do not use this option unless absoluately necessary as it should be an uncommon feature.

cel/env.go

@@ -116,6 +116,8 @@ type Env struct {
 
 	// Program options tied to the environment
 	progOpts []ProgramOption
+
+	locale string
 }
 
 // NewEnv creates a program environment configured with the standard library of CEL functions and

cel/options.go

@@ -17,6 +17,7 @@ package cel
 import (
 	"fmt"
 
+	"golang.org/x/text/language"
 	"google.golang.org/protobuf/proto"
 	"google.golang.org/protobuf/reflect/protodesc"
 	"google.golang.org/protobuf/reflect/protoreflect"
@@ -328,6 +329,19 @@ func registerFiles(reg ref.TypeRegistry, files *protoregistry.Files) error {
 	return err
 }
 
+// Locale configures the Env with the given locale tag. If this is not passed,
+// then string.format will behave as if the locale was set to en_US.
+func Locale(locale string) EnvOption {
+	return func(e *Env) (*Env, error) {
+		_, err := language.Parse(locale)
+		if err != nil {
+			return nil, fmt.Errorf("failed to parse locale: %w", err)
+		}
+		e.locale = locale
+		return e, nil
+	}
+}
+
 // ProgramOption is a functional interface for configuring evaluation bindings and behaviors.
 type ProgramOption func(p *prog) (*prog, error)