type checking and error handling for builtins improved.

builtins/id.go

@@ -3,13 +3,16 @@ package builtins
 import (
 	"fmt"
 	"monkey/object"
+	"monkey/typing"
 )
 
 // IdOf ...
 func IdOf(args ...object.Object) object.Object {
-	if len(args) != 1 {
-		return newError("wrong number of arguments. got=%d, want=1",
-			len(args))
+	if err := typing.Check(
+		"id", args,
+		typing.ExactArgs(1),
+	); err != nil {
+		return newError(err.Error())
 	}
 
 	arg := args[0]
@@ -32,7 +35,7 @@ func IdOf(args ...object.Object) object.Object {
 		return &object.String{Value: fmt.Sprintf("%p", c)}
 	} else if b, ok := arg.(*object.Builtin); ok {
 		return &object.String{Value: fmt.Sprintf("%p", b)}
-	} else {
-		return newError("argument 31 to `id()` unsupported got=%T", arg.Type())
 	}
+
+	return nil
 }