type checking and error handling for builtins improved.

builtins/lower.go

@@ -1,18 +1,22 @@
 package builtins
 
-import "monkey/object"
+import (
+	"monkey/object"
+	"monkey/typing"
+)
 
 import "strings"
 
 // Lower ...
 func Lower(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(
+		"lower", args,
+		typing.ExactArgs(1),
+		typing.WithTypes(object.STRING_OBJ),
+	); err != nil {
+		return newError(err.Error())
 	}
 
-	if str, ok := args[0].(*object.String); ok {
-		return &object.String{Value: strings.ToLower(str.Value)}
-	}
-	return newError("expected `str` argument to `lower` got=%T", args[0])
+	str := args[0].(*object.String)
+	return &object.String{Value: strings.ToLower(str.Value)}
 }