type checking and error handling for builtins improved.

builtins/find.go

@@ -2,6 +2,7 @@ package builtins
 
 import (
 	"monkey/object"
+	"monkey/typing"
 	"sort"
 )
 
@@ -11,19 +12,29 @@ import (
 
 // Find ...
 func Find(args ...object.Object) object.Object {
-	if len(args) != 2 {
-		return newError("wrong number of arguments. got=%d, want=2",
-			len(args))
+	if err := typing.Check(
+		"find", args,
+		typing.ExactArgs(2),
+	); err != nil {
+		return newError(err.Error())
 	}
 
+	// find substring in string
 	if haystack, ok := args[0].(*object.String); ok {
-		if needle, ok := args[1].(*object.String); ok {
-			index := strings.Index(haystack.Value, needle.Value)
-			return &object.Integer{Value: int64(index)}
-		} else {
-			return newError("expected arg #2 to be `str` got got=%T", args[1])
+		if err := typing.Check(
+			"find", args,
+			typing.WithTypes(object.STRING_OBJ, object.STRING_OBJ),
+		); err != nil {
+			return newError(err.Error())
 		}
-	} else if haystack, ok := args[0].(*object.Array); ok {
+
+		needle := args[1].(*object.String)
+		index := strings.Index(haystack.Value, needle.Value)
+		return &object.Integer{Value: int64(index)}
+	}
+
+	// find in array
+	if haystack, ok := args[0].(*object.Array); ok {
 		needle := args[1].(object.Comparable)
 		i := sort.Search(len(haystack.Elements), func(i int) bool {
 			return needle.Compare(haystack.Elements[i]) == 0
@@ -32,7 +43,10 @@ func Find(args ...object.Object) object.Object {
 			return &object.Integer{Value: int64(i)}
 		}
 		return &object.Integer{Value: -1}
-	} else {
-		return newError("expected arg #1 to be `str` or `array` got got=%T", args[0])
 	}
+
+	return newError(
+		"TypeError: find() expected argument #1 to be `array` or `str` got `%s`",
+		args[0].Type(),
+	)
 }