type checking and error handling for builtins improved.

builtins/int.go

@@ -1,13 +1,19 @@
 package builtins
 
-import "monkey/object"
+import (
+	"monkey/object"
+	"monkey/typing"
+)
 
 import "strconv"
 
 // Int ...
 func Int(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(
+		"int", args,
+		typing.ExactArgs(1),
+	); err != nil {
+		return newError(err.Error())
 	}
 
 	switch arg := args[0].(type) {
@@ -23,9 +29,8 @@ func Int(args ...object.Object) object.Object {
 		if err != nil {
 			return newError("could not parse string to int: %s", err)
 		}
-
 		return &object.Integer{Value: n}
 	default:
-		return newError("argument to `int` not supported, got=%s", args[0].Type())
+		return &object.Integer{}
 	}
 }