type checking and error handling for builtins improved.

builtins/input.go

@@ -1,6 +1,9 @@
 package builtins
 
-import "monkey/object"
+import (
+	"monkey/object"
+	"monkey/typing"
+)
 
 import (
 	"bufio"
@@ -11,15 +14,17 @@ import (
 
 // Input ...
 func Input(args ...object.Object) object.Object {
-	if len(args) > 0 {
-		obj, ok := args[0].(*object.String)
-		if !ok {
-			return newError(
-				"argument to `input` not supported, got %s",
-				args[0].Type(),
-			)
-		}
-		fmt.Fprintf(os.Stdout, obj.Value)
+	if err := typing.Check(
+		"input", args,
+		typing.RangeOfArgs(0, 1),
+		typing.WithTypes(object.STRING_OBJ),
+	); err != nil {
+		return newError(err.Error())
+	}
+
+	if len(args) == 1 {
+		prompt := args[0].(*object.String).Value
+		fmt.Fprintf(os.Stdout, prompt)
 	}
 
 	buffer := bufio.NewReader(os.Stdin)