type checking and error handling for builtins improved.

builtins/ffi.go

@@ -1,6 +1,9 @@
 package builtins
 
-import "monkey/object"
+import (
+	"monkey/object"
+	"monkey/typing"
+)
 
 import (
 	"fmt"
@@ -9,22 +12,16 @@ import (
 
 // FFI ...
 func FFI(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(
+		"ffi", args,
+		typing.ExactArgs(2),
+		typing.WithTypes(object.STRING_OBJ, object.STRING_OBJ),
+	); err != nil {
+		return newError(err.Error())
 	}
 
-	arg, ok := args[0].(*object.String)
-	if !ok {
-		return newError("argument #1 to `ffi` expected to be `str` got=%T", args[0].Type())
-	}
-	name := arg.Value
-
-	arg, ok = args[1].(*object.String)
-	if !ok {
-		return newError("argument #2 to `ffi` expected to be `str` got=%T", args[0].Type())
-	}
-	symbol := arg.Value
+	name := args[0].(*object.String).Value
+	symbol := args[1].(*object.String).Value
 
 	p, err := plugin.Open(fmt.Sprintf("%s.so", name))
 	if err != nil {
@@ -36,5 +33,5 @@ func FFI(args ...object.Object) object.Object {
 		return newError("error finding symbol: %s", err)
 	}
 
-	return &object.Builtin{Name: symbol, Fn: v.(func(...object.Object) object.Object)}
+	return &object.Builtin{Name: symbol, Fn: v.(object.BuiltinFunction)}
 }