fix(core): add WriterFrom and WriterTo helpers.

builtins_funcs.go

@@ -838,13 +838,8 @@ func BuiltinReadFunc(c Call) (ret Object, err error) {
 			Name: "reader",
 			TypeAssertion: &TypeAssertion{
 				Handlers: map[string]TypeAssertionHandler{
-					"reader": func(v Object) bool {
-						switch v.(type) {
-						case Reader:
-							return true
-						default:
-							return false
-						}
+					"reader": func(v Object) (ok bool) {
+						return ReaderFrom(v) != nil
 					},
 				},
 			},
@@ -897,13 +892,13 @@ func BuiltinReadFunc(c Call) (ret Object, err error) {
 		if buffered {
 			return Bytes{}, nil
 		}
-		b, err = io.ReadAll(reader.Value.(Reader))
+		b, err = io.ReadAll(ReaderFrom(reader.Value))
 		s = len(b)
 	} else {
 		if len(b) == 0 {
 			b = make([]byte, l)
 		}
-		s, err = reader.Value.(Reader).Read(b)
+		s, err = ReaderFrom(reader.Value).Read(b)
 	}
 
 	if err != nil {
@@ -930,7 +925,7 @@ func BuiltinWriteFunc(c Call) (ret Object, err error) {
 	}
 
 	arg := c.Args.Get(0)
-	if w2, ok := arg.(Writer); ok {
+	if w2 := WriterFrom(arg); w2 != nil {
 		w = w2
 		c.Args.Shift()
 	}

helper/namedargvar.go

@@ -8,8 +8,7 @@ func NamedArgOfWriter(name string) *gad.NamedArgVar {
 		TypeAssertion: &gad.TypeAssertion{
 			Handlers: map[string]gad.TypeAssertionHandler{
 				"writer": func(v gad.Object) (ok bool) {
-					_, ok = v.(gad.Writer)
-					return
+					return gad.WriterFrom(v) != nil
 				},
 			},
 		},
@@ -22,8 +21,7 @@ func NamedArgOfReader(name string) *gad.NamedArgVar {
 		TypeAssertion: &gad.TypeAssertion{
 			Handlers: map[string]gad.TypeAssertionHandler{
 				"reader": func(v gad.Object) (ok bool) {
-					_, ok = v.(gad.Reader)
-					return
+					return gad.ReaderFrom(v) != nil
 				},
 			},
 		},

objects_api.go

@@ -493,3 +493,31 @@ func IsTypeAssignableTo(a, b ObjectType) bool {
 	}
 	return false
 }
+
+type ToReaderConverter interface {
+	Reader() Reader
+}
+
+type ToWriterConverter interface {
+	Writer() Writer
+}
+
+func ReaderFrom(o Object) (r Reader) {
+	if r, _ = o.(Reader); r != nil {
+		return r
+	}
+	if tr, _ := o.(ToReaderConverter); tr != nil {
+		return tr.Reader()
+	}
+	return
+}
+
+func WriterFrom(o Object) (r Writer) {
+	if r, _ = o.(Writer); r != nil {
+		return r
+	}
+	if tr, _ := o.(ToWriterConverter); tr != nil {
+		return tr.Writer()
+	}
+	return
+}

objects_reflect.go

@@ -1054,6 +1054,20 @@ type ReflectStruct struct {
 	Interface            any
 }
 
+func (s *ReflectStruct) Reader() Reader {
+	if r, ok := s.Interface.(io.Reader); ok {
+		return NewReader(r)
+	}
+	return nil
+}
+
+func (s *ReflectStruct) Writer() Writer {
+	if w, ok := s.Interface.(io.Writer); ok {
+		return NewWriter(w)
+	}
+	return nil
+}
+
 var (
 	_ ReflectValuer     = (*ReflectStruct)(nil)
 	_ Iterabler         = (*ReflectStruct)(nil)