implemented nullable type notation

src/lib/objects/o_nullable.c

@@ -0,0 +1,62 @@
+#include "../common/defs.h" 
+#include "objects.h"
+
+static void walk(Object *self, void (*callback)(Object **referer, void *userp), void *userp);
+static bool istypeof(Object *self, Object *other);
+
+typedef struct {
+    Object  base;
+    Object *item;
+} NullableObject;
+
+static TypeObject t_nullable = {
+    .base = (Object) { .type = &t_type, .flags = Object_STATIC },
+    .name = TYPENAME_NULLABLE,
+    .size = sizeof(NullableObject),
+    .init = NULL,
+    .free = NULL,
+    .hash = NULL,
+    .copy = NULL,
+    .call = NULL,
+    .print = NULL,
+
+    .select = NULL,
+    .delete = NULL,
+    .insert = NULL,
+    .count  = NULL,
+
+    .istypeof = istypeof,
+
+    .op_eql = NULL,
+    .walk = walk,
+    .walkexts = NULL,
+};
+
+TypeObject *Object_GetNullableType()
+{
+    return &t_nullable;
+}
+
+Object *Object_NewNullable(Object *item, Heap *heap, Error *error)
+{
+    NullableObject *nullable = (NullableObject*) Heap_Malloc(heap, &t_nullable, error);
+    if (nullable != NULL)
+        nullable->item = item;
+    return (Object*) nullable;
+}
+
+static bool 
+istypeof(Object *self, 
+         Object *other)
+{
+    NullableObject *nullable = (NullableObject*) self;
+    if (Object_IsNone(other))
+        return true;
+    return Object_IsTypeOf(nullable->item, other);
+}
+
+static void walk(Object *self, void (*callback)(Object **referer, void *userp), void *userp)
+{
+    NullableObject *nullable = (NullableObject*) self;
+    callback(&nullable->item, userp);
+}