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Why can't immediate objects be extended?
- Richard Jones
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Date: | 2008-07-12 (14:12) |
From: | Richard Jones <rich@a...> |
Subject: | Why can't immediate objects be extended? |
I'm wondering if there's a reason why one cannot inherit from immediate objects? (See description at: http://caml.inria.fr/pub/docs/manual-ocaml/manual005.html#ss:immediate-objects) Is it just a syntax problem because the 'inherit' keyword currently needs to take a class type, or is there a deeper reason? This is the code I'd like to write (non-working, obviously): type 'a tree = Leaf of string | Node of 'a tree * 'a * 'a tree let none = object end let tree = Node (Leaf "1", none, Leaf "2") let rec string_of_tree = function | Leaf str -> str | Node (left, _, right) -> "(" ^ string_of_tree left ^ "," ^ string_of_tree right ^ ")" let rec annotate = function | (Leaf _ as t) -> t | (Node (left, parent_obj, right) as t) -> let obj = object inherit (typeof parent_obj) method str = string_of_tree t end in Node (annotate left, obj, annotate right) let tree = annotate tree The problematic function is 'annotate'. I believe I want annotate to have a type along these lines (again, this is not precisely OCaml code because I've used alpha to stand for '..'): val annotate : < 'a > tree -> < str : string; 'a > tree Rich. -- Richard Jones Red Hat