Re: Imperative list operations

From: John Prevost (
Date: Wed Sep 15 1999 - 15:33:33 MET DST

To: Steve Stevenson <>
Subject: Re: Imperative list operations
From: John Prevost <>
Date: 15 Sep 1999 09:33:33 -0400
In-Reply-To: Steve Stevenson's message of "Tue, 14 Sep 1999 15:36:18 -0400 (EDT)"

Steve Stevenson <> writes:

> I need a double-ended queue implementation. {...imperative...}

> I've tried all the na´ve type declarations --- all of which
> don't seem to work. I've tried the age old tricks. What am I not
> understanding? or doing right? I'm not fussy: tuples, records or
> objects are fine.

It'd help to know what you've tried. The following work for me:

type 'a dlist_node =
  | Nil
  | Node of 'a dlist_node ref * 'a * 'a dlist_node ref

Or better:

type 'a dlist_node =
  { mutable dlist_prev : 'a dlist_node option;
            dlist_val : 'a;
    mutable dlist_next : 'a dlist_node option }

Note that in the first case, you need to use value constructors, since:

type 'a dlist_node = 'a dlist_node option ref * 'a * 'a dlist_node option ref

is recursive, and O'Caml doesn't allow type aliases (as opposed to
union types or record types, which "create" new types) to be


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