To: Steve Stevenson <steve@cs.clemson.edu>
Subject: Re: Imperative list operations
From: John Prevost <prevost@maya.com>
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 <steve@cs.clemson.edu> writes:
> I need a double-ended queue implementation. {...imperative...}
> I've tried all the naïve 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
recursive.
John.
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