Re: [Caml-list] Pattern matching on refs

[Yotam Barnoy <yotambarnoy@gmail.com>]

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D'oh! I always forget about mutable fields somehow. Refs just take over in
my mind and I end up putting them everywhere I need mutability. Shows how
little I actually use mutability in ocaml.

And the reason for the linked lists is that I need a (low-performance)
queue/stack with random access. And the reason for implementing a
doubly-linked list myself is that my advisor is against using any library
that's not part of the ocaml distribution.

Sorry for the disturbance folks. Move along!

-Yotam


On Thu, Oct 10, 2013 at 3:42 PM, David Allsopp <dra-news@metastack.com>wrote:

> Yotam Barnoy wrote:
> > I recently found out how ugly it is to pattern-match on a ref,
> > using {contents=...}. This should be extremely easy to fix in
> > the parser. Can it please be put into the next version of ocaml?
>
> I imagine there are those who might suggest that the ugliness of pattern
> matching on refs is part of the discouragement against using them!
>
> > match x with
> > | ref y -> ...
>
> I'm guessing that you're really pattern matching with refs inside tuples
> or something which makes using !x impractical? That said, if you've ended
> with up (foo, bar, baz) where at least one of those is a reference, why not
> consider using records with mutable fields?
>
> While writing this, Yotam Barnoy wrote:
> > It wouldn't solve the problem, because in reality
> > I'm matching something like this piece of code
> > implementing a doubly-linked list:
> >
> > type 'a cell = { data : 'a;
> >                 next : 'a link ref;
> >                 last : 'a link ref;
> >               }
>
> Completely ignoring why you might be implementing linked lists in a
> list-processing language (I'm sure there's a good reason!), why not have
>
> type 'a cell = {data: 'a;
>                 next: mutable 'a link;
>                 last: mutable 'link}
>
> ?
>
> The parser change you propose is probably not trivial - for a start, "ref"
> is part of the Pervasives module, not part of the grammar, and [ref] itself
> can be redefined (try [let ref x = x] in the toplevel). Putting something
> into the grammar to allow pattern matching on ref like this would at best
> be a grim hack.
>
>
> David
>
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