Re: [Caml-list] Pattern matching on refs

[Arnaud Spiwack <aspiwack@lix.polytechnique.fr>]

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If you need queues with random access, and do not need highly tuned
performances, may I suggest you having a look at Okasaki's *Purely
functional datastructures*. It has a handful of these, which do not involve
assignment (they use laziness annotation, though, for good amortized
performances). It would make your life better. The book version has a
little more than the thesis, by the way.


On 10 October 2013 21:46, Yotam Barnoy <yotambarnoy@gmail.com> wrote:

> 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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>
>

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