Proposal: Change the signature of mkWeakMVar

[John Lato]

I think this is a good idea, but I wonder if we could do a bit better?

Weaks are already implemented as key/value pairs, what we really need is a
way to specify that a data value is a primitive that is appropriate to be
used as a weak key.  Would it make sense to introduce a new typeclass:

class WeakKey k where
    mkWeakOn :: k -> v -> Maybe (IO ()) -> IO (Weak v)

Then we could have implementations for MVar, IORef, TVar etc.

John L.

On Fri Nov 07 2014 at 9:02:25 AM Carter Schonwald <
carter.schonwald at gmail.com> wrote:

> after reading current
> <http://hackage.haskell.org/package/base-4.7.0.1/docs/Control-Concurrent-MVar.html>
>  haddocks
> <http://hackage.haskell.org/package/base-4.7.0.1/docs/System-Mem-Weak.html#t:Weak>,
> so i could understand the current and proposed apis, I'm +1 on this.
> this would align with the api provided by weak pointer too.
>
> On Fri, Nov 7, 2014 at 11:16 AM, Edward Kmett <ekmett at gmail.com> wrote:
>
>> We've had a ticket languishing in the trac for a couple of years that
>> probably belongs as a libraries proposal:
>>
>> https://ghc.haskell.org/trac/ghc/ticket/7285#comment:6
>>
>> To summarize it here:
>>
>> In base 4.6 addMVarFinalizer is deprecated in favour of mkWeakMVar of type
>>
>>     mkWeakMVar :: MVar a -> IO () -> IO (Weak (MVar a))
>>
>> This type makes it inherently non-compositional. For instance, if we have
>> a larger datatype T that contains an MVar somewhere inside then there in no
>> way to define mkWeakT in terms of mkWeakMVar; instead, mkWeakT would have
>> to be defined along the lines of
>>
>>     mkWeakT :: T a -> IO () -> IO (Weak (T a))
>>     mkWeakT m@(MkT (MVar m#) _) f = IO $ \s ->
>>       case mkWeak# m# m f s of (# s1, w #) -> (# s1, Weak w #)
>>
>> It would be better if the type of mkWeakMVar would change to
>>
>>     mkWeakMVar :: MVar a -> v -> Maybe (IO ()) -> IO (Weak v)
>>
>> (i.e., following mkWeak rather than mkWeakPtr).
>>
>> (The same comment goes for related functions such as mkWeakIORef.)
>>
>>
>>
>> I'm personally in favor of the change, but wanted to seek wider community
>> feedback.
>>
>> Thoughts?
>>
>> Discussion Period: 2 Weeks
>>
>> _______________________________________________
>> Libraries mailing list
>> Libraries at haskell.org
>> http://www.haskell.org/mailman/listinfo/libraries
>>
>>
> _______________________________________________
> Libraries mailing list
> Libraries at haskell.org
> http://www.haskell.org/mailman/listinfo/libraries
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://www.haskell.org/pipermail/libraries/attachments/20141107/33824bf3/attachment.html>