Proposal: Change the signature of mkWeakMVar

[Edward Kmett]

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
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