[Proposal] Unify various specialized mkWeak functions in base?

[Cheng Shao]

Thanks, David! That looks exactly like what I have in mind. Given its
dependency on primitive and primitive not being a part of base (or a boot
lib), I think I'd better give up upstreaming the idea and simply use that
package when really needed.

On Wed, Nov 11, 2020 at 10:36 PM David Feuer <david.feuer at gmail.com> wrote:

> You should look at the primitive-unlifted package which offers a class for
> wrapping and unwrapping unlifted things and functions for making weak
> pointers from (and to) unlifted things.
>
> On Wed, Nov 11, 2020, 4:34 PM Cheng Shao <astrohavoc at gmail.com> wrote:
>
>> Hi everyone,
>>
>> In System.Mem.Weak we have an mkWeak function to create weak pointers.
>> But we
>> also have a lot of specialized mkWeak* functions, e.g. mkWeakIORef,
>> mkWeakThreadId, etc, to ensure that we actually pass the unlifted closure
>> instead of the lifted one as the key to the underlying mkWeak# primop.
>>
>> The various specialized mkWeak* functions' signatures don't have a
>> coherent
>> style. Some functions like mkWeakIORef always take a Haskell finalizer
>> argument,
>> while mkWeakThreadId doesn't take a finalizer argument at all. This is
>> troublesome for users since:
>>
>> * Sometimes a user doesn't want to attach a finalizer at all. They could
>> pass a
>> "pure ()" dummy finalizer, but that'll still come with a minor bit of
>> extra
>> runtime overhead
>>
>> * For mkWeakThreadId, they may want to attach a finalizer. The runtime
>> supports
>> it, but the type signature doesn't say so.
>>
>> Of course, they can always add a few extensions and call mkWeak# as they
>> wish,
>> but it would be nicer if we have a uniform API to allow a Haskell
>> finalizer to
>> be added optionally. The simplest API could be something like (forgive my
>> terrible naming):
>>
>> class WeakKey k where
>>   mkWeakWithRealKey :: WeakKey k => k -> v -> Maybe (IO ()) -> IO (Weak v)
>>
>> The WeakKey class is just a way of exposing the unlifted field from a
>> single
>> datacon wrapper type, and I think this can be useful for other purposes
>> as well,
>> so maybe we can do something like:
>>
>> class Unlift a where
>>   type Unlifted a :: TYPE 'UnliftedRep
>>   unlift :: a -> Unlifted a
>>
>> mkWeakWithRealKey :: Unlift k => k -> v -> Maybe (IO ()) -> IO (Weak v)
>>
>> Thanks for reading so far, what are your thoughts on this? The proposed
>> extended
>> interfaces can be easily implemented in 3rd-party packages, but it would
>> be nice
>> to add to base.
>>
>> Cheers,
>> Cheng Shao
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>>
>
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