minad/writer-cps-transformers

Compatibility package for old transformers: Stricter drop in replacements for WriterT and RWST

Stricter drop in replacements for WriterT and RWST

• https://hackage.haskell.org/package/writer-cps-transformers
• https://hackage.haskell.org/package/writer-cps-mtl
• https://hackage.haskell.org/package/writer-cps-monads-tf

writer-cps-transformers's Issues

Provide pattern synonyms to access internal representations.

The current transformers (0.5.6.2) does not expose the internal representations of Control.Monad.Trans.Writer.CPS or Control.Monad.Trans.RWS.CPS. This makes it impossible to implement certain things, ex. MFunctor from the mmorph package. The transformers package has had an ongoing issue for about a year, but even in the case of an update, some packages may want to maintain backward-compatibility.

In version 0.5.5.0, this package exported the modules Control.Monad.Trans.Writer.CPS.Internal and Control.Monad.Trans.RWS.CPS.Internal, which exposed these representations.
My proposal is to re-add these modules, exporting the original interface with the addition of the following pattern synonyms:

{-# LANGUAGE PatternSynonyms, ViewPatterns #-}

pattern WriterT :: (w -> m (a, w)) -> WriterT w m a
pattern WriterT x = (unsafeCoerce -> x)
    where WriterT x = unsafeCoerce x
{-# COMPLETE WriterT #-}

pattern RWST :: (r -> s -> w -> m (a, s, w)) -> WriterT w m a
pattern RWST x = (unsafeCoerce -> x)
    where RWST x = unsafeCoerce x
{-# COMPLETE RWST #-}

Because these datatypes are implemented via newtype, this usage of unsafeCoerce is safe with ghc.

This allows for implementation of MFunctor:

instance MFunctor (WriterT w) where
    hoist :: Monad m => (forall a. m a -> n a) -> WriterT w m b -> WriterT w n b
    hoist nat (WriterT m) = WriterT $ \w -> nat (m w)

Add `exceptions` instances

I found myself needing the MonadThrow and MonadCatch instances of the CPS version of WriterT , so I understand they should appear here, though this will introduce a new exceptions dependency.

What do you think?

Create Control.Lens instances

https://github.com/louispan/writer-cps-lens can be used once the Internal module code is merged.

Do not provide liftCallCC'

The fact that this implementation of Writer is under the hood a State shouldn't show up in the public interface.

In particular, there shouldn't be a liftCallCC' function in the public module. Only liftCallCC should be provided and it should behave like the one for Writer, not like the one for State.

Current state of this package is confusing

I had to find #7 to understand why there are no module listed at http://hackage.haskell.org/package/writer-cps-transformers-0.5.6.0.

Would you mind properly deprecating the package and directing people to transformers?

Make this package a compatibility package for older transformers

See https://hub.darcs.net/ross/transformers/patch/b6743fad9f5ce8a9e13f15320f7d2752330084e2