No instances for recently-added monads from transformers

[bucklereed]

Control.Monad.Trans.Accum was added in transformers 0.5.3.0, but currently it's slightly painful to use because it doesn't have the usual MonadState, MonadReader, etc instances yet.

Whether it should get a MonadWriter instance that lifts the operations to the inner monad or handles them itself is a question.

SelectT was also added in transformers 0.5.3.0 and similarly lacks instances.

[RyanGlScott]

We should definitely add instances for AccumT and SelectT.

As far as how AccumT's instances should work, I'm not sure. @fizruk had proposed in #32 that we introduce a new type class for things that can support AccumT's operations (MonadAccum?), but it's not clear to me what its relationship with MonadWriter/MonadState would be.

I think there are enough design questions here that it would be beneficial to draft up a sketch of how this would work and propose it to the Haskell libraries mailing list for feedback.

[fizruk]

I think that AccumT should propagate MonadReader, MonadWriter and MonadState to inner monad and that we should introduce MonadAccum. This would be consistent with how other classes and transformers work.

[RyanGlScott]

@fizruk, that sounds reasonable to me. But the design of these Monad- classes are usually more involved than one would initially believe (at least in my experience), so I think it would help to come up with a concrete design and propose it to the mailing list. If the community likes it, I'll accept a PR which implements it.

[ekmett]

I'm also leaning towards adding MonadAccum and the like.

[treeowl]

I just got bitten by the lack of a MonadError instance for AccumT.

[minad]

See also #38

[chessai]

This is resolved in master, pending 2.3 release