No MaybeT

[adek05]

Why there is no MaybeT transformer in library? There is a implemenation of it here:
http://www.haskell.org/haskellwiki/New_monads/MaybeT
Why not to put it inside?

[ekmett]

It is already in transformers and mtl depends on transformers.

There is no class required for MaybeT, however. It is described entirely by MonadPlus

Just import Control.Monad.Trans.Maybe.

[ekmett]

A case might be made that perhaps we should offer a Control.Monad.Maybe to re-export it here though.

[adek05]

Actually the case is that it will not offer these lovely instances which avoid have usage of lifting common in transformers library.

[ekmett]

If we're missing instances for lifting over MaybeT then I should definitely fix that. It just doesn't have/need a class of its own.

[ekmett]

They are all here. e.g. there is already an

instance MonadState s m => MonadState s (MaybeT m)

etc.

[sdroege]

They are in the MaybeT package, but the version in transformers does not have them. Compare:
https://hackage.haskell.org/package/MaybeT-0.1.2/docs/Control-Monad-Maybe.html
https://hackage.haskell.org/package/transformers-0.4.2.0/docs/Control-Monad-Trans-Maybe.html

For example the above mentioned
MonadState s m => MonadState s (MaybeT m)
is not there in the transformers version.

[sdroege]

Ah nevermind. The instances are on MonadState (etc) in mtl.