[Haskell-cafe] Proposal: new function for lifting

Nick Vanderweit nick.vanderweit at gmail.com
Fri Sep 27 22:21:24 CEST 2013

Sorry for sending this twice; I didn't reply to the list initially.

I thought people [1] were generally talking about lift from

class MonadTrans t where
    lift :: Monad m => m a -> t m a

The idea being that lifting through a monad stack feels tedious. The
proposed solution is to use instances to do the lifting for you, like in
mtl. So we've got instances like:

MonadState s m => MonadState s (ReaderT r m)

Which let you automatically lift get/put/modify up a stack, without doing
any work.

This is different from liftM*, which are about applying a pure function
to monadic arguments. This can be done quite nicely with (<$>) and (<*>)
from Data.Functor and Control.Applicative, respectively. Your first
example can be written:

(+) <$> (Just 42) <*> Nothing



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