haskell - Monad transformers monad duplication -


i new monad transformers, sorry easy question. have value val :: maybet io string , function fn :: string -> io [string]. after binding, have val >>= liftm fn :: maybet io (io [string]). how can remove duplicate io monad , result of type maybet io [string]?

use lift (or liftio) instead of liftm.

> :t val >>= lift . fn val >>= lift . fn :: maybet io [string]

liftm applying pure functions in monad. lift , liftio lifting actions transformer.

liftm  :: monad m => (a -> b) -> m -> m b lift   :: (monad m, monadtrans t) => m -> t m liftio :: monadio m => io -> m

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