hub.darcs.net :: ross -> transformers -> issue

#49Maybe to MaybeT

Li-yao Xia

Can the following function be added to Control.Monad.Trans.Maybe?

liftMaybe :: Applicative m => Maybe a -> MaybeT m a

When using MaybeT, it's quite common to want to lift Maybe a to MaybeT m a. This is the equivalent of except, state, etc. for other transformers.

• andrewthadFri Feb 16 14:27:22 UTC 2018

  In the errors package, these have existed for years as:

  hoistMaybe :: Monad m => Maybe b -> MaybeT m b

  Ignoring the naming bikeshed, I really think this ought to be in transformers.

• treeowlWed Mar 21 05:00:19 UTC 2018

  The Applicative constraint is the right one these days.

• Ross PatersonThu Mar 22 18:09:02 UTC 2018

  Yes, the Applicative constraint is fine for new definitions.

• Li-yao XiaSun Apr 1 22:29:46 UTC 2018

  This can be generalized to Alternative m => Maybe a -> m a, in which case I would expect to find it in base first, but it would still be useful to have it exported in transformers, in one form or another, if only for discoverability. I have started a thread on the libraries mailing list.

  https://mail.haskell.org/pipermail/libraries/2018-April/028690.html

• Ross PatersonSun Apr 1 22:42:33 UTC 2018

  That's a different function from what was proposed here, I think:

  liftMaybe :: Applicative m => Maybe a -> MaybeT m a liftMaybe = MaybeT . pure

• fumievalSat Jun 6 14:24:54 UTC 2020

  It can be generalised further into (Alternative m, Foldable f) => f a -> m a, and I'm calling it foldAlt

• Ross PatersonMon Jul 5 19:49:53 UTC 2021
  • status set to closed

  fixed, thanks