Functor-Applicative-Monad Proposal

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Haskell calls a couple of historical accidents its own. While some of them, such as the "number classes" hierarchy, can be justified by pragmatism or lack of a strictly better suggestion, there is one thing that stands out as, well, not that: Applicative not being a superclass of Monad.

The topic has been discussed multiple times in the past (cf. link section at the bottom). This article was updated to describe the current, and very likely to succeed, Haskell 2014 Applicative => Monad proposal.

Pragmatic and short version

The following is a list of things you may have to change in your code so the AMP doesn't break it.

  • Add Applicative/Functor instances for all your Monads. If you don't care about efficiency, you can simply derive these instances from the Monad by adding the following code:
-- Monad m

import Control.Monad       (liftM, ap)
import Control.Applicative (Applicative(..))

instance Functor m where
    fmap = liftM

instance Applicative m where
    pure  = return
    (<*>) = ap
  • Add an Alternative instance for all instances of MonadZero. This can again be done easily using
-- MonadZero m

import Control.Monad       (mzero, mplus)
import Control.Applicative (Alternative(..))

instance Alternative m where
    (<|>) = mplus
    empty = mzero
  • Change your API to not define functions named <*>, join or pure

Detailed description

Main text of the Haskell 2014 AMP

Mailing list discussion of the proposal

Previous proposals

  • From early 2011: GHC ticket – Makes Applicative into a superclass of Monad, but does not deprecate any existing names
    • See [1] for the associated discussion.
  • The Other Prelude