[GHC] #11692: Suggest default definition of Applicative

[GHC]

#11692: Suggest default definition of Applicative
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        Reporter:  Iceland_jack      |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  GHCi              |              Version:  8.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Incorrect         |  Unknown/Multiple
  warning at compile-time            |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by hvr):

 Your suggested warning enhancement is in contradiction to the
 recommendation expressed in several places (migration guide, AMP wiki page
 etc, `-Wnoncanonical-monad-instances`) to define `return` in terms of
 `pure` rather than the other way round.

 In fact, since GHC 7.10/base-4.8 you don't even need to define
 `Monad(return)` at all anymore, as it's got a default implementation
 `return = pure`. Suggesting to define `pure = return` risks ending up with
 circular definition of `return`/`pure` when refactoring.

 In fact, together with the MonadFail proposal the proper (minimal, i.e.
 excluding non-mandatory methods) mental model of the F-A-M-MF hierarchy to
 have is more or less:

 {{{#!hs
 class Functor f where
   fmap :: (a -> b) -> f a -> f b

 class Functor f => Applicative f where
   pure :: a -> f a
   (<*>) :: f (a -> b) -> f a -> f b

 class Applicative m => Monad m  where
   (>>=) :: m a -> (a -> m b) -> m b

 class Monad m => MonadFail m
   fail :: String -> m a
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11692#comment:1>
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