[GHC] #8004: Applicative/Monad proposal related warnings (AMP phase 1)

[GHC]

#8004: Applicative/Monad proposal related warnings (AMP phase 1)
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        Reporter:  quchen            |                Owner:  quchen
            Type:  feature request   |               Status:  closed
        Priority:  high              |            Milestone:  7.8.1
       Component:  Compiler          |              Version:  7.6.3
      Resolution:  fixed             |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #4834             |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by ekmett):

 Replying to [comment:23 songzh]:
 > I am so curious that I want to ask for the case "You have an instance of
 Monad, but not Applicative"
 > why cann't we add default implementation to Applicative typeclass?
 > {{{
 > class Functor f => Applicative f where
 >     pure :: a -> f a
 >     default pure :: Monad f => a -> f a
 >     pure = return

 At least in the case of `pure` and `return` we have the default definition
 of `return` in terms of `pure`. That flow the 'right' way through the
 class hierarchy and doesn't require an extension. Doing both that and this
 yields a cycle with no way to detect it through `MINIMAL` pragmas and the
 like, which can't straddle two classes.

 >     (<*>) :: f (a -> b) -> f a -> f b
 >     default (<*>) :: Monad f => f (a -> b) -> f a -> f b
 >     (<*>) f a = ap
 > }}}

 This one could be done. It'd require a willingness to have an attempt to
 implement `Applicative` that doesn't supply a `(<*>)` to be a compile
 error rather than a warning, however. (A state that can arise as you write
 incrementally.) It is a fairly minor concern, but given the difficulty of
 pushing through Prelude related changes one that __may__ be a sufficient
 blocker.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8004#comment:24>
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