Re: [GHC] #13876: Check 'pure' method of 'Applicative (WrappedMonad m)'

26 Jun 2017

      ...
Perhaps. However, given that `pure` and `return` shouldn't differ, and
#13876: Check 'pure' method of 'Applicative (WrappedMonad m)'
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        Reporter:  Iceland_jack      |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by Iceland_jack):

 Replying to [comment:1 bgamari]:
 that the "`Monad` of no `return`" was seriously considered, it's not clear
 to me whether it matters enough to change it. The moment `pure /= return`
 you have bigger problems.

 It matters in the following code:

 {{{#!hs
 {-# Language TypeApplications, ScopedTypeVariables, InstanceSigs,
 RankNTypes, DeriveFunctor, GeneralizedNewtypeDeriving #-}

 import Control.Monad
 import Data.Coerce

 -- As in Control.Applicative
 newtype WrappedMonad m a = WrapMonad { unwrapMonad :: m a }
   deriving (Monad)

 instance Monad m => Functor (WrappedMonad m) where
   fmap :: (a -> b) -> (WrappedMonad m a -> WrappedMonad m b)
   fmap f (WrapMonad v) = WrapMonad (liftM f v)

 instance Monad m => Applicative (WrappedMonad m) where
   pure :: a -> WrappedMonad m a
   pure = WrapMonad . pure

   (<*>) :: WrappedMonad m (a -> b) -> WrappedMonad m a -> WrappedMonad m b
   WrapMonad f <*> WrapMonad v = WrapMonad (f `ap` v)

 -- Definition defining `Applicative' roundtripping through `WrappedMonad'
 data V3 a = V3 a a a
   deriving (Functor, Show)

 instance Applicative V3 where
   pure :: forall a. a -> V3 a
   pure = coerce (pure @(WrappedMonad V3) @a)

   (<*>) :: forall a b. V3 (a -> b) -> V3 a -> V3 b
   (<*>) = coerce ((<*>) @(WrappedMonad V3) @a @b)

 instance Monad V3 where
   return :: a -> V3 a
   return a = V3 a a a

   (>>=) :: V3 a -> (a -> V3 b) -> V3 b
   V3 a b c >>= f = V3 a' b' c' where
     V3 a' _  _  = f a
     V3 _  b' _  = f b
     V3 _  _  c' = f c
 }}}

 If we try to run `pure @V3 'a'` it loops..

 but replace `pure = WrapMonad . pure` by `pure = WrapMonad . return` and
 it works as expected.
 but comment `Control.Applicative` out and un-comment the definition of
 `WrapMonad` (th

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13876#comment:3
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