Re: [GHC] #11152: GND accepts ill-roled coercion when manually defining it won't typecheck

1 Dec 2015

      #11152: GND accepts ill-roled coercion when manually defining it won't typecheck
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        Reporter:  RyanGlScott       |                Owner:
            Type:  bug               |               Status:  closed
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  7.10.2
  checker)                           |
      Resolution:  invalid           |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC accepts       |  Unknown/Multiple
  invalid program                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #9123             |  Differential Rev(s):
       Wiki Page:                    |
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Changes (by RyanGlScott):

 * status:  new => closed
 * resolution:   => invalid
 * related:   => #9123

Comment:

 Oops, I overlooked the `forall (a_axs :: *)` part. It turns out that (and
 some language extensions) are needed to make the generated code compile
 when typed out manually:

 {{{#!hs
 {-# LANGUAGE GeneralizedNewtypeDeriving #-}
 {-# LANGUAGE KindSignatures #-}
 {-# LANGUAGE ScopedTypeVariables #-}
 module CoerceJoin where

 import qualified Control.Monad (join)
 import           Data.Coerce (coerce)

 class MyMonad m where
   join :: m (m a) -> m a

 instance MyMonad Maybe where
   join = Control.Monad.join

 newtype MyMaybe a = MyMaybe (Maybe a)
 instance MyMonad MyMaybe where
   join = coerce (join :: Maybe (Maybe a) -> Maybe a) :: forall (a :: *).
 MyMaybe (MyMaybe a) -> MyMaybe a
 }}}

 Also, I think the issue I was trying to reproduce is #9123, so I'll add
 that to the related tickets as evidence that adding `join` to `Monad`
 ''would'' cause some trouble.

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11152#comment:3
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