[GHC] #7594: GHCi becomes confused about IO type

16 Jan 2013

      #7594: GHCi becomes confused about IO type
-----------------------------+----------------------------------------------
Reporter:  Khudyakov         |          Owner:                  
    Type:  bug               |         Status:  new             
Priority:  normal            |      Component:  GHCi            
 Version:  7.6.1             |       Keywords:                  
      Os:  Unknown/Multiple  |   Architecture:  Unknown/Multiple
 Failure:  None/Unknown      |      Blockedby:                  
Blocking:                    |        Related:                  
-----------------------------+----------------------------------------------
 {{{
 {-# LANGUAGE FlexibleInstances     #-}
 {-# LANGUAGE UndecidableInstances  #-}
 {-# LANGUAGE MultiParamTypeClasses #-}
 {-# LANGUAGE KindSignatures        #-}
 {-# LANGUAGE TypeOperators         #-}
 {-# LANGUAGE ConstraintKinds #-}
 {-# LANGUAGE GADTs           #-}
 {-# LANGUAGE Rank2Types      #-}
 import GHC.Prim (Constraint)

 class    (c1 t, c2 t) => (:&:) (c1 :: * -> Constraint) (c2 :: * ->
 Constraint) (t :: *)
 instance (c1 t, c2 t) => (:&:) c1 c2 t

 data ColD c where
   ColD :: (c a) => a -> ColD c

 app :: (forall a. (c a) => a -> b) -> ColD c -> b
 app f (ColD x) = f x

 q :: ColD (Show :&: Real)
 q = ColD (1.2 :: Double)
 }}}

 In the interactive mode it's possible to confuse GHCi about IO type. It
 tries to show expression instread of executing it.

 {{{
 *Main> app print q

 <interactive>:3:1:
     No instance for (Show (IO ())) arising from a use of `print'
     Possible fix: add an instance declaration for (Show (IO ()))
     In a stmt of an interactive GHCi command: print it
 }}}

-- 
Ticket URL: http://hackage.haskell.org/trac/ghc/ticket/7594
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