[GHC] #9897: Couldn't match type `Id (Id Char)' with `Char'

17 Dec 2014


      #9897: Couldn't match type `Id (Id Char)' with `Char'
-------------------------------------+-------------------------------------
       Reporter:  erisco             |                   Owner:
           Type:  bug                |                  Status:  new
       Priority:  normal             |               Milestone:
      Component:  Compiler (Type     |                 Version:  7.8.3
  checker)                           |        Operating System:  Windows
       Keywords:                     |         Type of failure:  GHC
   Architecture:  x86_64 (amd64)     |  rejects valid program
     Difficulty:  Unknown            |               Test Case:
     Blocked By:                     |                Blocking:
Related Tickets:                     |  Differential Revisions:
-------------------------------------+-------------------------------------
 'foo' is expected to type check given that 'bar' and 'qux' type check.

 {{{
 ghc_bug.hs:21:7:
     Couldn't match type `Id (Id Char)' with `Char'
     Expected type: (:.:) Id Id Char
       Actual type: Char
     In the expression: 'x'
     In an equation for `foo': foo = 'x'
 }}}

 {{{#!hs
 {-# LANGUAGE TypeOperators, TypeFamilies #-}
 module Main where

 type family (:.:) g f a where
  (:.:) g f a = g (f a)
 --

 type family Id x where
   Id x = x
 --

 -- Type checks
 bar :: Id (Id Char)
 bar = 'x'

 -- Type checks
 qux = 'x' :: (Id :.: Id) Char

 -- Couldn't match type `Id (Id Char)' with `Char'
 foo :: (Id :.: Id) Char
 foo = 'x'
 }}}

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9897
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