Re: [GHC] #14203: GHC-inferred type signature doesn't actually typecheck

2 Nov 2017

      #14203: GHC-inferred type signature doesn't actually typecheck
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.2.1
  checker)                           |             Keywords:  TypeInType,
      Resolution:                    |  TypeFamilies
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by RyanGlScott):

 Actually, it's not just limited to datatypes: the same effect can be
 observed in this program:

 {{{#!hs
 {-# LANGUAGE AllowAmbiguousTypes #-}
 {-# LANGUAGE ScopedTypeVariables #-}
 {-# LANGUAGE TypeFamilies #-}
 {-# LANGUAGE TypeInType #-}
 module Bug where

 type family F a
 data Foo (z :: F a)

 bar :: forall a (z :: F a). Foo z -> Int
 bar _ = 42
 }}}
 {{{
 $ /opt/ghc/8.2.1/bin/ghci Bug.hs
 GHCi, version 8.2.1: http://www.haskell.org/ghc/  :? for help
 Loaded GHCi configuration from /home/ryanglscott/.ghci
 [1 of 1] Compiling Bug              ( Bug.hs, interpreted )

 Bug.hs:10:33: error:
     • Expected kind ‘F a0’, but ‘z’ has kind ‘F a’
     • In the first argument of ‘Foo’, namely ‘z’
       In the type signature: bar :: forall a (z :: F a). Foo z -> Int
    |
 10 | bar :: forall a (z :: F a). Foo z -> Int
    |                                 ^
 }}}

 Here, I would expect `AllowAmbiguousTypes` to squash the error message
 about the ambiguous variable `a0` in `F a0`, but that doesn't happen.

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14203#comment:6
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