Re: [GHC] #11187: Odd interaction between type families and coercions (regression)

9 Dec 2015

      #11187: Odd interaction between type families and coercions (regression)
-------------------------------------+-------------------------------------
        Reporter:  dfeuer            |                Owner:
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:
       Component:  Compiler          |              Version:  7.10.2
      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:                    |
-------------------------------------+-------------------------------------
Description changed by dfeuer:

Old description:
...
{{{#!hs
{-# LANGUAGE TypeFamilies #-}
module Wonk where
import Data.Type.Coercion
type family X
coercionXX :: Coercion X X
coercionXX = Coercion
}}}
This works just fine in 7.8.3, but in 7.10.2 it gives the following
peculiar error:
{{{
    Couldn't match representation of type ‘X’ with that of ‘X’
    NB: ‘X’ is a type function, and may not be injective
    In the expression: Coercion
    In an equation for ‘coercionXX’: coercionXX = Coercion
}}}
I don't even know what it means for a ''nullary'' type function to be
injective.
I tried the following workarounds:
{{{#!hs
coercionXX1 :: Coercion X X
coercionXX1 = c where
  c :: x ~ X => Coercion x x
  c = Coercion
coercionXX2 :: Coercion X X
coercionXX2 = c where c = Coercion
}}}
Interestingly, `coercionXX1` seems to work under all circumstances,
with or without the top-level type signature; `coercionXX2` works
if it's in a `.hs` file, but does ''not'' work if entered by hand
at the `ghci` prompt (it produces a similar error).
New description:

 {{{#!hs
 {-# LANGUAGE TypeFamilies #-}

 module Wonk where
 import Data.Type.Coercion

 type family X

 coercionXX :: Coercion X X
 coercionXX = Coercion
 }}}

 This works just fine in 7.8.3, but in 7.10.2 it gives the following
 peculiar error:

 {{{
     Couldn't match representation of type ‘X’ with that of ‘X’
     NB: ‘X’ is a type function, and may not be injective
     In the expression: Coercion
     In an equation for ‘coercionXX’: coercionXX = Coercion
 }}}

 I don't even know what it means for a ''nullary'' type family to be
 injective.
 I tried the following workarounds:

 {{{#!hs
 coercionXX1 :: Coercion X X
 coercionXX1 = c where
   c :: x ~ X => Coercion x x
   c = Coercion

 coercionXX2 :: Coercion X X
 coercionXX2 = c where c = Coercion
 }}}

 Interestingly, `coercionXX1` seems to work under all circumstances,
 with or without the top-level type signature; `coercionXX2` works
 if it's in a `.hs` file, but does ''not'' work if entered by hand
 at the `ghci` prompt (it produces a similar error).

--

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11187#comment:2
GHC http://www.haskell.org/ghc/
The Glasgow Haskell Compiler