#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

#11187: Odd interaction between type families and coercions (regression) -------------------------------------+------------------------------------- Reporter: dfeuer | Owner: Type: bug | Status: closed Priority: high | Milestone: Component: Compiler | Version: 7.10.2 Resolution: fixed | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: indexed- | types/should_compile/T11187 Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by Simon Peyton Jones <simonpj@…>): In [changeset:"f4f00c0f28f3c21eb6f1396f48058c430c4e9b30/ghc" f4f00c0/ghc]: {{{ #!CommitTicketReference repository="ghc" revision="f4f00c0f28f3c21eb6f1396f48058c430c4e9b30" Test Trac #11187 }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11187#comment:4> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler