#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 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- {{{#!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). -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11187 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler