#15561: TypeInType: Type error conditioned on ordering of GADT and type family definitions -------------------------------------+------------------------------------- Reporter: Bj0rn | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: 8.4.3 Keywords: TypeInType, | Operating System: Unknown/Multiple TypeFamilies, GADTs | Architecture: | Type of failure: GHC rejects Unknown/Multiple | valid program Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Consider this code which successfully compiles: {{{#!hs {-# LANGUAGE TypeInType, TypeFamilies, GADTs #-} module Bug where class HasIndex a where type Index a emptyIndex :: IndexWrapper a instance HasIndex [a] where type Index [a] = Int emptyIndex = Wrap 0 data IndexWrapper a where Wrap :: Index a -> IndexWrapper a type family UnwrapAnyWrapperLikeThing (a :: t) :: k type instance UnwrapAnyWrapperLikeThing ('Wrap a :: IndexWrapper [b]) = a }}} The mere act of moving the definition of `IndexWrapper` anywhere below the definition of `UnwrapAnyWrapperLikeThing` makes the type family instance at the bottom of the example fail compilation, with this error: {{{ Bug.hs:17:15: error: • Illegal type synonym family application in instance: Index [b] • In the type instance declaration for ‘UnwrapAnyWrapperLikeThing’ | 17 | type instance UnwrapAnyWrapperLikeThing ('Wrap a :: IndexWrapper [b]) = a | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ }}} This is the smallest example that I could come up with; my real scenario of course has more things going on, but I can share if it would help. The problem for me (other than that I'm pretty sure reordering definitions in Haskell should never affect anything) is that I would like just the definition of the type family (`UnwrapAnyWrapperLikeThing` in this example) in module `A` and all of the other definitions in module `B` that imports `A`. Ideally, I would have liked to add a `HasIndex a` constraint to the `Wrap` constructor, but that disqualifies use of `'Wrap` on the type level. This does make me feel like I'm on shaky ground to begin with. I have reproduced this bug on 8.2.2, 8.4.3 and 8.6.0.20180810 (NixOS). I should note that 8.0.2 rejects even the code that I pasted here. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15561 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler