#11416: GHC mistakenly believes datatype with type synonym in its type can't be eta-reduced -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: Type: bug | Status: closed Priority: normal | Milestone: 8.0.1 Component: Compiler (Type | Version: 8.0.1-rc1 checker) | Resolution: fixed | Keywords: TypeInType Operating System: Unknown/Multiple | Architecture: Type of failure: GHC rejects | Unknown/Multiple valid program | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Phab:D1772 Wiki Page: | -------------------------------------+------------------------------------- Comment (by RyanGlScott): Hm, there's another tricky case that I hadn't previously considered: {{{#!hs type ConstT a b = a data family Fam a b data instance Fam (ConstT a b) b = Fam b deriving Functor }}} Currently, the eta-reduction check rejects this. Should it? On on hand, the `b` in `ConstT a b` will go away if you expand the `ConstT` type synonym, so one could argue this is no different from `data instance Fam a b = Fam b deriving Functor` (which is accepted). On the other hand, if you consider what the derived instance would look like: {{{#!hs instance Functor (Fam (ConstT a b)) where ... }}} This feels a bit off, because now the instance head is mentioning an eta- reduced type variable! But it does typecheck if you type it in manually... -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11416#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler