#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 simonpj): Maybe. If you type it in manually you'll get a spuriously polymorphic instance, which will result in some uses of `Any`. I think the same will happen if you allow this data instance. Perhaps it's no more than using `exactTyCoVarsofType` in `eta_reduce` in `tcDataFamInstDecl`. You'd need to make sure you quantify over the tyvar anyway; but I think that will happen. I don't feel strongly. It would be good to document it. Crumbs if I had a pound for every hour I've spent on `GeneralisedNewtypeDeriving` I'd be a rich man. Thanks for digging into this -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11416#comment:6 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler