I hate the fact that a type-system question forces us to lose a
#9291: Don't reconstruct sum types if the type subtly changes -------------------------------------+------------------------------------- Reporter: schyler | Owner: Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.2 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Runtime | Unknown/Multiple performance bug | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by nomeata): I was considering this problem a bit more today, and my best shot at this so far turned out to be quite similar to what [comment:6 rwbarton] already suggested above. Richard thinks this might be interesting to get right (with type safety proofs and all that). Replying to [comment:13 simonpj]: potential optimisation. Me too! Although any solution will likely incur some changes to Core, and that probably needs to be justified by more than the unhappy feeling I have about Core’s type system getting in the way of producing the obviously right code. Does anyone have a practical example where this bug bytes a lot? Maybe something with lenses, or with generic programs? It seems to affect, for example, every `Functor` instance for a sum type where the type parameter is not used in at least one of the branches. Anyways, I started writing down a possible approach with more detail at CaseBinderGeneralisation. Comments welcome. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9291#comment:27 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler