#12025: Order of constraints forced (in pattern synonyms, type classes in comments) -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 8.1 checker) | Keywords: Resolution: | TypeApplications PatternSynonyms Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: 11513, 10928 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by goldfire): This ordering problem is very apparent in GADT definitions. If you say {{{#!hs data X b where MkX :: forall a b. a -> b -> X b }}} you get `MkX :: forall b a. a -> b -> X b`. See the last bullet [https://downloads.haskell.org/~ghc/8.0.1-rc4/docs/html/users_guide/glasgow_e... #visible-type-application in the manual]. It would be straightforward, syntactically, to fix this for GADTs. The implementation is a bit more involved, because GHC assumes that universals come before existentials. This is surmountable with some engineering. We should consider this problem in concert with allowing visible type patterns (#11350), because we want patterns to be able to match existentials but not universals. It's very unclear how to do this in a way that will be sane to users. I don't have a good idea for a new syntax for dealing with this in pattern synonyms. As for classes, I'm not as bothered. The workaround is straightforward. Furthermore, we don't currently give full type signatures for class methods, always leaving off the class constraint. (I don't see how #11620 relates to this.) If we are going to be bothered about classes, we should also be bothered about record seleectors. But I vote not to be bothered about anything except data constructors and pattern synonyms. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12025#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler