#12018: Equality constraint not available in pattern type signature (GADTs/ScopedTypeVariables) -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: (none) Type: bug | Status: new Priority: lowest | Milestone: Component: Compiler (Type | Version: 7.10.3 checker) | Keywords: GADTs, Resolution: | ScopedTypeVariables Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj):
it's been drilled into my skull that the act of pattern matching on a GADT constructor in a case changes the typing rules
Correct -- but it only brings those constraints into scope in ''parts'' of the program. For example {{{ data T a where TBool :: TBool TOther :: T a f :: a -> T a -> Int f True TBool = 3 -- Rejected g :: T a -> a -> Int g TBool True = 3 -- Accepted }}} `f` is rejected because pattern matching goes left-to-right and outside- in. So when we meet the `True` we are not in the scope of `a~Bool`. But `g` is fine. In the case of a type signature, when matching `(p :: ty)` we first meet `ty` and only then match `p`. You want to match `p` and only then match `ty` with the refined type of the pattern. But currently `ty` signature may restrict the type of the pattern; I'm not sure that would be so easy if the signature was only matched after matching the pattern. In short, I don't see an easy, compositional way to give you what you want. Maybe someone else does. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12018#comment:8 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler