#12187: Clarify the scoping of existentials for pattern synonym signatures -------------------------------------+------------------------------------- Reporter: mpickering | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Keywords: | Operating System: Unknown/Multiple PatternSynonyms | Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: #12108 Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- What are scoping rules for type variables in pattern synonym signatures? We said that existentials variables are scoped as follows: 1. Existentials scope over only the provided constraints and the arguments, not over the result type. {{{ type Disguised a b = b -> RP a pattern Q :: () => Eq b => Disguised a b }}} However, Simon argues that `Q` should be accepted as if we expand the type synonym then the existentially quantified `b` is no longer in the result type. Richard disagrees, he considers the result type to be `Disguised a b` and so `b` is in the result type and not in-scope. Therefore he wishes to reject this example. The purpose of this ticket is to decide the fate of `Q`. This kind of problem is related to #12108 -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12187 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler