#14288: ScopedTypeVariables with nested foralls broken since 8.0.2 -------------------------------------+------------------------------------- Reporter: MikolajKonarski | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 8.2.1 checker) | Resolution: | Keywords: Operating System: Linux | Architecture: x86_64 Type of failure: GHC rejects | (amd64) valid program | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by RyanGlScott): As far as specifying how GHC should bring variables from nested `forall`s into scope, I don't think it's nearly as difficult as you're making it out to be. The algorithm would be as follows: when `ScopedTypeVariables` is enabled and an identifier with an explicit type signature `T` is being typechecked, then the set of variables `foralls(T)` is brought into scope, where: * `foralls(forall a_1 ... a_k. T) = {a_1, ..., a_k} ∪ foralls (T)` * `foralls(C => T) = foralls(T)` * `foralls(b -> c) = foralls(c)` Note that this doesn't expand type synonyms, so we needn't worry about your `Foo` example in comment:5 bringing variables into scope. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14288#comment:7 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler