#15592: Type families without CUSKs cannot be given visible kind variable binders -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: 8.4.3 Resolution: | Keywords: | TypeApplications, TypeFamilies, | CUSKs 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):
Bottom line: any use of visible kind application should be considered to be an instance of polymorphic recursion.
Yes, and we (presumably) have exactly same at the term level {{{ reverse (xs :: [a]) = ...(reverse @a ys).... }}} This is illegal. When type checking `reverse`'s RHS we have `reverse :: alpha` in the envt, and you can't type-apply that to anything. If we had a signature {{{ reverse :: [b] -> [b] reverse (xs :: [a]) = ...(reverse @a ys).... }}} that's fine, because the signature puts `reverse :: forall b. [b] -> [b]` into the envt. This might be a useful example to add to the user-manual documetation for visible type application. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15592#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler