#15119: Program involving existentials and type class constraint gets rejected -------------------------------------+------------------------------------- Reporter: sgraf | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.1 Component: Compiler (Type | Version: 8.2.2 checker) | Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: GHC rejects | Unknown/Multiple valid program | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by RyanGlScott): Right, you're going to need //something// which determines `a`, be it `Proxy`, `Tagged`, or something else. There may come a day in which you can write: {{{#!hs forall a -> C a => Int }}} That is, where one can make `a` a visible argument (i.e., a "big lambda", as you've put it). But until then, you'll have to resort to baser tricks to accomplish what you seek. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15119#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler