#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): This is indeed expected. `a` is completely ambiguous in your program. The only type it ever appears in is `C a`, which is to the left of `=>`, and such types are always resolved through instance lookup, so GHC has trouble figuring out which particular instance you meant. The only type to the right of `=>`, `Int`, does not determine `a` either, so GHC throws up its hands and gives up. One way to make this program compile is to introduce an additional argument which determines `a`, such as in: {{{#!hs {-# LANGUAGE AllowAmbiguousTypes #-} {-# LANGUAGE RankNTypes #-} {-# LANGUAGE ScopedTypeVariables #-} {-# LANGUAGE TypeApplications #-} module Foo where import Data.Proxy class C a where getInt :: Int instance C Char where getInt = 42 f :: (forall a. C a => Proxy a -> Int) -> Bool f x = even $ x $ Proxy @Char g :: (forall a. C a => Proxy a -> Int) -> Bool g = f }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15119#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler