#15649: Errors about ambiguous untouchable variable when using constraint variable in RankN type -------------------------------------+------------------------------------- Reporter: infinity0 | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: 8.4.3 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- The following code compiles and works: {{{#!haskell {-# LANGUAGE ConstraintKinds , FlexibleInstances , MultiParamTypeClasses , GADTs , RankNTypes , ScopedTypeVariables , UndecidableSuperClasses #-} import GHC.Types (Constraint) main :: IO () main = do return () class (r a) => DynPS r a where data PSAny r = forall a. DynPS r a => PSAny a class Unconstrained a instance Unconstrained a instance DynPS Unconstrained () newtype DynLoad' r = DynLoad' { unDynLoad' :: forall ref. r ref => ref -> PSAny r } loadAll :: forall a r . (DynPS r a) => DynLoad' r -> a -> Maybe a loadAll loader r = undefined testCallable :: IO (Maybe ()) testCallable = return $ loadAll (undefined :: DynLoad' Unconstrained) () }}} However it's ugly having to expose `DynLoad'` in the API. Ideally we'd like to have: {{{#!haskell loadAll2 :: forall a r . (DynPS r a) => (forall ref. r ref => ref -> PSAny r) -> a -> Maybe a loadAll2 loader r = loadAll (DynLoad' loader :: DynLoad' r) r }}} But this fails with: {{{ Test3.hs:37:6: error: • Couldn't match type ‘r0’ with ‘r’ ‘r0’ is untouchable inside the constraints: r0 ref bound by the type signature for: loadAll2 :: forall ref. r0 ref => ref -> PSAny r0 at Test3.hs:(37,6)-(39,17) ‘r’ is a rigid type variable bound by the type signature for: loadAll2 :: forall a (r :: * -> Constraint). DynPS r a => (forall ref. r ref => ref -> PSAny r) -> a -> Maybe a at Test3.hs:(37,6)-(39,17) Expected type: ref -> PSAny r0 Actual type: ref -> PSAny r • In the ambiguity check for ‘loadAll2’ To defer the ambiguity check to use sites, enable AllowAmbiguousTypes In the type signature: loadAll2 :: forall a r. (DynPS r a) => (forall ref. r ref => ref -> PSAny r) -> a -> Maybe a | 37 | :: forall a r . (DynPS r a) | ^^^^^^^^^^^^^^^^^^^^^^^^... }}} It compiles if we enable `AllowAmbiguousTypes`, but are then forced to use `TypeApplications` as well to actually call it. :( {{{#!haskell -- as above, but add: {-# LANGUAGE AllowAmbiguousTypes, TypeApplications #-} -- and then: testCallable2 :: IO (Maybe ()) --testCallable2 = return $ loadAll2 (undefined :: forall ref. Unconstrained ref => ref -> PSAny Unconstrained) () -- ^ doesn't work either testCallable2 = return $ loadAll2 @() @Unconstrained undefined () }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15649 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler