#14877: QuantifiedConstraints: Can't deduce `xx' from `(xx => a, xx)' -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.2 Keywords: | Operating System: Unknown/Multiple QuantifiedConstraints, wipT2893 | Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- This works with the [https://ghc.haskell.org/trac/ghc/ticket/2893#comment:34 latest change to the -XQuantifiedConstraints branch]. {{{#!hs {-# Language QuantifiedConstraints, FlexibleInstances, MultiParamTypeClasses, AllowAmbiguousTypes, RankNTypes, PolyKinds, ConstraintKinds, UndecidableInstances #-} class (forall xx. (xx => a) => Implies xx b) => F a b instance (forall xx. (xx => a) => Implies xx b) => F a b class (a => b) => Implies a b instance (a => b) => Implies a b }}} but replacing `Implies xx b` with `(xx => b)` causes it to fail. I don't know if the cause of this overlaps with an existing ticket. {{{#!hs class (forall xx. (xx => a) => (xx => b)) => F a b instance (forall xx. (xx => a) => (xx => b)) => F a b -- $ ghci -ignore-dot-ghci /tmp/ASD.hs -- GHCi, version 8.5.20180128: http://www.haskell.org/ghc/ :? for help -- [1 of 1] Compiling Main ( /tmp/ASD.hs, interpreted ) -- -- /tmp/ASD.hs:4:10: error: -- • Could not deduce: xx0 -- arising from the superclasses of an instance declaration -- from the context: forall (xx :: Constraint). (xx => a, xx) => b -- bound by the instance declaration at /tmp/ASD.hs:4:10-53 -- or from: (xx => a, xx) -- bound by a quantified context at /tmp/ASD.hs:1:1 -- • In the instance declaration for ‘F a b’ -- | -- 4 | instance (forall xx. (xx => a) => (xx => b)) => F a b -- | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ -- Failed, no modules loaded. -- Prelude> }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14877 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler