id,summary,reporter,owner,description,type,status,priority,milestone,component,version,resolution,keywords,cc,os,architecture,failure,testcase,blockedby,blocking,related,differential,wikipage
14877,"QuantifiedConstraints: Can't deduce `xx' from `(xx => a, xx)'",Iceland_jack,,"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>
}}}",bug,new,normal,,Compiler,8.5,,QuantifiedConstraints,,Unknown/Multiple,Unknown/Multiple,None/Unknown,,,,,,