QuantifiedConstraints: Can't deduce `xx' from `(xx => a, xx)'
This works with the latest change to the -XQuantifiedConstraints branch.
{-# 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.
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>
Trac metadata
Trac field | Value |
---|---|
Version | 8.2.2 |
Type | Bug |
TypeOfFailure | OtherFailure |
Priority | normal |
Resolution | Unresolved |
Component | Compiler |
Test case | |
Differential revisions | |
BlockedBy | |
Related | |
Blocking | |
CC | |
Operating system | |
Architecture |