QuantifiedConstraints: Odd superclass constraint
This code works
{-# Language QuantifiedConstraints, RankNTypes, PolyKinds, ConstraintKinds, UndecidableInstances #-}
import Data.Semigroup
newtype Free cls a = Free ()
instance (forall xx. cls xx => Semigroup xx) => Semigroup (Free cls a) where
(<>) = undefined
stimes = undefined
sconcat = undefined
-- instance (forall xx. cls xx => Semigroup xx) => Monoid (Free cls a) where
-- mempty = undefined
but uncomment the Monoid
instance and we get
$ ghci -ignore-dot-ghci /tmp/I.hs
GHCi, version 8.5.20180128: http://www.haskell.org/ghc/ :? for help
[1 of 1] Compiling Main ( /tmp/I.hs, interpreted )
/tmp/I.hs:12:10: error:
• Could not deduce: cls (Free cls a)
arising from the superclasses of an instance declaration
from the context: forall xx. cls xx => Semigroup xx
bound by the instance declaration at /tmp/I.hs:12:10-67
• In the instance declaration for ‘Monoid (Free cls a)’
|
12 | instance (forall xx. cls xx => Semigroup xx) => Monoid (Free cls a) where
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Failed, no modules loaded.
Prelude>
Is this correct behavior?
Trac metadata
Trac field | Value |
---|---|
Version | 8.5 |
Type | Bug |
TypeOfFailure | OtherFailure |
Priority | normal |
Resolution | Unresolved |
Component | Compiler |
Test case | |
Differential revisions | |
BlockedBy | |
Related | |
Blocking | |
CC | |
Operating system | |
Architecture |