QuantifiedConstraints: Can't deduce "(a, b)" from "a" and "b"
{-# Language QuantifiedConstraints, RankNTypes, PolyKinds, ConstraintKinds, UndecidableInstances, GADTs, LiberalTypeSynonyms, TypeOperators, MultiParamTypeClasses, FlexibleInstances #-}
data Dict c where
Dict :: c => Dict c
infixr |-
class (a => b) => (a |- b)
instance (a => b) => (a |- b)
type a :- b = Dict (a |- b)
class ((a, b) => c) => Curried a b c
instance ((a, b) => c) => Curried a b c
curryC :: forall a b c. ((a, b) |- c) :- (a |- b |- c)
curryC = Dict
This fails with
GHCi, version 8.5.20180128: http://www.haskell.org/ghc/ :? for help
[1 of 1] Compiling Main ( /tmp/Test.hs, interpreted )
/tmp/Test.hs:16:10: error:
• Could not deduce (a, b) arising from a use of ‘Dict’
from the context: (a, b) |- c
bound by a quantified context at /tmp/Test.hs:1:1
or from: a bound by a quantified context at /tmp/Test.hs:1:1
or from: b bound by a quantified context at /tmp/Test.hs:1:1
Possible fix:
add (a, b) to the context of
the type signature for:
curryC :: forall (a :: Constraint) (b :: Constraint) (c :: Constraint).
((a, b) |- c) :- (a |- (b |- c))
• In the expression: Dict
In an equation for ‘curryC’: curryC = Dict
|
16 | curryC = Dict
| ^^^^
Failed, no modules loaded.
but it works by changing the type of curryC
:
curryC :: forall a b c. Curried a b c :- (a |- b |- c)
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 |