GHC could not deduce constraint from itself
Summary
following code:
{-# LANGUAGE GADTs, DataKinds, PolyKinds, ConstraintKinds, TypeApplications, RankNTypes, TypeFamilies, TypeOperators, MultiParamTypeClasses, UndecidableSuperClasses, FlexibleInstances, PatternSynonyms, FlexibleContexts, AllowAmbiguousTypes, ScopedTypeVariables #-}
module Lib where
import GHC.Exts
import GHC.TypeLits
import Data.Proxy
data a ::: b = a ::: b
class Pair f where
type Left f :: a
type Right f :: b
instance Pair (a '::: b) where
type Left (a '::: b) = a
type Right (a '::: b) = b
class NC (x :: k)
instance NC (x :: k)
data L1 c f xs where
LC :: c x => f x -> L1 c f xs -> L1 c f (x ': xs)
LN :: L1 c f '[]
class (c1 (Left f), c2 (Right f)) => PairC c1 c2 f
instance (c1 x, c2 y) => PairC c1 c2 (x '::: y)
data f1 :*: f2 :: (kx ::: ky) -> * where
(:*:) :: f1 x -> f2 y -> (f1 :*: f2) (x '::: y)
pairC :: forall c1 c2 x y e . (c1 x, c2 y) => (PairC c1 c2 (x '::: y) => e) -> e
pairC e = e
type Foo t = L1 (PairC KnownSymbol NC) (Proxy :*: t)
pattern Foo :: forall s a t xs . KnownSymbol s => KnownSymbol s => Proxy s -> t a -> Foo t xs -> Foo t (s '::: a ': xs)
pattern Foo p t xs <- LC (p :*: t) xs where
Foo p t xs = pairC @KnownSymbol @NC @s @a (LC (p :*: t) xs)Results in this compile error:
/home/ryba/ghc-bug/src/Lib.hs:38:46: error:
• Could not deduce (PairC KnownSymbol NC (s '::: a))
arising from a use of ‘LC’
from the context: KnownSymbol s
bound by the signature for pattern synonym ‘Foo’
at src/Lib.hs:(37,1)-(38,61)
or from: PairC KnownSymbol NC (s '::: a)
bound by a type expected by the context:
PairC KnownSymbol NC (s '::: a) =>
L1 (PairC KnownSymbol NC) (Proxy :*: t) ((s '::: a) : xs)
at src/Lib.hs:38:45-61
• In the fifth argument of ‘pairC’, namely ‘(LC (p :*: t) xs)’
In the expression: pairC @KnownSymbol @NC @s @a (LC (p :*: t) xs)
In an equation for ‘Foo’:
Foo p t xs = pairC @KnownSymbol @NC @s @a (LC (p :*: t) xs)
|
38 | Foo p t xs = pairC @KnownSymbol @NC @s @a (LC (p :*: t) xs)
| ^^^^^^^^^^^^^^^Which basically boils down to "could not deduce a from a"
Environment
- GHC version used: 8.10.2
- stack resolver=nightly-2020-09-14
Optional:
- Operating System: Ubuntu
- System Architecture: x86-64