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{-# LANGUAGE DerivingVia, StandaloneDeriving, PolyKinds, RankNTypes #-} | |
module DerivingViaPolyKinds where | |
class Foo (f :: k -> *) | |
newtype Bar (f :: k -> *) (a :: k) = Bar (f a) | |
-- deriving Foo via f -- Works | |
-- Standalone deriving instances only work if you instantiate the polykinded variable: | |
-- deriving via (f :: * -> *) instance Foo f => Foo (Bar f) | |
-- deriving via (f :: (* -> *) -> *) instance Foo f => Foo (Bar f) | |
-- None of these work: | |
{- | |
deriving via f instance Foo f => Foo (Bar f) | |
deriving via (f :: k -> *) instance Foo f => Foo (Bar f) | |
deriving via (f :: k -> *) instance Foo (f :: k -> *) => Foo (Bar f) | |
deriving via (f :: k -> *) instance Foo f => Foo (Bar (f :: k -> *)) | |
deriving via (f :: k -> *) instance Foo (f :: k -> *) => Foo (Bar f :: k -> *) | |
deriving via f instance Foo (f :: k -> *) => Foo (Bar f) | |
deriving via f instance Foo (f :: k -> *) => Foo (Bar f :: k -> *) | |
forall-ing won't help. | |
forall k. on the LHS interprets it as an RankNType | |
forall k. on the RHS interprets it a universally quantified variable free from the LHS. | |
deriving via (forall k. f :: k -> *) instance Foo (f :: k -> *) => Foo (Bar f :: k -> *) | |
deriving via (forall k. f :: k -> *) instance Foo f => Foo (Bar f) | |
deriving via (forall f k. f :: k -> *) instance Foo (f :: k -> *) => Foo (Bar f :: k -> *) | |
deriving via (forall f k. f :: k -> *) instance Foo f => Foo (Bar f) | |
-} |
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