I am almost sure this is a known issue, but I noticed some erroneous (?)
interaction between datatype promotion and existential quantification. Consider
the following program:
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE ExistentialQuantification #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE KindSignatures #-}
module Test where
data K = forall a. T a -- promotion gives 'T :: * -> K
data G :: K -> * where
D :: G (T []) -- kind error!
I would expect the type checker to reject it, but GHC (version 7.6.1) compiles
it happily. Is this indeed a (known) bug?
On a related note: is there a way to promote a type that involves an
existential type variable of a kind other than *?
Thanks,
Stefan
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