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{-# LANGUAGE KindSignatures, ConstraintKinds, MultiParamTypeClasses, FlexibleInstances, UndecidableInstances #-}
module Ctx where
import GHC.Prim( Constraint )
data Proxy (ctxt :: * -> Constraint) = Proxy
-- At one time, this one worked:
nonmeth :: ctxt Int => Proxy ctxt -> a
nonmeth prox = nonmeth prox
class Foo (ctxt :: * -> Constraint) a where
meth :: ctxt a => Proxy ctxt -> a
instance ctxt Int => Foo ctxt Int where
-- But this one didn't:
meth prox = meth prox
-- The error was:
-- Could not deduce (ctxt Int) arising from a use of `meth'
-- from the context (ctxt Int)
-- The problem was that irreducible evidence did not interact with
-- evidence of equal type.
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