Ticket #4008: Pain.hs

{-# LANGUAGE RankNTypes, EmptyDataDecls, LiberalTypeSynonyms #-}

module Pain
where

data O
data C

type Counter n e x = n e x -> Int
type ExTriple a = (a C O, a O O, a O C) -- ^ entry/exit triple

scalar :: forall a . (forall e x . a e x) -> ExTriple a
scalar a = (a, a, a)

-- Claim: the type of 'wrap' is an instance of the type of 'scalar'
-- obtained by substituting 'counter n' for 'a'.
wrap :: (forall e x . Counter n e x) -> ExTriple (Counter n)
wrap = scalar