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{-# LANGUAGE ExistentialQuantification, FlexibleContexts,
MultiParamTypeClasses #-}
-- !!! Existential data tyes
-- Hugs didn't like this one
module Main (main) where
class MyClass a b where
foo :: a -> b -> Int
data Special = forall b. (MyClass Int b)=> MkSpecial b
data General a = forall b. (MyClass a b)=> MkGeneral b
instance MyClass Int Bool where
foo x False = -x
foo x True = x
xs :: [General Int]
xs = [MkGeneral True, MkGeneral False]
main = print [foo (3::Int) x | MkGeneral x <- xs]
-- Without the (::Int) part we get an
-- incomprehensible error message :-(
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