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{-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies #-}
{-# LANGUAGE ScopedTypeVariables, FlexibleContexts #-}
-- Trac #1783
-- Like Trac #1781 you could argue that this one should succeed
-- but we stick with the old behaviour for now. When we do
-- fundeps properly it'll probably start to work
module ShouldCompile where
import Prelude hiding (foldr, foldr1)
import Data.Maybe
class Elem a e | a -> e
class Foldable a where
foldr :: Elem a e => (e -> b -> b) -> b -> a -> b
-- foldr1 :: forall e. Elem a e => (e -> e -> e) -> a -> e -- WORKS!
foldr1 :: Elem a e => (e -> e -> e) -> a -> e
foldr1 f xs = fromMaybe (error "foldr1: empty structure")
(foldr mf Nothing xs)
where mf :: Elem a e => (e -> Maybe e -> Maybe e)
mf x Nothing = Just x
mf x (Just y) = Just (f x y)
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