#1776: type families : couldn't match type "Elem a" against type "Elem a"
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Reporter: guest | Owner:
Type: bug | Status: new
Priority: normal | Milestone:
Component: Compiler | Version: 6.8
Severity: normal | Keywords:
Difficulty: Unknown | Os: Unknown
Testcase: | Architecture: Unknown
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I received the error
{{{
GHCi, version 6.8.0.20071012: http://www.haskell.org/ghc/ :? for help
Loading package base ... linking ... done.
[1 of 1] Compiling Test ( Test.hs, interpreted )
Test.hs:17:27:
Couldn't match expected type `Elem a'
against inferred type `Elem a'
Expected type: Elem a -> Maybe (Elem a) -> Maybe (Elem a)
Inferred type: Elem a -> Maybe (Elem a) -> Maybe (Elem a)
In the first argument of `foldr', namely `mf'
In the second argument of `fromMaybe', namely
`(foldr mf Nothing xs)'
Failed, modules loaded: none.
}}}
when running the following module in GHCi:
{{{
{-# LANGUAGE TypeFamilies #-}
module Test where
import qualified Prelude as P
import Prelude hiding (foldr, foldr1)
import Data.Maybe
type family Elem a
class Foldable a where
foldr :: (Elem a -> b -> b) -> b -> a -> b
foldr1 :: (Elem a -> Elem a -> Elem a) -> a -> Elem a
foldr1 f xs = fromMaybe (error "foldr1: empty structure")
(foldr mf Nothing xs)
where mf x Nothing = Just x
mf x (Just y) = Just (f x y)
}}}
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/1776>
GHC <http://www.haskell.org/ghc/>
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