Wrong result for element check

On the beginners list, Luca Ciciriello reported that

{-# LANGUAGE PArr #-}
{-# OPTIONS -fdph-seq #-}

module Main where

import Control.Parallel
import GHC.PArr

parArr :: [:String:]
parArr = [: "1", "2", "3", "4" :]

isElement :: String -> Bool
isElement x = x `elemP` parArr

main :: IO ()
main = do
          putStrLn $ (show . isElement) "5"

prints True. Obviously it should print False. I've replicated the behaviour with 7.0.4 and 6.10.4 on 64-bit linux (Luca tested on Mac and Windows).

As far as I can tell, elemP no longer exists in dph, so I had to modify to test with HEAD, and

{-# LANGUAGE ParallelArrays #-}
{-# OPTIONS -fdph-seq #-}

module Main where

import GHC.PArr
import Data.Array.Parallel

parArr :: [:String:]
parArr = [: "1", "2", "3", "4" :]

isElement :: String -> Bool
isElement x = x `elemP1` parArr
  where
    elemP1 :: Eq a => a -> [:a:] -> Bool
    elemP1 v ar = lengthP (filterP (== v) ar) /= 0

main :: IO ()
main = do
          putStrLn $ (show . isElement) "5"

also prints True with HEAD, but prints False with 6.10.4 and 7.0.4 (with the necessary modifications, !ParallelArrays -> PArr, don't import Data.Array.Parallel).

Trac metadata

Trac field	Value
Version	7.0.4
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Data Parallel Haskell
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