I found a strange behavior in my code Here a (very) simplified version of my code: {-# 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" The aspected result is "False" ("5" isn't a member of parArr), but the result I get is "True". I've compiled this code with the command: ghc --make -Odph -threaded Test.hs -rtsopts -o test and launched with: test +RTS -N2 My system is MacOS X 10.7.1 with Xcode 4.1 and GHC 7.0.4 (I've found the same behavior on Windows). Its obvious I'm missing something. Could someone help me? Thanks. Luca.