Hi again! I cut the example a bit further down. Read the comments at the end of the file. ---------------------------------------------------------------------------------------------------------------

import Array

data CM a = ZM | M (Array (Int,Int) a) deriving (Show,Eq)

data RD = NB !(Double,Int)

main = do let arr = foo (NB (1.0,1)) putStr ("arr==arrS "++show (arr==arrS)++"\n") putStr ("arrS==arr "++show (arrS==arr)++"\n") putStr ("bnds arr arrS "++show ((bounds arr)==(bounds arrS))++"\n") putStr ("bnds +id arr arrS "++show (((bounds.id) arr)==((bounds) arrS))++"\n") putStr ("id +bnds arr arrS "++show (((id.bounds) arr)==((bounds) arrS))++"\n")

foo :: RD -> Array (Int,Int) (CM Double) foo rd = case rd of NB (f,p) -> h where h=hCMt [(p,listArray ((1,1),(1,1)) [f])] where --h0CMt :: Array (Int, Int) (CM Double) h0CMt = accumArray (\a _-> a) ZM ((1,1),(1,2)) [] hCMt po_arL = h0CMt // (map (\(po,ar) -> ((1,po),M ar)) po_arL)

arrS :: Array (Int,Int) (CM Double) arrS = listArray ((1,1),(1,2)) [M (listArray ((1,1),(1,1)) [1.0]),ZM] -- This is the array that foo (NB (1.0,1)) shows in Hugs. -- But (foo (NB (1.0,1)))==arrS is False. -- All in main is only to show the strange behaviour. -- If I write NB (f,p) -> hCMt [(p,listArray ((1,1),(1,1)) [f])] instead of line 16 -- the bug disappears. That is also the reason why I have to keep the data declaration RD. -- If I put the type signature of line 18 in scope the bug also disappears. -- If I write -- hCMt po_arL = (accumArray (\a _-> a) ZM ((1,1),(1,2)) []) // (map (\(po,ar) -> ((1,po),M ar)) po_arL) -- instead of line 19 and 20 it also vanishes. -- So I thing now it is the shortest example that I can find. (Apart from deleting main.)