I've seen 'error "foo"' or simply 'undefined' used as a placeholder for elements in a structure which we, as programmers, know will be filled in soon. Often this makes the code clearer because the creation of the data structure and filling in its values is separated (you could argue with this, but that's not the point). This troubles me, though, because even though *we* know the elements will be filled in, maybe the strictness analyser doesn't. So, to test this, I wrote the following program: module Main where import Array sumArray :: Array Int Integer -> Integer sumArray arr = sumArray' 0 low where (low,high) = bounds arr sumArray' acc pos | pos > high = acc | otherwise = sumArray' (acc + (arr!pos)) (pos+1) mkArray1 :: Array Int Integer mkArray1 = fillInArr 1 (listArray (1,9999) (repeat undefined)) where fillInArr 5000 arr = arr // [(5000,arr!4999 + arr!5001)] fillInArr 1 arr = fillInArr 2 (arr // [(1,1),(9999,2)]) fillInArr n arr = fillInArr (n+1) (arr // [(n,(arr!(n-1))+(arr!(10001-n))),(10000-n,(arr!(n-1))+(arr!(10001-n))+1)]) mkArray2 :: Array Int Integer mkArray2 = fillInArr 1 (listArray (1,9999) (repeat 0)) where fillInArr 5000 arr = arr // [(5000,arr!4999 + arr!5001)] fillInArr 1 arr = fillInArr 2 (arr // [(1,1),(9999,2)]) fillInArr n arr = fillInArr (n+1) (arr // [(n,(arr!(n-1))+(arr!(10001-n))),(10000-n,(arr!(n-1))+(arr!(10001-n))+1)]) main = print $ (sumArray mkArray1) `mod` 1024 And compiled it using both mkArray1 (which has undefined elements, supposedly) and mkArray2, which doesn't. (I tried to come up with a function with which to fill in the array that was nontrivial, in the sence that it would be difficult to code using accumulators without the explicit recursive 'fillInArr' function.) The timing results were: mkArray1: 11.42u 0.79s 0:13.47 90.6% mkArray2: 24.55u 2.31s 0:30.12 89.1% Which is actually *slower*. Any ideas why? (These were compiled with ghc 5.02.3 -O2 -fvia-c -fall-strict) - Hal -- Hal Daume III "Computer science is no more about computers | hdaume@isi.edu than astronomy is about telescopes." -Dijkstra | www.isi.edu/~hdaume