Hello, I've been trying to optimise the following code.. -- Search Tree data type newtype STree = STree (Array Int (STree,[Match])) -- Initial value for Search Tree sTree0 :: STree sTree0 = STree (array (0,9) [(n,(sTree0,[]))| n <- [0..9]]) -- Make the search tree from a list of words makeSTree :: [String] -> STree makeSTree ws = foldl' putWord sTree0 pairs where pairs = [let ps = packString w in ps `seq` (word2keys w, MatchW ps) | w<-ws] word2keys cs = [getKey (toUpper c) | c <- cs, c /= '"' , c /= '-' ] putWord stree (keys,m) = put keys stree where put [] _ = error "makeSTree: empty Keys" put [k] (STree a) = let (t,ms) = a ! k in STree (a // [(k,(t,m:ms))]) put (k:ks) (STree a) = let (t,ms) = a ! k t' = put ks t in t' `seq` STree (a // [(k,(t',ms))]) This seems to be taking about 4.8 seconds (of 5.1 seconds total program execution time) for the input I'm using. I thought using DiffArrays might be faster, but no such luck. Execution time rose to 9.5 *minutes*. Is this what I should expect to see? I'm using ghc 6.0, BTW. Thanks -- Adrian Hey