Another newbie question. The following is a naive attempt at a word count function, that takes a string and returns a list of word/count pairs. import Char isLetter c = isAlphaNum c || c=='\'' normalize = map toLower . filter isLetter isWord [] = False isWord s = True nwords = filter isWord . map normalize . words update w [] = [(w, 1)] update w ((x, n):xs) = if x==w then (x, n+1) : xs else (x, n) : (update w xs) wordCount = foldr update [] . nwords It works fine, but it's obviously very inefficient. I could make it less inefficient by implementing the word/count dictionary as a hash table or binary tree, but that's what standard libraries are for. So - what is there in the standard libraries that I could use in this scenario? I've looked at the Array module, but don't think it's quite suitable (we don't know the array bounds ahead of time, and the indices are strings). Data.HashTable looks better, but involves IO as it mutates the dictionary entries in place. Is there an alternative that could be used with a fold as in the code above? Dominic