Hi. I'm still working on the Netflix Prize; now I have managed to implement a parsing function for the qualifying data set (or quiz set). The quiz set is in the format: 1: 10 20 30 2: 100 200 3: 1000 2000 3000 4000 5000 Where 1, 2, 3 are movie ids, and the others are user ids. The parsing program is at: http://hpaste.org/fastcgi/hpaste.fcgi/view?id=2300 The program reads the file using lazy IO. One of the feature I want is, for the quiz function, to be a *good producer*. I'm quite satisfied with the result (this is the first "complex" parsing function I have written in Haskell), and I have also managed to avoid the use of an accumulator. However I'm interested to know it there is a more efficient solution. The qualifying.txt file is 51MB, 2834601 lines. On my laptop, the function performance are: real 1m14.117s user 0m2.496s sys 0m0.136s CPU usage is about 3%, system load is about 0.20, memory usage is 4956 KB. What I'm worried about is: quiz' ((id, ":") : l) = (id, quiz'' l) : quiz' l quiz' ((id, _) : l) = quiz' l the problem here is that the same elements in the list are processed multiple times. I have written alternate versions. The first using foldl http://hpaste.org/fastcgi/hpaste.fcgi/view?id=2303 (that, however, builds the entire data structure in memory, and in reverse order) The latter using foldr http://hpaste.org/fastcgi/hpaste.fcgi/view?id=2304 (that, however, is incorrect and I'm unable to fix). The performances of the foldr version are very similar to the performances of the first implementation (it make use, however, of 3704 KB, and it is about 3 seconds faster). P.S: the expected result for the sample quiz set I have posted is: [(1,[10,20,30]),(2,[100,200]),(3,[1000,2000,3000,4000,5000])] The foldl version produces: [(3,[5000,4000,3000,2000,1000]),(2,[200,100]),(1,[30,20,10])] The foldr version produces: [(1,[]),(2,[10,20,30]),(3,[100,200]),(5000,[1000,2000,3000,4000])] Thanks Manlio Perillo