Hi Manuel, >> PID USER PRI NI SIZE RSS SHARE STAT %CPU %MEM TIME COMMAND 23542 >> herrmann 20 0 250M 130M 97500 R 66.3 52.4 0:21 ghc-5.02 Manuel> Is this what I think it is? Do you benchmark the Manuel> interpreter? Interpreted code isn't optimised. not really. I wanted to have a strict foldl (for a different purpose than sum) and could not figure out how to tell the compiler. I tried different things, precompile just one module, compile the entire program, using -fglasgow-exts, but nothing seems to work. Thus, I reduced the problem to sum to make it simpler for you. However, the sum computation based on my sfold definition did not take too long even with the interpreter. Thus, I wondered why the predefined sum computation behaves that bad. Manuel> GHC's Prelude does not define `sum' in terms of foldl; Manuel> instead, it uses the definition ... Where can I find the ghc prelude? Is there also the definition of the mysterious foldl' function which I saw from time to time before it disappeared again? Manuel> I haven't checked the Core code produced for the above Manuel> definition, but as I know GHC, I am pretty sure that it Manuel> compiles the Prelude definition into a nice tight loop Manuel> making use of all available strictness. One would expect that ghci uses a compiled version of sum. Does it or not? Cheers -- Christoph