On Thu, Oct 2, 2008 at 12:45 AM, Matthew Williams
I'm using GHC to run it. My problem / question is this: It's running quite a lot slower than equivalent programs in erlang and python. I suspect it's down to the way I've written it. Any thoughts (or comments in general)
Although Ben's algorithm suggestions are wise (just test up to the square root and add the matching divisor), even without those improvements, I get huge speed-up just using Int instead of Integer and compiling with -O2 instead of using ghci. On my machine, just compiling it takes the time down from about 30 seconds to 4.5. Then switching to Int takes it down to 1.2 seconds. I guess maybe the Python interpreter is a bit more deft at this type of problem than the GHC interpreter. In case you're wondering, since you said you're new to Haskell, the Int type uses machine storage (like 'int' in C, with the same bound restrictions), whereas the Integer type is a bit more abstract and can increase without limit. That's why Integer is a bit more costly to calculate with. Another style tip: if your list comprehension is only used to filter out elements (like in your 'do_perfect' function), it's clearer to use the Prelude function 'filter': do_perfect = filter is_perfect Kurt