This implementation of calculating 10000 primes (compiled with GHC -O2) is 25% slower than the naive python implementation. Shouldn't it be faster? Am I doing something obviously stupid? primes = map (\(x,_,_)->x) $ filter (\(_,isP,_)->isP) candidates candidates = (2,True,[]):(3,True,[]): map next (tail candidates) next (candidate,isP,modCounts) = let newCounts = map incrMod2 modCounts in -- accumulate mods (candidate+2 -- we only need to bother with odd numbers ,(isPrime newCounts) -- track whether this one is prime ,if isP then (candidate,2):newCounts else newCounts) -- add if prime isPrime = and . map ((/=0).snd) incrMod2 (p,mc) = let mc' = mc+2 in if mc'
p then (p,1) else (p,0) Note: It is shorter than the python, but I would have assumed that GHC could deliver faster as well. -Alex-