[Haskell-cafe] Re: computing lists of pairs

Daniel Fischer schrieb:

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allPossibilities :: [[a]] -> [[a]] allPossibilities [] = [[]] allPossibilities (l:ls) = [ x : xs | xs <- allPossibilites ls, x <- l ] I cannot really observe a speed up, with this version, but there are probably examples where any version is faster than the other.

I can,

Oh yes, I can too.

aP1 [] = [[]] aP1 (h:t) = do x <- h xs <- aP1 t return (x:xs)

for every x in h, we calculate the combinations of t anew.

Do we? Isn't "aP1 t" one closure that's being evaluated only once?

aP2 [] = [[]] aP2 (h:t) = do xs <- aP2 t x <- h return (x:xs)

now we first calculate the combinations of t, for each of those, we cons the elements of h to it in turn and never reuse it afterwards.

Thanks for explaining. C.