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Re: Challenge: Another Infinite Lazy List

by tmoertel (Chaplain)
on Mar 19, 2005 at 01:25 UTC ( #440841=note: print w/replies, xml ) Need Help??

in reply to Challenge: Another Infinite Lazy List

For comparision with Haskell, we can define R for any given list of factors by folding the merge operator through the list of lists of multiples:
r = foldr merge [] . map (\n -> map (*n) [1..])
We use a duplicate-eating merge:
merge [] ys = ys merge xs [] = xs merge (x:xs) (y:ys) | x < y = x : merge xs (y:ys) | y < x = y : merge (x:xs) ys | otherwise = x : merge xs ys
Sample output:
> take 60 (r [2,3,5]) [2,3,4,5,6,8,9,10,12,14,15,16,18,20,21,22,24,25,26,27,28,30,32,33,34,3 +5,36,38,39,40,42,44,45,46,48,50,51,52,54,55,56,57,58,60,62,63,64,65,6 +6,68,69,70,72,74,75,76,78,80,81,82]

Replies are listed 'Best First'.
Re^2: Challenge: Another Infinite Lazy List
by kelan (Deacon) on Mar 19, 2005 at 15:20 UTC

    I guess I don't have enough practice with Haskell yet. When trying to think of an easy way to merge a list of lists I completely forgot about folding. ++ to you.

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