Re: [Haskell-cafe] Weaving fun

This reminded me of "interleaving" as in: Backtracking, Interleaving, and Terminating Monad Transformers http://www.cs.rutgers.edu/~ccshan/logicprog/LogicT-icfp2005.pdf On 4/10/07, Dave Feustel wrote:

Talk about synchronicity! I was just wondering whether 'weaving' of infinite lists is possible.

eg weave the infinite lists [2,4..], [3,6..], [5,10..] to get [2,3,4,5,6,8,9,10,..]

Is this kind of lazy evaluation possible?

Thanks, Dave Feustel

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From: Bas van Dijk Sent: Apr 10, 2007 6:13 PM To: haskell-cafe@haskell.org Subject: [Haskell-cafe] Weaving fun

Hello,

For my own exercise I'm writing a function 'weave' that "weaves" a list of lists together. For example:

weave [[1,1,1], [2,2,2], [3,3]] ==> [1,2,3,1,2,3,1,2] weave [[1,1,1], [2,2], [3,3,3]] ==> [1,2,3,1,2,3,1]

Note that 'weave' stops when a list is empty. Right now I have:

weave :: [[a]] -> [a] weave ll = work ll [] [] where work ll = foldr f (\rst acc -> work (reverse rst) [] acc) ll f [] g = \_ acc -> reverse acc f (x:xs) g = \rst acc -> g (xs:rst) (x:acc)

However I find this definition hard to read and I'm questioning its efficiency especially due to the 'reverse' parts (how do they impact performance and can they be removed?)

So I'm wondering if 'weave' can be defined more "elegantly" (better readable, shorter, more efficient, etc.)?

happy hacking,

Bas van Dijk _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

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