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3 points by conanite 5168 days ago | link | parent

  arc> (tuples (range 0 19) 3)
  ((0 1 2) (3 4 5) (6 7 8) (9 10 11) (12 13 14) (15 16 17) (18 19))
tuples is defined in arc.arc:

  (def tuples (xs (o n 2))
    (if (no xs)
        nil
        (cons (firstn n xs)
              (tuples (nthcdr n xs) n))))


1 point by hasenj 5168 days ago | link

Thanks!

Interestingly, it's smaller than 'pair' even though it does more.

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3 points by akkartik 5167 days ago | link

I first encountered this idea in a theorem-proving class - that if you have a hard time proving a theorem, often a really good approach is to find a more general 'lemma' to try to prove. Stronger theorems are often easier because they let you assume more in the induction step. Recursion has a similar 1-1 correspondence with inductive proofs.

http://www.cs.utexas.edu/users/moore/classes/cs389r/index.ht...

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1 point by hasenj 5167 days ago | link

Actually I just realized, 'tuple' uses 'firstn' and 'nthcdr', where as 'pair' sort of inlines these concept and has to deal with nils.

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