[Haskell-cafe] beginner's problem about lists

Matthias Fischmann fis at wiwi.hu-berlin.de
Tue Oct 10 11:02:43 EDT 2006

On Tue, Oct 10, 2006 at 08:58:05AM -0400, Lennart Augustsson wrote:
> >a function that takes two lists and decides whether one of them is
> >finite or not , without being given further information on the lists,
> >does not exist.
> A function that takes two lists and decides if one is finite does  
> indeed exist.  But if both are infinite you'll get partial  
> information out.
> The example
>   shorter [1..5] (shorter [2..] [3..])
> is a little tricky, but certainly doable.

right, my implementation wouldn't cover this case.

i was meaning termination, though: if a function accepts two possibly
infinte lists, it cannot unconditionally return with the shorter one.

i think the best you can do is Hennings solution (and mine, but i was
too slow :) that hits bottom once you touch any element in (shorter
[2..] [3..]), instead of merely the lengths of its prefices.


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