Re: [Haskell-cafe] Re: Why is this strict in its arguments?

On Dec 5, 2007 1:43 PM, Benja Fallenstein wrote:

Hi Paolo,

On Dec 5, 2007 2:09 PM, Paulo J. Matos wrote:

...

I'm glad that my initial post generated such an interesting discussion but I'm still not understanding why the first version of findAllPath seems to be computing the whole list even when I just request the head, while the second one doesn't.

Because the function starts its work with

if isNothing lfpaths && isNothing rtpaths then Nothing else ...

which forces the evaluation of 'lfpaths' and 'rtpaths' to see whether they are Just or Nothing, which recursively forces the evaluation of findAllPath for the whole tree.

Oh, but lfpaths is not nothing so that means that isNothing rtpaths shouldn't be evaluated, right?

Hope this helps, - Benja

-- Paulo Jorge Matos - pocm at soton.ac.uk http://www.personal.soton.ac.uk/pocm PhD Student @ ECS University of Southampton, UK