[Haskell-beginners] Question about Ord for lists and multi-dimensional lists

[Tyler Huffman]

Awesome, that helps a lot.

Thanks a bunch, Mateusz!


Regards,
Tyler Huffman


On Sun, Mar 2, 2014 at 10:44 PM, Mateusz Kowalczyk
<fuuzetsu at fuuzetsu.co.uk>wrote:

> On 03/03/14 05:33, Tyler Huffman wrote:
> > Hi all,
> >
> > I'm having trouble finding resources explaining how Ord for two lists
> works.
> >
> > I've created a public gist to explain what I mean.
> >
> > https://gist.github.com/iduhetonas/9318958
> >
> > Could anybody shed some light on what I'm seeing?
> >
> >
> > Regards,
> > Tyler Huffman
> >
> >
> >
> > _______________________________________________
> > Beginners mailing list
> > Beginners at haskell.org
> > http://www.haskell.org/mailman/listinfo/beginners
> >
>
> It does element-wise comparisons and in case one of the lists is empty,
> the other one is considered greater (unless they are both empty).
>
> So:
> Prelude> max [1] [2]
>
> Here 2 is bigger than 1 so we get the result straight away
>
> Prelude> max [1,2] [1]
>
> Here the first element is equal so we compare the rest of lists: [2] and
> []. As the second list is empty, we return the first list.
>
> Prelude> max [2,1] [1]
>
> No problem at all here, 2 is greater than 1 so we just return the first
> list on the first comparison.
>
> Prelude> max [1,2] [1,2,4]
>
> This is exactly the same case as ‘max [1, 2] [1]’: we first compare the
> 1s, find they are equal and compare the rest of the list. Same with 2s.
> Then the base case is ‘max [] [4]’ so the second list is treated as
> greater.
>
> Prelude> max [1,2,4] [1,2]
>
> Same as above but with arguments switched. I believe that the law here
> is ‘max x y ≡ max y x’.
>
> Prelude> max [[1,2,3],[2,3,4]] [[1,2],[2,3],[2,4]]
>
> This is just like all our other examples except one more level of
> nesting. We first end up comparing the first elements of the argument
> (so we call out ‘max [1, 2, 3] [1, 2]’, and then check each element of
> these lists. 1 and 2 are equal and we reach the ‘max [3] []’ case: the
> first list was greater so our whole call returns ‘[[1, 2, 3], [2, 3, 4]]’.
>
> I hope that sheds some light.
>
> --
> Mateusz K.
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> Beginners at haskell.org
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>
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