[Haskell-beginners] Need some understanding on this behavior.

[Ut Primum]

Hi,

You can't sort an infinite list. It is easy to see this because you can't
even decide what the first element of the ordered list should be, without
checking all the list elements. Since they are infinite, you can't do that.

Best,
Ut

Il mar 4 feb 2020, 15:00 Alexander Chen <alexander at chenjia.nl> ha scritto:

> Hi,
>
> thank you for the explanation. I know now how not to use it but for deeper
> understanding.
>
> question:
> Can a lazy list be ordered or is that just not possible? And how can you
> create a lazy list like this [1..] without there being any implicit order
> in it?
>
> I don't want to be nitt-picking, i am trying to reconcile this paradox for
> deeper understanding.
>
> thank you.
>
> best,
>
>
> February 4, 2020 2:44:27 PM CET David McBride <toad3k at gmail.com> wrote:
>
> You can absolutely use a lazy list in a list comprehension.  What you
> cannot do is evaluate an infinitely long list.
>
> When you evaluate it so that it will be printed you are saying, print the
> *entire* list.  You may know that this list will never contain an element
> over 100, but as far as haskell is concerned, there could be an element
> after 100 and it will keep crunching to find it, by incrementing x and
> testing it against x <= 100 forever, even though it will never succeed.
>
> On Mon, Feb 3, 2020 at 3:14 AM Alexander Chen <alexander at chenjia.nl>
> wrote:
>
>
> hi,
>
> I need some understanding on the following behavior:
>
> a = [1..]
> [x | x <-a, x <= 100]
>
> [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,84,85,86,87,88,89,90,91,92,93,94,95,96,97,98,99,100
>
> (note the missing ']' which results in the command not finishing)
>
> b = take 1000 a
> [x | x <-b, x <= 100]
>
> [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,84,85,86,87,88,89,90,91,92,93,94,95,96,97,98,99,100]
>
> Note, I took more for b  just for ease sake. But now the command finishes
> correctly.
>
>
> Question:
> why can i not use a lazy list directly in a list comprehension?
>
> best,
>
>
>
>
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