Proposed new syntax

[Chris Angelico]

On Tue, Aug 15, 2017 at 1:33 AM, Ian Kelly <ian.g.kelly at gmail.com> wrote:
> On Sun, Aug 13, 2017 at 8:36 AM, Steve D'Aprano
>> Sure. In Haskell, comprehensions are *implicit* loops, rather than explicit like
>> in Python.
>
> No, they aren't. Haskell list comprehensions use lazy evaluation.
> Here's an example of an infinite list comprehension:
>
> Prelude> let squares = [ x ** 2 | x <- [1 ..] ] :: [Float]
> Prelude> :print squares
> squares = (_t1::[Float])
>
> You might say that this is more like a generator expression but the
> result is in fact a list. We can evaluate the first four elements:
>
> Prelude> print $ take 4 squares
> [1.0,4.0,9.0,16.0]
>
> And then see that these have been lazily evaluated in the list:
>
> Prelude> :print squares
> squares = 1.0 : 4.0 : 9.0 : 16.0 : (_t2::[Float])
>
> A Haskell list comprehension is not a loop at all.

What if you don't take the first four, but instead take just the tenth
element? Will the preceding elements be calculated too, or do you have
a sparse list? If the latter, it's not really comparable to a Python
list, but to some sort of cached mapping from input values to output
values, which in Python I would implement as a dict with a __missing__
method. And if the former, well, that's still going to have a loop,
and it's definitely like a genexp, but genexps have more functionality
than they do in Python.

ChrisA