So what the hell is gather/take

[AlexDaniel]

Inspired by #2135 which says:

Note: Please feel free to reword the title. I'm just frustrated with how little our docs say about such an important feature of perl 6.

This is another case when an amazing feature is not explained enough.

Docs: https://docs.perl6.org/syntax/gather%20take

It says:

gather/take can generate values lazily, depending on context. If you want to force lazy evaluation use the lazy subroutine or method. Binding to a scalar or sigilless container will also force laziness.

(let's ignore questionable “force laziness” stuff for now)

In some way it is right, but to me this is not enough. Surely it can generate values lazily, but how? What does it mean exactly? What should I write in order to get lazy behavior?

See my mumblings here: http://colabti.org/irclogger/irclogger_log/perl6?date=2018-10-17#l34

If I get it right (please correct me if I'm wrong), then if you have code like this:

say ‘before’;
take 42;
say ‘after’;
…

Then say ‘after’ will not be executed unless you pull another value from the Seq. So your code just magically stops executing before returning from take. This means that you can write any kind of code, like infinite recursion and the like, and it won't run more than you ask it to. That's freaking awesome.

[lichtkind]

not only that what i like about that feature you can have normal loop but in it you not take every iteration a value or sometimes two and so press irregular ebehaviour into seemingly regular frame

[zoffixznet]

So your code just magically stops executing before returning from take

I believe the technical term is "continuation"

[Xliff]

Moreover, you are not limited to the type of the value of the original list when forming the results,
consider.

my @a = gather for %results {
   if <some condion> {
      take <do somehing complex with $_ and return the result>
   }
}

Please note that this is different than:

%results.values.grep(<some condition>).map(<complex function>)

Because no mater what you are doing, you are still looping twice, and the data you have in .grep() may no longer be available in .map. Why do in two loops what can be done in one?

I'll look through some of my code and see if I can give a more concrete example.

[FCO]

But that would be the same as:

%results.values.map: { <complex function> if <some condition> }