2020/12/23

2020 Perl Advent Calendar - Day 23

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For today's article, I'd like to take a look at yet another of my syntax-providing CPAN modules, Syntax::Keyword::Dynamically. This provides a single new keyword, dynamically. To quote its documentation:

Syntactically and semantically it is similar to the built-in perl keyword local, but is implemented somewhat differently to give two key advantages over regular local:
  • You can dynamically assign to lvalue functions and accessors.
  • You can dynamically assign to regular lexical variables.

This is important to us when working with Object::Pad because of the way slot variables work. Within a method body a slot looks like a regular lexical variable. This means that Perl's regular local keyword refuses to interact with one. If we want to assign a new value temporarily, only for the duration of one block of code and have it restored automatically afterwards, we must use dynamically instead.

For example, both Syntax::Keyword::Dynamically and Object::Pad contain a copy of a unit test which asserts that their interaction works as expected.:

has $value = 1;
method value { $value }

method test
{
    is $self->value, 1, 'value is 1 initially';

    {
        dynamically $value = 2;
        is $self->value, 2, 'value is 2';
    }

    is $self->value, 1, 'value is 1 finally';
}

If instead we were to try this using core Perl's local it fails to compile:

...
    {
        local $value = 2;
        ...
$ perl -c example.pl
Can't localize lexical variable $value at ...

When a variable is dynamically assigned a new value inside an asynchronous function it has to be swapped back to its original value while that function is suspended, and its new value put back when the function resumes. This may have to happen several times before the function eventually returns. The way that dynamically is implemented means it is supported by Future::AsyncAwait and can detect the times it needs to swap values back and forth.

There is also a unit test which checks this interaction in both Syntax::Keyword::Dynamically and Future::AsyncAwait:

my $var = 1;

async sub with_dynamically
{
    my $f = shift;

    dynamically $var = 2;

    is $var, 2, '$var is 2 before await';
    await $f;
    is $var, 2, '$var is 2 after await';
}

my $f1 = Future->new;
my $fret = with_dynamically( $f1 );

is $var, 1, '$var is 1 while suspended';

$f1->done;
is $var, 1, '$var is 1 after finish';

Given these three modules are now known to be working nicely in each of the three pairwise combinations, you might wonder if all three can be combined at once - can you dynamically change the value of an object slot during an async method? The answer is still yes.

All three of these module distributions contain a copy of a unit test which checks this behaviour:

class Logger {
    has $_level = 1;

    method level { $_level }

    async method verbosely {
        my ( $code ) = @_;
        dynamically $_level = $_level + 1;
        is $self->level, 2, 'level is 2 before code';
        await $code->();
        is $self->level, 2, 'level is 2 after code';
    }
}

my $logger = Logger->new;

my $f1 = Future->new;
my $fret = $logger->verbosely(async sub {
    is $logger->level, 2, 'level is 2 before await';
    await $f1;
    is $logger->level, 2, 'level is 2 after await';
});

is $logger->level, 1, 'level is 1 outside';

$f1->done;

is $logger->level, 1, 'level is 1 finally';

Each of these syntax modules has provided something useful on its own, but as we have seen both yesterday and today they can be combined with each other to provide even more useful behaviours. It is easily possible to create CPAN modules that operate together to extend the Perl language with new syntax and semantics, and have those extensions work and feel every bit as convenient and powerful as all of the native syntax built into the language.

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