NAME
PerlX::ScopeFunction - new keywords for creating scopes.
SYNOPSIS
use PerlX::ScopeFunction qw( let with );
use List::Util qw( sum0 );
use List::MoreUtils qw( part minmax );
with ( part { $_ % 2 } @input ) {
my ($evens, $odds) = @_;
say "There are " . scalar(@$evens) . " even numbers: " . join(" ", @$evens);
say "There are " . scalar(@$odds) . " odd numbers: " . join(" ", @$odds);
}
let ( ($min,$max) = minmax(@input); $mean = sum0(@input)/@input ) {
...
}
DESCRIPTION
Scope functions can be used to create small lexical scope, inside which the results of an given expression are used, but not outside.
This module provide extra keywords / methods / symbols for creating scopes that help grouping related statements together, or generally help on making the code look more fluent.
By use
-ing this module without a import list, all keywords are imported. To import only wanted keywords, specify them in the import list:
# Import all keywords
use PerlX::ScopeFunction;
# Import just `let`
use PerlX::ScopeFunction qw(let);
# Import just `with`, and name it differently
use PerlX::ScopeFunction 'with' => { -as 'withThese' }
Imported keywords can be removed by a no
statement.
no PerlX::ScopeFunction;
Importable keywords / methods / symbols.
with
The with
keyword can be used to bring the result of an given EXPR to a smaller scope (code block):
with ( EXPR ) BLOCK
The EXPR are evaluated in list context, and the result (a list) is available inside BLOCK as @_
. The conventional topic variable $_
is also assigned the last value of the list ($_[-1]
).
let
The let
keyword can be used to create readonly my
- variables in a smaller scope (code block).
The keyword let
should be followed by a list of variable declarations, then a block.
let ( DECLARATIONS ) BLOCK
The word DECLARATIONS
here means a list of variable declaration statements seperated by semicolons, except there must be a RHS. They must be given without any of my
, our
, state
keywords.
For example, if in the BLOCK you would do these to prepare 3 convenient variables:
my $mean = mean(@input);
my ($min, $max) = minmax(@input);
With let
statements, you do this instead:
let (
$mean = mean(@input);
($min,$max) = minmax(@input);
) {
...
}
Declaration are evaluated in the same order as they are given and all variables declarated by this are made readonly inside the BLOCK. The underlying library to make variables readonly is Const::Fast. Variables created in the beginning of this list of can be used in the latter positions.
If in the current scope, there are variables with identical names as the ones in the DECLARATIONS, they are masked in the let-block.
For example, in the following example code, 3 new variables are created in the let-block and they the ones with identical names outside of the let-block.
my ($foo, $bar, $baz) = (10, 20, 30);
let ($foo = 1; $bar = 2; $baz = $foo + $bar) {
say "$foo $bar $baz"; #=> 1 2 3
}
say "$foo $bar $baz"; #=> 10 20 30
Array and Hash can also be created:
let (@foo = (1,2,3); %bar = (bar => 1); $baz = 42) {
...
}
$do
$do
is a method that can be used as a call on any objects. It takes a CodeRef, evaluates the CodeRef on the context of the object being called on, and returns the result.
Syntax-wise, $do
is used like this:
EXPR -> $do(sub BLOCK)
For example, the following code would take an object and compute a sha1 digest:
my $digest = $o->$do(sub {
sha1($_->header->title . $_->body->content)
});
Inside the CodeRef BLOCK, both $_
and $_[0]
are an alias to the object being called on.
$tap
, and $also
$tap
is a method that takes a CodeRef can be inserted of into a chain of method calls, do some side actions, then resume.
$also
is an alternative name of $tap
. They are completely identical. Import and use which ever that is more comprehensible to you.
Syntax-wise $tap
is supposed to used like these:
EXPR -> $tap(sub BLOCK)
EXPR -> $tap(sub BLOCK) -> EXPR
For example, the following code would produce a warning message before calling send()
method on object $o
:
my $o = Example::Mail->new( body => $args{body}, to => $args{to} )
->$tap(sub { warn "Mail sening to: " . $_->to ) })
->send();
In side the tap code block, $_
(and $_[0]
) refers to the object from the EXPR beforehand. The return value of tap code block is thrown away, and the $tap
itself always evaulates to the same object $_
it is called on. This makes it easier to do some side-effects in the middle of a call chain, but without having to rewrite the call chain as 2 separate statements. It can also be useful to work around methods with "inconvenient" return values, or to group a sequence of object-setup statements together, in a tighter scope:
For example, here we construct an Example::Mail
, fill it a recipient and body, but we want to check some external factors before actully send it:
my $o = Example::Mail->new()
->$tap(sub {
$_->set_body( $args{body} );
$_->set_to( $args{to} );
$_->ping_mail_server() or die "No internet";
$_->check_recipient_mood() or die "Bad timing";
})
->send();
The $tap
can be think as a method that can be invoked on any objects. But it would not work on plain scalar values, or anything you couldn't call methods on.
Since it is just a scalar variable, it can also be copied to a lexical variable, under whatever more sensible names:
sub run ($self) {
my $byTheWay = $PerlX::ScopeFunction::tap;
$self->$byTheWay(sub { warn "Star running" })
->do_run();
}
See also the tap
method from Mojo::Base. Or the scope function also
in Kotlin programming language: Kotlin Scope Function
Importing as different names
Since the keywords provided in this module are commonly defined in other CPAN modules, this module also provides a way to let users to import those keywords as different names, with a conventional spec also seen in Sub::Exporter.
For example, to import with
as given_these
, you say:
use PerlX::ScopeFunction "with" => { -as => "given_these" };
Basically HashRef in import list becomes modifiers of their previous entries. However, This module supports only the modifier -as
but not other ones as seen in Sub::Exporter.
CAVEATS
Due to the fact this module hooks into perl parser, the keywords cannot be used without being imported into current namespace. Statements like the following do not compile:
PerlX::ScopeFunction::let( ... ) {
...
}
PerlX::ScopeFunction::with( ... ) {
...
}
AUTHOR
Kang-min Liu <gugod@gugod.org>
LICENCE
The MIT License
DISCLAIMER OF WARRANTY
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