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PerlX-ScopeFunction-0.05
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/ PerlX::ScopeFunction

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

BECAUSE THIS SOFTWARE IS LICENSED FREE OF CHARGE, THERE IS NO WARRANTY FOR THE SOFTWARE, TO THE EXTENT PERMITTED BY APPLICABLE LAW. EXCEPT WHEN OTHERWISE STATED IN WRITING THE COPYRIGHT HOLDERS AND/OR OTHER PARTIES PROVIDE THE SOFTWARE "AS IS" WITHOUT WARRANTY OF ANY KIND, EITHER EXPRESSED OR IMPLIED, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. THE ENTIRE RISK AS TO THE QUALITY AND PERFORMANCE OF THE SOFTWARE IS WITH YOU. SHOULD THE SOFTWARE PROVE DEFECTIVE, YOU ASSUME THE COST OF ALL NECESSARY SERVICING, REPAIR, OR CORRECTION.

IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MAY MODIFY AND/OR REDISTRIBUTE THE SOFTWARE AS PERMITTED BY THE ABOVE LICENCE, BE LIABLE TO YOU FOR DAMAGES, INCLUDING ANY GENERAL, SPECIAL, INCIDENTAL, OR CONSEQUENTIAL DAMAGES ARISING OUT OF THE USE OR INABILITY TO USE THE SOFTWARE (INCLUDING BUT NOT LIMITED TO LOSS OF DATA OR DATA BEING RENDERED INACCURATE OR LOSSES SUSTAINED BY YOU OR THIRD PARTIES OR A FAILURE OF THE SOFTWARE TO OPERATE WITH ANY OTHER SOFTWARE), EVEN IF SUCH HOLDER OR OTHER PARTY HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES.