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chocolateboy

NAME

Devel::Pragma - helper functions for developers of lexical pragmas

SYNOPSIS

    package MyPragma;

    use Devel::Pragma qw(:all);

    sub import {
        my ($class, %options) = @_;
        my $hints  = hints;        # the builtin (%^H) used to implement lexical pragmas
        my $caller = ccstash();    # the name of the currently-compiling package (stash)

        unless ($hints->{MyPragma}) { # top-level
            $hints->{MyPragma} = 1;
        }

        if (new_scope($class)) {
            ...
        }

        my $scope_id = scope();
    }

DESCRIPTION

This module provides helper functions for developers of lexical pragmas (and a few functions that may be useful to non-pragma developers as well).

Pragmas can be used both in older versions of perl (from 5.8.1), which had limited support, and in the most recent versions, which have improved support.

EXPORTS

Devel::Pragma exports the following functions on demand. They can all be imported at once by using the :all tag. e.g.

    use Devel::Pragma qw(:all);

hints

This function enables the scoped behaviour of the hints hash (%^H) and then returns a reference to it.

The hints hash is a compile-time global variable (which is also available at runtime in recent perls) that can be used to implement lexically-scoped features and pragmas. This function provides a convenient way to access this hash without the need to perform the bit-twiddling that enables it on older perls. In addition, this module loads Lexical::SealRequireHints, which implements bugfixes that are required for the correct operation of the hints hash on older perls (< 5.12.0).

Typically, hints should be called from a pragma's import (and optionally unimport) method:

    package MyPragma;

    use Devel::Pragma qw(hints);

    sub import {
        my $class = shift;
        my $hints = hints;

        if ($hints->{MyPragma}) {
            # ...
        } else {
            $hints->{MyPragma} = ...;
        }

        # ...
    }

new_scope

This function returns true if the currently-compiling scope differs from the scope being compiled the last time new_scope was called. Subsequent calls will return false while the same scope is being compiled.

new_scope takes an optional parameter that is used to uniquely identify its caller. This should usually be supplied as the pragma's class name unless new_scope is called by a module that is not intended to be subclassed. e.g.

    package MyPragma;

    sub import {
        my ($class, %options) = @_;

        if (new_scope($class)) {
            ...
        }
    }

If not supplied, the identifier defaults to the name of the calling package.

scope

This returns an integer that uniquely identifies the currently-compiling scope. It can be used to distinguish or compare scopes.

A warning is issued if scope (or new_scope) is called in a context in which it doesn't make sense i.e. if the scoped behaviour of %^H has not been enabled - either by explicitly modifying $^H, or by calling "hints".

ccstash

Returns the name of the currently-compiling package (stash). It only works inside code that's being required, either in a BEGIN block via use or at runtime. In practice, its use should be restricted to compile-time i.e. import methods and any other methods/functions that can be traced back to import.

When called from code that isn't being required, it returns undef.

It can be used as a replacement for the scalar form of caller to provide the name of the package in which use MyPragma is called. Unlike caller, it returns the same value regardless of the number of intervening calls before MyPragma::import is reached.

    package Caller;

    use Callee;

    package Callee;

    use Devel::Pragma qw(ccstash);

    sub import {
        A();
    }

    sub A() {
        B();
    }

    sub B {
        C();
    }

    sub C {
        say ccstash; # Caller
    }

fqname

Takes a subroutine name and an optional caller (package name). If no caller is supplied, it defaults to "ccstash", which requires fqname to be called from import (or a function/method that can be traced back to import).

It returns the supplied name in package-qualified form. In addition, old-style ' separators are converted to new-style ::.

If the name contains no separators, then the caller/ccstash package name is prepended. If the name is already package-qualified, it is returned unchanged.

In list context, fqname returns the package and unqualified subroutine name (e.g. 'Foo::Bar' and 'baz'), and in scalar context it returns the package and sub name joined by '::' (e.g. 'Foo::Bar::baz').

e.g.

    package MyPragma::Loader;

    use MyPragma (\&coderef, 'foo', 'MyPragmaLoader::bar');

    package MyPragma;

    sub import {
        my ($class, @listeners) = @_;
        my @subs;

        for my $listener (@listeners) {
            push @subs, handle_sub($listener);
        }
    }

    sub handle_sub {
        my $sub = shift

        if (ref($ub) eq 'CODE') {
            return $sub;
        } else {
            handle_name($sub);
        }
    }

    sub handle_name {
        my ($package, $name) = fqname($name); # uses ccstash e.g. foo -> MyPragma::Loader::foo
        my $sub = $package->can($name);
        die "no such sub: $package\::$name" unless ($sub);
        return $sub;
    }

VERSION

1.1.0

SEE ALSO

AUTHOR

chocolateboy <chocolate@cpan.org>

COPYRIGHT AND LICENSE

Copyright (C) 2008-2016 by chocolateboy

This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself, either Perl version 5.8.1 or, at your option, any later version of Perl 5 you may have available.