=head1 NAME

Lexical::Sub - subroutines without namespace pollution


	use Lexical::Sub quux => sub { $_[0] + 1 };
	use Lexical::Sub carp => \&Carp::carp;


This module implements lexical scoping of subroutines.  Although it can
be used directly, it is mainly intended to be infrastructure for modules
that manage namespaces.

This module influences the meaning of single-part subroutine names that
appear directly in code, such as "C<&foo>" and "C<foo(123)>".
Normally, in the absence of
any particular declaration, these would refer to the subroutine of that
name located in the current package.  A C<Lexical::Sub> declaration
can change this to refer to any particular subroutine, bypassing the
package system entirely.  A subroutine name that includes an explicit
package part, such as "C<&main::foo>", always refers to the subroutine
in the specified package, and is unaffected by this module.  A symbolic
reference through a string value, such as "C<&{'foo'}>", also looks in
the package system, and so is unaffected by this module.

Bareword references to subroutines, such as "C<foo(123)>", only work on
Perl 5.11.2 and later.  On earlier Perls you must use the C<&> sigil,
as in "C<&foo(123)>".

A name definition supplied by this module takes effect from the end of the
definition statement up to the end of the immediately enclosing block,
except where it is shadowed within a nested block.  This is the same
lexical scoping that the C<my>, C<our>, and C<state> keywords supply.
These lexical definitions propagate into string C<eval>s, on Perl versions
that support it (5.9.3 and later).

This module is implemented through the mechanism of L<Lexical::Var>.
Its distinct name and declaration syntax exist to make lexical subroutine
declarations clearer.


package Lexical::Sub;

{ use 5.006; }
use warnings;
use strict;

our $VERSION = "0.009";

require Lexical::Var;
die "mismatched versions of Lexical::Var and Lexical::Sub modules"
	unless $Lexical::Var::VERSION eq $VERSION;


These methods are meant to be invoked on the C<Lexical::Sub> package.


=item Lexical::Sub->import(NAME => REF, ...)

Sets up lexical subroutine declarations, in the lexical environment that
is currently compiling.  Each I<NAME> must be a bare subroutine name
(e.g., "B<foo>"), and each I<REF> must be a reference to a subroutine.
The name is lexically associated with the referenced subroutine.

=item Lexical::Sub->unimport(NAME [=> REF], ...)

Sets up negative lexical subroutine declarations, in the lexical
environment that is currently compiling.  Each I<NAME> must be a bare
subroutine name (e.g., "B<foo>").  If the name is given on its own, it is
lexically dissociated from any subroutine.  Within the resulting scope,
the subroutine name will not be recognised.  If a I<REF> (which must
be a reference to a subroutine) is specified with a name, the name
will be dissociated if and only if it is currently associated with
that subroutine.


=head1 BUGS

Subroutine invocations without the C<&> sigil cannot be correctly
processed on Perl versions earlier than 5.11.2.  This is because
the parser needs to look up the subroutine early, in order to let any
prototype affect parsing, and it looks up the subroutine by a different
mechanism than is used to generate the call op.  (Some forms of sigilless
call have other complications of a similar nature.)  If an attempt
is made to call a lexical subroutine via a bareword on an older Perl,
this module will probably still be able to intercept the call op, and
will throw an exception to indicate that the parsing has gone wrong.
However, in some cases compilation goes further wrong before this
module can catch it, resulting in either a confusing parse error or
(in rare situations) silent compilation to an incorrect op sequence.
On Perl 5.11.2 and later, sigilless subroutine calls work correctly,
except for an issue noted below.

Subroutine calls that have neither sigil nor parentheses (around the
argument list) are subject to an ambiguity with indirect object syntax.
If the first argument expression begins with a bareword or a scalar
variable reference then the Perl parser is liable to interpret the call as
an indirect method call.  Normally this syntax would be interpreted as a
subroutine call if the subroutine exists, but the parser doesn't look at
lexically-defined subroutines for this purpose.  The call interpretation
can be forced by prefixing the first argument expression with a C<+>,
or by wrapping the whole argument list in parentheses.

Package hash entries get created for subroutine names that are used,
even though the subroutines are not actually being stored or looked
up in the package.  This can occasionally result in a "used only once"
warning failing to occur when it should.

On Perls prior to 5.15.5,
if this package's C<import> or C<unimport> method is called from inside 
a string C<eval> inside a C<BEGIN> block, it does not have proper
access to the compiling environment, and will complain that it is being
invoked outside compilation.  Calling from the body of a C<require>d
or C<do>ed file causes the same problem
on the same Perl versions.  Other kinds of indirection
within a C<BEGIN> block, such as calling via a normal function, do not
cause this problem.

=head1 SEE ALSO


=head1 AUTHOR

Andrew Main (Zefram) <zefram@fysh.org>


Copyright (C) 2009, 2010, 2011, 2012, 2013
Andrew Main (Zefram) <zefram@fysh.org>

=head1 LICENSE

This module is free software; you can redistribute it and/or modify it
under the same terms as Perl itself.