Lexical::Sub(3pm)

NAME
Lexical::Sub - subroutines without namespace pollution

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

DESCRIPTION
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 "&foo" and "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 "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 "&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 ""&{'foo'}"", also looks in the package system, and so is unaffected by this module.

Bareword references to subroutines, such as "foo(123)", only work on Perl 5.11.2 and later. On earlier Perls you must use the "&" sigil, as in "&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 "my", "our", and "state" keywords supply. Definitions from Lexical::Sub and from "my"/"our"/"state" can shadow each other, on Perl versions where these duration keywords can be applied to subroutines (5.17.4 and later), except that Lexical::Sub can't shadow a "my"/"our"/"state" subroutine prior to Perl 5.19.1. These lexical definitions propagate into string "eval"s, on Perl versions that support it (5.9.3 and later).

This module only manages subroutines of static duration (the kind of duration that subroutines declared without "my" have). To get a fresh subroutine for each invocation of a function, use "my sub", on a Perl version that supports it (5.17.4 and later).

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

PACKAGE METHODS
These methods are meant to be invoked on the "Lexical::Sub" package.

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

Lexical::Sub->unimport(NAME [=> REF], ...)
    Sets up negative lexical subroutine declarations, in the lexical environment that is currently compiling. Each NAME must be a bare subroutine name (e.g., "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 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.

BUGS
Subroutine invocations without the "&" 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::Sub 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 "+", or by wrapping the whole argument list in parentheses.

In the earlier Perl versions that support "my"/"our"/"state" subroutines, starting from Perl 5.17.4, the mechanism for core lexical subroutines suffers a couple of bugs that mean that Lexical::Sub can't shadow subroutines declared that way. This was fixed in Perl 5.19.1.

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 "import" or "unimport" method is called from inside a string "eval" inside a "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 "require"d or "do"ed file causes the same problem on the same Perl versions. Other kinds of indirection within a "BEGIN" block, such as calling via a normal function, do not cause this problem.

When judging whether the "unimport" method should hide a subroutine, this module can't distinguish between a lexical subroutine established by this module and a "state" subroutine. This may change in the future.

SEE ALSO
Lexical::Import, Lexical::Var

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

COPYRIGHT
Copyright (C) 2009, 2010, 2011, 2012, 2013, 2023 Andrew Main (Zefram) <zefram@fysh.org>

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