Lexical::Var(3pm) User Contributed Perl Documentation Lexical::Var(3pm)
NAME
Lexical::Var - static variables without namespace pollution
SYNOPSIS
use Lexical::Var '$foo' => \$Remote::foo;
use Lexical::Var '$const' => \123;
use Lexical::Var '@bar' => [];
use Lexical::Var '%baz' => { a => 1, b => 2 };
use Lexical::Var '&quux' => sub { $_[0] + 1 };
use Lexical::Var '*wibble' => Symbol::gensym();
DESCRIPTION
This module implements lexical scoping of static variables and
subroutines. Although it can be used directly, it is mainly intended
to be infrastructure for modules that manage namespaces.
This module influences the meaning of single-part variable names that
appear directly in code, such as "$foo". Normally, in the absence of
any particular declaration, or under the effect of an "our"
declaration, this would refer to the scalar variable of that name
located in the current package. A "Lexical::Var" declaration can
change this to refer to any particular scalar, bypassing the package
system entirely. A variable name that includes an explicit package
part, such as "$main::foo", always refers to the variable in the
specified package, and is unaffected by this module. A symbolic
reference through a string value, such as ""${'foo'}"", also looks in
the package system, and so is unaffected by this module.
The types of name that can be influenced are scalar ("$foo"), array
("@foo"), hash ("%foo"), subroutine ("&foo"), and glob ("*foo"). A
definition for any of these names also affects code that logically
refers to the same entity, even when the name is spelled without its
usual sigil. For example, any definition of "@foo" affects element
references such as "$foo[0]". Barewords in filehandle context actually
refer to the glob variable. 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)".
Where a scalar name is defined to refer to a constant (read-only)
scalar, references to the constant through the lexical namespace can
participate in compile-time constant folding. This can avoid the need
to check configuration values (such as whether debugging is enabled) at
runtime.
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::Var and from "my"/"our"/"state" can shadow
each other (except that Lexical::Var 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 variables of static duration (the kind of
duration that "our" and "state" variables have). To get a fresh
variable for each invocation of a function, use "my".
PACKAGE METHODS
These methods are meant to be invoked on the "Lexical::Var" package.
Lexical::Var->import(NAME => REF, ...)
Sets up lexical variable declarations, in the lexical environment
that is currently compiling. Each NAME must be a variable name
(e.g., "$foo") including sigil, and each REF must be a reference to
a variable/value of the appropriate type. The name is lexically
associated with the referenced variable/value.
Scalar::Construct can be helpful in generating appropriate REFs,
especially to create constants. There are Perl core bugs to beware
of around compile-time constants; see "BUGS".
Lexical::Var->unimport(NAME [=> REF], ...)
Sets up negative lexical variable declarations, in the lexical
environment that is currently compiling. Each NAME must be a
variable name (e.g., "$foo") including sigil. If the name is given
on its own, it is lexically dissociated from any value. Within the
resulting scope, the variable name will not be recognised. If a
REF (which must be a reference to a value of the appropriate type)
is specified with a name, the name will be dissociated if and only
if it is currently associated with that value.
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::Var 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::Var can't
shadow subroutines declared that way. This was fixed in Perl 5.19.1.
On Perls built for threading (even if threading is not actually used),
scalar constants that are defined by literals in the Perl source don't
reliably maintain their object identity. What appear to be multiple
references to a single object can end up behaving as references to
multiple objects, in surprising ways. The multiple objects all
initially have the correct value, but they can be writable even though
the original object is a constant. See Perl bug reports [perl #109744]
and [perl #109746]. This can affect objects that are placed in the
lexical namespace, just as it can affect those in package namespaces or
elsewhere. "Lexical::Var" avoids contributing to the problem itself,
but certain ways of building the parameters to "Lexical::Var" can
result in the object in the lexical namespace not being the one that
was intended, or can damage the named object so that later referencing
operations on it misbehave. Scalar::Construct can be used to avoid
this problem.
Bogus redefinition warnings occur in some cases when "our" declarations
and "Lexical::Var" declarations shadow each other.
Package hash entries get created for subroutine and glob names that are
used, even though the subroutines and globs 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
Attribute::Lexical(3), Lexical::Import(3), Lexical::Sub(3),
Scalar::Construct(3)
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.
perl v5.34.3 2024-03-12 Lexical::Var(3pm)
lexical-var 0.10.0 - Generated Mon Aug 4 16:32:10 CDT 2025