1Lexical::Var(3) User Contributed Perl Documentation Lexical::Var(3)
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6 Lexical::Var - static variables without namespace pollution
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9 use Lexical::Var '$foo' => \$Remote::foo;
10 use Lexical::Var '$const' => \123;
11 use Lexical::Var '@bar' => [];
12 use Lexical::Var '%baz' => { a => 1, b => 2 };
13 use Lexical::Var '&quux' => sub { $_[0] + 1 };
14 use Lexical::Var '*wibble' => Symbol::gensym();
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17 This module implements lexical scoping of static variables and
18 subroutines. Although it can be used directly, it is mainly intended
19 to be infrastructure for modules that manage namespaces.
20 This module influences the meaning of single-part variable names that
22 appear directly in code, such as "$foo". Normally, in the absence of
23 any particular declaration, or under the effect of an "our"
24 declaration, this would refer to the scalar variable of that name
25 located in the current package. A "Lexical::Var" declaration can
26 change this to refer to any particular scalar, bypassing the package
27 system entirely. A variable name that includes an explicit package
28 part, such as "$main::foo", always refers to the variable in the
29 specified package, and is unaffected by this module. A symbolic
30 reference through a string value, such as ""${'foo'}"", also looks in
31 the package system, and so is unaffected by this module.
32 The types of name that can be influenced are scalar ("$foo"), array
34 ("@foo"), hash ("%foo"), subroutine ("&foo"), and glob ("*foo"). A
35 definition for any of these names also affects code that logically
36 refers to the same entity, even when the name is spelled without its
37 usual sigil. For example, any definition of "@foo" affects element
38 references such as "$foo[0]". Barewords in filehandle context actually
39 refer to the glob variable. Bareword references to subroutines, such
40 as ""foo(123)"", only work on Perl 5.11.2 and later; on earlier Perls
41 you must use the "&" sigil, as in ""&foo(123)"".
42 Where a scalar name is defined to refer to a constant (read-only)
44 scalar, references to the constant through the lexical namespace can
45 participate in compile-time constant folding. This can avoid the need
46 to check configuration values (such as whether debugging is enabled) at
47 runtime.
48 A name definition supplied by this module takes effect from the end of
50 the definition statement up to the end of the immediately enclosing
51 block, except where it is shadowed within a nested block. This is the
52 same lexical scoping that the "my", "our", and "state" keywords supply.
53 Definitions from Lexical::Var and from "my"/"our"/"state" can shadow
54 each other. These lexical definitions propagate into string "eval"s,
55 on Perl versions that support it (5.9.3 and later).
56 This module only manages variables of static duration (the kind of
58 duration that "our" and "state" variables have). To get a fresh
59 variable for each invocation of a function, use "my".
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62 These methods are meant to be invoked on the "Lexical::Var" package.
63 Lexical::Var->import(NAME => REF, ...)
65 Sets up lexical variable declarations, in the lexical environment
66 that is currently compiling. Each NAME must be a variable name
67 (e.g., "$foo") including sigil, and each REF must be a reference to
68 a variable/value of the appropriate type. The name is lexically
69 associated with the referenced variable/value.
70 Scalar::Construct can be helpful in generating appropriate REFs,
72 especially to create constants. There are Perl core bugs to beware
73 of around compile-time constants; see "BUGS".
74 Lexical::Var->unimport(NAME [=> REF], ...)
76 Sets up negative lexical variable declarations, in the lexical
77 environment that is currently compiling. Each NAME must be a
78 variable name (e.g., "$foo") including sigil. If the name is given
79 on its own, it is lexically dissociated from any value. Within the
80 resulting scope, the variable name will not be recognised. If a
81 REF (which must be a reference to a value of the appropriate type)
82 is specified with a name, the name will be dissociated if and only
83 if it is currently associated with that value.
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86 Subroutine invocations without the "&" sigil cannot be correctly
87 processed on Perl versions earlier than 5.11.2. This is because the
88 parser needs to look up the subroutine early, in order to let any
89 prototype affect parsing, and it looks up the subroutine by a different
90 mechanism than is used to generate the call op. (Some forms of
91 sigilless call have other complications of a similar nature.) If an
92 attempt is made to call a lexical subroutine via a bareword on an older
93 Perl, this module will probably still be able to intercept the call op,
94 and will throw an exception to indicate that the parsing has gone
95 wrong. However, in some cases compilation goes further wrong before
96 this module can catch it, resulting in either a confusing parse error
97 or (in rare situations) silent compilation to an incorrect op sequence.
98 On Perl 5.11.2 and later, sigilless subroutine calls work correctly,
99 except for an issue noted below.
100 Subroutine calls that have neither sigil nor parentheses (around the
102 argument list) are subject to an ambiguity with indirect object syntax.
103 If the first argument expression begins with a bareword or a scalar
104 variable reference then the Perl parser is liable to interpret the call
105 as an indirect method call. Normally this syntax would be interpreted
106 as a subroutine call if the subroutine exists, but the parser doesn't
107 look at lexically-defined subroutines for this purpose. The call
108 interpretation can be forced by prefixing the first argument expression
109 with a "+", or by wrapping the whole argument list in parentheses.
110 On Perls built for threading (even if threading is not actually used),
112 scalar constants that are defined by literals in the Perl source don't
113 reliably maintain their object identity. What appear to be multiple
114 references to a single object can end up behaving as references to
115 multiple objects, in surprising ways. The multiple objects all
116 initially have the correct value, but they can be writable even though
117 the original object is a constant. See Perl bug reports [perl #109744]
118 and [perl #109746]. This can affect objects that are placed in the
119 lexical namespace, just as it can affect those in package namespaces or
120 elsewhere. "Lexical::Var" avoids contributing to the problem itself,
121 but certain ways of building the parameters to "Lexical::Var" can
122 result in the object in the lexical namespace not being the one that
123 was intended, or can damage the named object so that later referencing
124 operations on it misbehave. Scalar::Construct can be used to avoid
125 this problem.
126 Bogus redefinition warnings occur in some cases when "our" declarations
128 and "Lexical::Var" declarations shadow each other.
129 Package hash entries get created for subroutine and glob names that are
131 used, even though the subroutines and globs are not actually being
132 stored or looked up in the package. This can occasionally result in a
133 "used only once" warning failing to occur when it should.
134 On Perls prior to 5.15.5, if this package's "import" or "unimport"
136 method is called from inside a string "eval" inside a "BEGIN" block, it
137 does not have proper access to the compiling environment, and will
138 complain that it is being invoked outside compilation. Calling from
139 the body of a "require"d or "do"ed file causes the same problem on the
140 same Perl versions. Other kinds of indirection within a "BEGIN" block,
141 such as calling via a normal function, do not cause this problem.
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144 Attribute::Lexical, Lexical::Import, Lexical::Sub, Scalar::Construct
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147 Andrew Main (Zefram) <zefram@fysh.org>
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150 Copyright (C) 2009, 2010, 2011, 2012, 2013 Andrew Main (Zefram)
151 <zefram@fysh.org>
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154 This module is free software; you can redistribute it and/or modify it
155 under the same terms as Perl itself.
156perl v5.34.0 2021-07-22 Lexical::Var(3)