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.\" ========================================================================
.\"
.IX Title "Syntax::Keyword::Dynamically 3pm"
.TH Syntax::Keyword::Dynamically 3pm "2021-02-04" "perl v5.32.1" "User Contributed Perl Documentation"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
.if n .ad l
.nh
.SH "NAME"
"Syntax::Keyword::Dynamically" \- dynamically change the value of a variable
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
.Vb 1
\&   use Syntax::Keyword::Dynamically;
\&
\&   my $logger = ...;
\&
\&   sub operate
\&   {
\&      dynamically $logger\->level = LOG_DEBUG;
\&
\&      do_things();
\&   }
.Ve
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
This module provides a syntax plugin that implements a single keyword,
\&\f(CW\*(C`dynamically\*(C'\fR, which alters the behaviour of a scalar assignment operation.
Syntactically and semantically it is similar to the built-in perl keyword
\&\f(CW\*(C`local\*(C'\fR, but is implemented somewhat differently to give two key advantages
over regular \f(CW\*(C`local\*(C'\fR:
.IP "\(bu" 2
You can \f(CW\*(C`dynamically\*(C'\fR assign to lvalue functions and accessors.
.IP "\(bu" 2
You can \f(CW\*(C`dynamically\*(C'\fR assign to regular lexical variables.
.PP
Semantically, the behaviour can be considered equivalent to
.PP
.Vb 3
\&   {
\&      my $old = $VAR;
\&      $VAR = "new value";
\&
\&      ...
\&
\&      $VAR = $old;
\&   }
.Ve
.PP
Except that the old value will also be restored in the case of exceptions,
\&\f(CW\*(C`goto\*(C'\fR, \f(CW\*(C`next/last/redo\*(C'\fR or similar ways to leave the controlling block
scope.
.SH "KEYWORDS"
.IX Header "KEYWORDS"
.SS "dynamically"
.IX Subsection "dynamically"
.Vb 4
\&   {
\&      dynamically LVALUE = EXPR;
\&      ...
\&   }
.Ve
.PP
The \f(CW\*(C`dynamically\*(C'\fR keyword modifies the behaviour of the following expression.
which must be a scalar assignment. Before the new value is assigned to the
lvalue, its current value is captured and stored internally within the Perl
interpreter. When execution leaves the controlling block for whatever reason,
as part of block scope cleanup the saved value is restored.
.PP
The \s-1LVALUE\s0 may be any kind of expression that allows normal scalar assignment;
lexical or package scalar variables, elements of arrays or hashes, or the
result of calling an \f(CW\*(C`:lvalue\*(C'\fR function or method.
.PP
If the \s-1LVALUE\s0 has any \s-1GET\s0 magic associated with it (including a \f(CW\*(C`FETCH\*(C'\fR
method of a tied scalar) then this will be executed exactly once when the
\&\f(CW\*(C`dynamically\*(C'\fR expression is evaluated.
.PP
If the \s-1LVALUE\s0 has any \s-1SET\s0 magic associated with it (including a \f(CW\*(C`STORE\*(C'\fR
method of a tied scalar) then this will be executed exactly once when the
\&\f(CW\*(C`dynamically\*(C'\fR expression is evaluated, and again a second time when the
controlling scope is unwound.
.PP
When the \s-1LVALUE\s0 being assigned to is a hash element, e.g. one of the following
forms
.PP
.Vb 2
\&   dynamically $hash{key} = EXPR;
\&   dynamically $href\->{key} = EXPR;
.Ve
.PP
the assignment additionally ensures to remove the key if it is newly-added,
and restores by adding the key back again if it had been deleted in the
meantime.
.SH "WITH Future::AsyncAwait"
.IX Header "WITH Future::AsyncAwait"
As of Future::AsyncAwait version 0.32, cross-module integration tests
assert that the \f(CW\*(C`dynamically\*(C'\fR correctly works across an \f(CW\*(C`await\*(C'\fR boundary.
.PP
.Vb 2
\&   use Future::AsyncAwait;
\&   use Syntax::Keyword::Dynamically;
\&
\&   our $var;
\&
\&   async sub trial
\&   {
\&      dynamically $var = "value";
\&
\&      await func();
\&
\&      say "Var is still $var";
\&   }
.Ve
.PP
When context-switching between scopes in which a variable is \f(CW\*(C`dynamically\*(C'\fR
modified, the value of the variable will be swapped in and out, possibly
multiple times if necessary, to ensure the visible value remains as expected.
.SH "AUTHOR"
.IX Header "AUTHOR"
Paul Evans <leonerd@leonerd.org.uk>