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Author image Paul Evans


Syntax::Keyword::Dynamically - dynamically change the value of a variable


   use Syntax::Keyword::Dynamically;

   my $logger = ...;

   sub operate
      dynamically $logger->level = LOG_DEBUG;



This module provides a syntax plugin that implements a single keyword, dynamically, which alters the behaviour of a scalar assignment operation. Syntactically and semantically it is similar to the built-in perl keyword local, but is implemented somewhat differently to give two key advantages over regular local:

  • You can dynamically assign to lvalue functions and accessors.

  • You can dynamically assign to regular lexical variables.

Semantically, the behaviour can be considered equivalent to

      my $old = $VAR;
      $VAR = "new value";


      $VAR = $old;

Except that the old value will also be restored in the case of exceptions, goto, next/last/redo or similar ways to leave the controlling block scope.



      dynamically LVALUE = EXPR;

The dynamically 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.

The LVALUE 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 :lvalue function or method.

If the LVALUE has any GET magic associated with it (including a FETCH method of a tied scalar) then this will be executed exactly once when the dynamically expression is evaluated.

If the LVALUE has any SET magic associated with it (including a STORE method of a tied scalar) then this will be executed exactly once when the dynamically expression is evaluated, and again a second time when the controlling scope is unwound.

When the LVALUE being assigned to is a hash element, e.g. one of the following forms

   dynamically $hash{key} = EXPR;
   dynamically $href->{key} = EXPR;

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.

WITH Future::AsyncAwait

As of Future::AsyncAwait version 0.32, cross-module integration tests assert that the dynamically correctly works across an await boundary.

   use Future::AsyncAwait;
   use Syntax::Keyword::Dynamically;

   our $var;

   async sub trial
      dynamically $var = "value";

      await func();

      say "Var is still $var";

When context-switching between scopes in which a variable is dynamically 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.


Paul Evans <leonerd@leonerd.org.uk>