NAME

PerlX::Let - Syntactic sugar for lexical state constants

VERSION

version v0.2.8

SYNOPSIS

  use PerlX::Let;

  {

      let $x = 1;
      let $y = "string";

      if ( ($a->($y} - $x) > ($b->{$y} + $x) )
      {
        something( $y, $x );
      }

  }

DESCRIPTION

This module allows you to define lexical constants using a new let keyword, for example, code such as

  if (defined $arg{username}) {
    $row->update( { username => $arg{username} );
  }

is liable to typos. You could simplify it with

  {
      let $key = "username";

      if (defined $arg{$key}) {
          $row->update( { $key => $arg{$key} );
      }

  }

This is roughly equivalent to using

  use Const::Fast ();

  {
      use feature 'state';

      state $key = "username";

      unless (state $_flag = 0) {
          Const::Fast::_make_readonly( \$key );
          $_flag = 1;
      }

      if (defined $arg{$key}) {
          $row->update( { $key => $arg{$key} );
      }

  }

However, if the value contains a sigil, or (for versions of Perl before 5.28) the value is not a scalar, then this uses a my variable

  use Const::Fast ();

  {
      Const::Fast::const my $key => "username";

      if (defined $arg{$key}) {
          $row->update( { $key => $arg{$key} );
      }
  }

The reason for using state variables is that it takes time to mark a variable as read-only, particularly for deeper data structures. However, the tradeoff for using this is that the variables remain allocated until the process exits.

DEPRECATED SYNTAX

Adding a code block after the let assignment is deprecated:

  let $x = "foo" {
    ...
  }

Instead, put the assignment inside of the block.

Specifying multiple assignments is also deprecated:

  let $x = "foo",
      $y = "bar";

Instead, use multiple let statements.

KNOWN ISSUES

A let assignment will enable the state feature inside of the current context.

The parsing of assignments is rudimentary, and may fail when assigning to another variable or the result of a function. Because of this, you may get unusual error messages for syntax errors, e.g. "Transliteration pattern not terminated".

Because this modifies the source code during compilation, the line numbers may be changed, particularly if the let assignment(s) are on multiple lines.

SEE ALSO

feature

Const::Fast

Keyword::Simple

AUTHOR

Robert Rothenberg <rrwo@cpan.org>

COPYRIGHT AND LICENSE

This software is Copyright (c) 2019-2020 by Robert Rothenberg.

This is free software, licensed under:

  The Artistic License 2.0 (GPL Compatible)

cpanm PerlX::Let

perl -MCPAN -e shell
install PerlX::Let