Features

You want to use say in your program.

• You can specify

  use feature qw{ say };
  

• You can also search the documentation (feature) to find out the first Perl version that supports it:

  use feature ':5.10';
  
  # or
  
  use v5.10;
  

• Or, you can stuff everything to a one-liner with -E.
  

Features (2)

• feature is lexically scoped.
• feature doesn't work for pre-5.10 Perls.
• Using a version enables all the features available in the given version.

Non-features

You want to use the defined-or operator (//).

• perlop doesn't tell you what Perl version you need
• You have to search through perldelta's to find out
• The correct answer is 5.10 (see perl5100delta). Some people add a comment explaining why

  use v5.10;  # defined-or //
  

• You can also specify 5.18. Users of 5.10 .. 5.16 will be out of luck.
• If you are too lazy (you are, aren't you?), you can omit any declaration. 5.8 users will get strange errors.

  Search pattern not terminated at ...
  Scalar found where operator expected at ...

Non-features (2)

But...

• Branch reset pattern (?|...) isn't documented in any perldelta.
• Nevertheless, perlre says:

  It is available starting from Perl 5.10.0.

Non-features (3)

Well, branch reset is an exception. Who would ever use it, anyway?

• The preserve regex option is not documented in any perldelta.

  $string =~ /:/p;
  my ($before, $after) = ( ${^PREMATCH}, ${^POSTMATCH} );
  

• In perlre, we can read

  ... Perl 5.10.0 introduces...

Solution?

#!/usr/bin/perl
use warnings;
use strict;

use Syntax::Construct qw(  //  ?|  );

Benefits for

• an author: no more searching, everything in one place (i.e. Syntax::Construct's documentation)
• their users: meaningful error message containing required Perl version
• another programmer: what construct to replace

Removals

5.24 will be the first Perl version to remove a non-feature.

my $h = { a => 12, b => 14 };
print keys $h;

Experimental keys on scalar is now forbidden at ...

Removals

5.24 will be the first Perl version to remove a non-feature.

my $h = { a => 12, b => 14 };
print keys $h;

Experimental keys on scalar is now forbidden at ...

But after prepending

use Syntax::Construct qw{ auto-deref };

The error message in 5.23.3 is different:

Faking version 5.024 to test removals.
auto-deref removed in 5.024 at ...
BEGIN failed--compilation aborted at ...

The Name

What's in a name? that which we call a rose
By any other name would smell as sweet

• Some non-features are rather semantic than syntactic (length-undef, computed-labels)
• Unfortunately, the most fitting name is already taken: feature

The Implementation

Very simple.

We store all the non-features in a hash with their corresponding minimal Perl versions. In the import method, we just require the maximal version in a block eval.

UTSL

Testing

Each construct is

• unique (and exists in one version only)
• implemented
• documented
• tested

See GitHub.

Should Syntax::Construct Replace feature?

• No.
• Syntax::Construct must stay non-CORE to be able to tell you about newer Perl versions.

Current Status

• CPAN
• github
• Problems: Links from the documentation to sections in Perl documentation on CPAN / MetaCPAN