Experimental features now warn (reaching back to v5.10)

Perl 5.18 provides a new way to introduce experimental features in a program, augmenting the feature pragma that v5.10 added. This change marks certain broken v5.10 features as experimental with an eye toward possible removal from the language.

Smart matching in v5.10 led to several broken and conflated features. The given used a lexical version of $_, which broke many other common uses of that variable inside the given, which I explain in Use for() instead of given() and you can see in given/when and lexical $_ ….

Under v5.18, when you use given, when, or ~~, you get a warning, even if there is no smart match involved:

# given_warning.pl
use v5.10; # earliest occurance of feature
for( 'Buster' ) { 
    when( 1 == 1 ) { say "Hello" } 
    }

These warnings might cause test suites to fail when people try to install modules on the new perl, like it does for Unicode::Tussle.

% perl5.10.1 given_warning.pl
Hello
% perl5.18.0 given_warning.pl
when is experimental at given_warning.pl line 4.
Hello

Using the diagnostics shows the sort of warning it is:

% perl5.18.0 -Mdiagnostics given_warning.pl
when is experimental at -e line 1 (#1)
    (S experimental::smartmatch) when depends on smartmatch, which is
    experimental.  Additionally, it has several special cases that may
    not be immediately obvious, and their behavior may change or
    even be removed in any future release of perl.
    See the explanation under "Experimental Details on given and when"
    in perlsyn.
    
Hello

To get rid of this warning, you do the same thing you do with other warnings. Take the category of the warning and turn it off with no (Item 100: Use lexical warnings to selectively turn on or off complaints):

# given_warning.pl
use v5.10; # earliest occurance of feature
no warnings 'experimental::smartmatch';
for( 'Buster' ) { 
    when( 1 == 1 ) { say "Hello" } 
    }

The lexical $_ is another broken fature that’s now marked as experimental.

# lexical_.pl
use v5.10;

sub cat { my $_ }

Any use in v5.18 gives a warning:

% perl5.18.0 lexical_.pl
Use of my $_ is experimental at lexcial_.pl line 3.

The category is different:

% perl5.18.0 -Mdiagnostics lexical_.pl
Use of my $_ is experimental at lexcial_.pl line 4 (#1)
    (S experimental::lexical_topic) Lexical $_ is an experimental 
    feature and its behavior may change or even be removed in any 
    future release of perl. See the explanation under "$_" in perlvar.

That takes care of the two retro features. Perl v5.18 introduces two new experimental features, set logic in character classes (for complete Unicode Level 1 regular expression compliance), and lexical subroutines, which I’ll cover in other items.

# regex.pl
use v5.18;

print "Match" if 'foo' =~ /(?[ \p{Thai} & \p{Digit} ])/;

Without turning off the warning, perl knows about the feature and points it out:

% perl5.18.0 regex.pl
The regex_sets feature is experimental in regex; marked by <-- HERE in m/(?[ <-- HERE  \p{Thai} & \p{Digit} ])/ at regex.pl line 4.

In this case, diagnostics is not any help:

% perl5.18.0 -Mdiagnostics regex.pl
The regex_sets feature is experimental in regex; marked by <-- HERE in m/(?[
        <-- HERE  \p{Thai} & \p{Digit} ])/ at regex.pl line 3 (#1)
The regex_sets feature is experimental in regex; marked by <-- HERE in m/(?[ <-- HERE  \p{Thai} & \p{Digit} ])/ at regex.pl line 3.

For lexical named subroutines, you have explicitly enable the feature but you then have to explicitly turn off its warnings.

# lexical_sub.pl
use v5.18;
no warnings 'experimental::lexical_subs';
use feature "lexical_subs";

my sub foo { say "Hello" }

Handling older perls

In v5.18, that's all fine and good, but older versions don't understand those warnings categories and will stop your program.

% perl5.10.1 -e 'no warnings qw(smartmatch)'
Unknown warnings category 'smartmatch' at -e line 1
BEGIN failed--compilation aborted at -e line 1.

Instead of using warnings, you can use the non-core (until v5.20) experimental module that handles that for you:

use experimental qw(smartmatch);

For versions without that warning category, nothing happens. For versions with that feature, it turns off the warning.

However, this is an additional dependency prior to v5.20. You should limit the features you use to the minimum version of Perl you allow.

Summary

This table summarizes the new experimental warnings categories and the features they affect.

Category Features
experimental::smartmatch given, when, ~~
experimental::lexical_topic my $_
experimental::regex_sets (?[ ])
experimental::lexical_subs my sub NAME {}, our sub NAME {}

Things to remember

  • Some v5.10 features now warn under v5.18
  • Some new experimental features must be explicitly enabled
  • Even explicitly enabled features still warn
  • The experimental module is version safe
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