Daftar Isi

• NAME
• SYNOPSIS
• DESCRIPTION
  • Lexical effect
  • no feature
• AVAILABLE FEATURES
  • The 'say' feature
  • The 'state' feature
  • The 'switch' feature
  • The 'unicode_strings' feature
  • The 'unicode_eval' and 'evalbytes' features
  • The 'current_sub' feature
  • The 'array_base' feature
  • The 'fc' feature
• FEATURE BUNDLES
• IMPLICIT LOADING

NAME

feature - Perl pragma to enable new features

SYNOPSIS

 use feature qw(say switch);
 
 given ($foo) {
 
 when (1)  { say "\$foo == 1" }
 
 when ([2,3])  { say "\$foo == 2 || \$foo == 3" }
 
 when (/^a[bc]d$/) { say "\$foo eq 'abd' || \$foo eq 'acd'" }
 
 when ($_ > 100)   { say "\$foo > 100" }
 
 default   { say "None of the above" }
 
 }
 
 
 use feature ':5.10'; # loads all features available in perl 5.10
 
 
 use v5.10;   # implicitly loads :5.10 feature bundle
 

DESCRIPTION

It is usually impossible to add new syntax to Perl without breakingsome existing programs. This pragma provides a way to minimize thatrisk. New syntactic constructs, or new semantic meanings to olderconstructs, can be enabled by use feature 'foo', and will be parsedonly when the appropriate feature pragma is in scope. (Nevertheless, theCORE:: prefix provides access to all Perl keywords, regardless of thispragma.)

Lexical effect

Like other pragmas (use strict, for example), features have a lexicaleffect. use feature qw(foo) will only make the feature "foo" availablefrom that point to the end of the enclosing block.

 {
 
 use feature 'say';
 
 say "say is available here";
 
 }
 
 print "But not here.\n";
 

no feature

Features can also be turned off by using no feature "foo". This toohas lexical effect.

 use feature 'say';
 
 say "say is available here";
 
 {
 
 no feature 'say';
 
 print "But not here.\n";
 
 }
 
 say "Yet it is here.";
 

no feature with no features specified will reset to the default group. Todisable all features (an unusual request!) use no feature ':all'.

AVAILABLE FEATURES

The 'say' feature

use feature 'say' tells the compiler to enable the Perl 6 stylesay function.

See say for details.

This feature is available starting with Perl 5.10.

The 'state' feature

use feature 'state' tells the compiler to enable statevariables.

See Persistent Private Variables in perlsub for details.

This feature is available starting with Perl 5.10.

The 'switch' feature

use feature 'switch' tells the compiler to enable the Perl 6given/when construct.

See Switch Statements in perlsyn for details.

This feature is available starting with Perl 5.10.

The 'unicode_strings' feature

use feature 'unicode_strings' tells the compiler to use Unicode semanticsin all string operations executed within its scope (unless they are alsowithin the scope of either use locale or use bytes). The same appliesto all regular expressions compiled within the scope, even if executed outsideit.

no feature 'unicode_strings' tells the compiler to use the traditionalPerl semantics wherein the native character set semantics is used unless it isclear to Perl that Unicode is desired. This can lead to some surpriseswhen the behavior suddenly changes. (SeeThe Unicode Bug in perlunicode for details.) For this reason, if you arepotentially using Unicode in your program, theuse feature 'unicode_strings' subpragma is strongly recommended.

This feature is available starting with Perl 5.12; was almost fullyimplemented in Perl 5.14; and extended in Perl 5.16 to cover quotemeta.

The 'unicode_eval' and 'evalbytes' features

Under the unicode_eval feature, Perl's eval function, when passed astring, will evaluate it as a string of characters, ignoring anyuse utf8 declarations. use utf8 exists to declare the encoding ofthe script, which only makes sense for a stream of bytes, not a string ofcharacters. Source filters are forbidden, as they also really only makesense on strings of bytes. Any attempt to activate a source filter willresult in an error.

The evalbytes feature enables the evalbytes keyword, which evaluatesthe argument passed to it as a string of bytes. It dies if the stringcontains any characters outside the 8-bit range. Source filters workwithin evalbytes: they apply to the contents of the string beingevaluated.

Together, these two features are intended to replace the historical evalfunction, which has (at least) two bugs in it, that cannot easily be fixedwithout breaking existing programs:

• eval behaves differently depending on the internal encoding of thestring, sometimes treating its argument as a string of bytes, and sometimesas a string of characters.

• Source filters activated within eval leak out into whichever filescope is currently being compiled. To give an example with the CPAN moduleSemi::Semicolons:

   BEGIN { eval "use Semi::Semicolons;  # not filtered here " }
   
   # filtered here!
   

  evalbytes fixes that to work the way one would expect:

   use feature "evalbytes";
   
   BEGIN { evalbytes "use Semi::Semicolons;  # filtered " }
   
   # not filtered
   

These two features are available starting with Perl 5.16.

The 'current_sub' feature

This provides the __SUB__ token that returns a reference to the currentsubroutine or undef outside of a subroutine.

This feature is available starting with Perl 5.16.

The 'array_base' feature

This feature supports the legacy $[ variable. See $[ in perlvar andarybase. It is on by default but disabled under use v5.16 (seeIMPLICIT LOADING, below).

This feature is available under this name starting with Perl 5.16. Inprevious versions, it was simply on all the time, and this pragma knewnothing about it.

The 'fc' feature

use feature 'fc' tells the compiler to enable the fc function,which implements Unicode casefolding.

See fc for details.

This feature is available from Perl 5.16 onwards.

FEATURE BUNDLES

It's possible to load multiple features together, usinga feature bundle. The name of a feature bundle is prefixed witha colon, to distinguish it from an actual feature.

  use feature ":5.10";
 

The following feature bundles are available:

  bundle features included
 
  --------- -----------------
 
  :default  array_base
 
 
  :5.10 say state switch array_base
 
 
  :5.12 say state switch unicode_strings array_base
 
 
  :5.14 say state switch unicode_strings array_base
 
 
  :5.16 say state switch unicode_strings
 
 unicode_eval evalbytes current_sub fc
 

The :default bundle represents the feature set that is enabled beforeany use feature or no feature declaration.

Specifying sub-versions such as the 0 in 5.14.0 in feature bundles hasno effect. Feature bundles are guaranteed to be the same for all sub-versions.

  use feature ":5.14.0"; # same as ":5.14"
 
  use feature ":5.14.1"; # same as ":5.14"
 

IMPLICIT LOADING

Instead of loading feature bundles by name, it is easier to let Perl doimplicit loading of a feature bundle for you.

There are two ways to load the feature pragma implicitly:

• By using the -E switch on the Perl command-line instead of -e.That will enable the feature bundle for that version of Perl in themain compilation unit (that is, the one-liner that follows -E).

• By explicitly requiring a minimum Perl version number for your program, withthe use VERSION construct. That is,

   use v5.10.0;
   

  will do an implicit

   no feature ':all';
   
   use feature ':5.10';
   

  and so on. Note how the trailing sub-versionis automatically stripped from theversion.

  But to avoid portability warnings (see use), you may prefer:

   use 5.010;
   

  with the same effect.

  If the required version is older than Perl 5.10, the ":default" featurebundle is automatically loaded instead.