Pragmas

Special modules for special use

In Perl 6, pragmas are directive used to either identify a specific version of Perl 6 to be used or to modify the compiler's normal behavior in some way. The use keyword enables a pragma (similar to how you can use a module). To disable a pragma, use the no keyword:

use v6.c;   # use 6.c language version 
no worries# don't issue compile time warnings 

Following is a list of pragmas with a short description of each pragma's purpose or a link to more details about its use. (Note: Pragmas marked "[NYI]" are not yet implemented, and those marked "[TBD]" are to be defined later.)

v6.x

This pragma states the version of the compiler that is going to be used, and turns on its features if they are optional.

    use v6;   # Load latest supported version (non-PREVIEW). 
              # Also, useful for producing better errors when accidentally 
              # executing the program with `perl` instead of `perl6` 
    use v6.c;         # Use the "Christmas" version of Perl 6 
    use v6.d;         # Use the "Diwali" version of Perl 6 
    use v6.d.PREVIEW# On 6.d-capable compilers, enables 6.d features, 
                      # otherwise enables the available experimental 
                      # preview features for 6.d language 

Since these pragmas turn on the compiler version, they should be the first statement in the file (preceeding comments and Pod are fine).

MONKEY-GUTS

This pragma is not currently part of any Perl 6 specification, but is present in Rakudo as a synonym to use nqp (see below).

MONKEY-SEE-NO-EVAL

EVAL

MONKEY-TYPING

augment

MONKEY

use MONKEY;

Turns on all available MONKEY pragmas, currently the three above; thus, it would be equivalent to

use MONKEY-TYPING;
use MONKEY-SEE-NO-EVAL;
use MONKEY-GUTS;

experimental

Allows use of experimental features

fatal

A lexical pragma that makes Failures returned from routines fatal. For example, prefix + on a Str coerces it to Numeric, but will return a Failure if the string contains non-numeric characters. Saving that Failure in a variable prevents it from being sunk, and so the first code block below reaches the say $x.^name; line and prints Failure in output.

In the second block, the use fatal pragma is enabled, so the say line is never reached because the Exception contained in the Failure returned from prefix + gets thrown and the CATCH block gets run, printing the Caught... line. Note that both blocks are the same program and use fatal only affects the lexical block it was used in:

{
    my $x = +"a";
    say $x.^name;
    CATCH { default { say "Caught {.^name}" } }
} # OUTPUT: «Failure␤» 
 
{
    use fatal;
    my $x = +"a";
    say $x.^name;
    CATCH { default { say "Caught {.^name}" } }
} # OUTPUT: «Caught X::Str::Numeric␤» 

Inside try blocks, the fatal pragma is enabled by default, and you can disable it with no fatal:

try {
    my $x = +"a";
    say $x.^name;
    CATCH { default { say "Caught {.^name}" } }
} # OUTPUT: «Caught X::Str::Numeric␤» 
 
try {
    no fatal;
    my $x = +"a";
    say $x.^name;
    CATCH { default { say "Caught {.^name}" } }
} # OUTPUT: «Failure␤» 

internals

[NYI]

invocant

[NYI]

isms

[2018.09 and later]

Allow for some other language constructs that were deemed to be a trap that warranted a warning and/or an error in normal Perl 6 programming. Currently, Perl5 and C++ are allowed.

    sub abs() { say "foo" }
    abs;
    # Unsupported use of bare "abs"; in Perl 6 please use .abs if you meant 
    # to call it as a method on $_, or use an explicit invocant or argument, 
    # or use &abs to refer to the function as a noun 

In this case, providing an abs sub that doesn't take any arguments, did not make the compilation error go away.

use isms <Perl5>;
sub abs() { say "foo" }
abs;   # foo 

With this, the compiler will allow the offending Perl 5 construct, allowing the code to actually be executed.

If you do not specify any language, all known language constructs are allowed.

use isms;   # allow for Perl5 and C++ isms 

lib

This pragma adds subdirectories to the library search path so that the interpreter can find the modules.

use lib <lib /opt/lib /usr/local/lib>;

This will search the directories passed in a list. Please check the modules documentation for more examples.

newline

Set the value of the $?NL constant in the scope it is called. Possible values are :lf (which is the default, indicating Line Feed), :crlf (indicating Carriage Return, Line Feed) and :cr (indicating Carriage Return).

nqp

Use at your own risk.

This is a Rakudo-specific pragma. With it, Rakudo provides access to the nqp opcodes in a top level namespace:

use nqp;
nqp::say("hello world");

This uses the underlying nqp say opcode instead of the Perl 6 routine. This pragma may make your code rely on a particular version of nqp, and since that code is not part of the Perl 6 specification, it's not guaranteed to be stable. You may find a large number of usages in the Rakudo core, which are used to make the core functionality as fast as possible. Future optimizations in the code generation of Rakudo may obsolete these usages.

parameters

[NYI]

precompilation

The default allows precompilation of source code, specifically if used in a module. If for whatever reason you do not want the code (of your module) to be precompiled, you can use no precompilation. This will prevent the entire compilation unit (usually a file) from being precompiled.

soft

Re-dispatching, inlining

strict

strict is the default behavior, and requires that you declare variables before using them. You can relax this restriction with no.

no strict$x = 42# OK 

trace

When use trace is activated, any line of code executing will be written to STDERR. You can use no trace to switch off the feature, so this only happens for certain sections of code.

v6

Writing Tests

variables

Defined Variables Pragma

worries

Lexically controls whether compile-time warnings generated by the compiler get shown. Enabled by default.

$ perl6 -e 'say :foo<>.Pair'
Potential difficulties:
  Pair with <> really means an empty listnot null stringuse :foo(''to represent the null string,
    or :foo() to represent the empty list more accurately
  at -e:1
  ------> say :foo<>.Pair
foo => Nil
 
$ perl6 -e 'no worries; say :foo<>.Pair'
foo => Nil