Pragmas
In Raku, pragmas are directive used to either identify a specific
version of Raku 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.
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.
Since these pragmas turn on the compiler version, they should be the first statement in the file (preceding comments and Pod are fine).
use v6; # Load latest supported version (non-PREVIEW).
Historically, use v6; was intended to signal to Perl interpreters to
error out and warn the user to use Perl 6. This is no longer a consideration,
so this statement has therefore become meaningless.
use v6.c; # Use the "Christmas" version of Raku
use v6.d; # Use the "Diwali" version of Raku
From 2018.11, which implemented 6.d, this pragma does not do anything.
use v6.d.PREVIEW; # On 6.d-capable compilers, enables 6.d features,
# otherwise enables the available experimental
# preview features for 6.d language
use v6.e.PREVIEW; # Enables the available experimental preview features
# for 6.e language
use v6.*; # Enables the available experimental preview features
# for the language version currently in development
MONKEY-GUTS
This pragma is not currently part of any Raku specification, but is present
in Rakudo as a synonym to use nqp (see below).
MONKEY-SEE-NO-EVAL
MONKEY-TYPING
MONKEY
use MONKEY;
Turns on all available MONKEY pragmas, equivalent to
use MONKEY-TYPING;
use MONKEY-SEE-NO-EVAL;
use MONKEY-GUTS;
dynamic-scope
Applies the is dynamic trait to variables in the pragma's lexical scope. The effect can be restricted to a subset of variables by listing their names as arguments. By default applies to all variables.
# Apply `is dynamic` only to $x, but not to $y
use dynamic-scope <$x>;
sub poke {
say $CALLER::x;
say $CALLER::y;
}
my $x = 23;
my $y = 34;
poke;
# OUTPUT:
# 23
# Cannot access '$y' through CALLER, because it is not declared as dynamic
This pragma is not currently part of any Raku specification and was added in Rakudo 2019.03.
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»
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 Raku programming. Currently,
Perl and C++ are allowed.
sub abs() { say "foo" }
abs;
# Unsupported use of bare "abs"; in Raku 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 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 Perl 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>;
#or a mixed list of IO::Path and Str
use lib ('.', '.'.IO, './lib'.IO);
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 Raku routine. This
pragma may make your code rely on a particular version of nqp, and since
that code is not part of the Raku 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.
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
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
variables
worries
Lexically controls whether compile-time warnings generated by the compiler get shown. Enabled by default.
$ raku -e 'say :foo<>.Pair'
Potential difficulties:
Pair with <> really means an empty list, not null string; use :foo('') to represent the null string,
or :foo() to represent the empty list more accurately
at -e:1
------> say :foo<>⏏.Pair
foo => Nil
$ raku -e 'no worries; say :foo<>.Pair'
foo => Nil