class Routine is Block { }

A Routine is a code object meant for units of code larger than Block. Routine is the common superclass for Sub (and therefore operators) and Method, the two primary code objects for code reuse, as well as Macro and Submethod.

Routines serve as a scope limiter for return (i.e. a return returns from the innermost outer Routine).

The routine level is also the one at which multiness (multi subs and multi methods) are handled. Subroutines can also be declared anon. See the documentation on the anon declarator for more information.

Routines are the lowest objects in the Code hierarchy that can introspect through the &?ROUTINE variable, which is defined automatically, and contains the corresponding Routine object.

class Foo {
    submethod bar { &?ROUTINE.^name }
};
say Foo.bar; # OUTPUT: «Submethod␤» 

Methods

method name

method name(Routine:D: --> Str:D)

Returns the name of the sub or method.

method package

method package(Routine:D:)

Returns the package in which the routine is defined.

method multi

method multi(Routine:D: --> Bool:D)

Returns True if the routine is a multi sub or method. Note that the name of a multi sub refers to its proto and this method would return false if called on it. It needs to be called on the candidates themselves:

multi foo ($, $) {};
say &foo.multi;             # OUTPUT: «False␤» 
say &foo.candidates».multi; # OUTPUT: «(True)␤»

method candidates

method candidates(Routine:D: --> Positional:D)

Returns a list of multi candidates, or a one-element list with itself if it's not a multi

method cando

method cando(Capture $c)

Returns a possibly-empty list of candidates that can be called with the given Capture, ordered by narrowest candidate first. For methods, the first element of the Capture needs to be the invocant:

.signature.say for "foo".^can("comb")[0].cando: \(Cool, "o");
# OUTPUT: «(Cool $: Str $matcher, $limit = Inf, *%_)␤»

method wrap

method wrap(Routine:D: &wrapper)

Wraps (i.e. in-place modifies) the routine. That means a call to this routine first calls &wrapper, which then can (but doesn't have to) call the original routine with the callsame, callwith, nextsame and nextwith dispatchers. The return value from the routine is also the return value from the wrapper.

wrap returns an instance of a private class called Routine::WrapHandle, which you can pass to unwrap to restore the original routine.

method unwrap

method unwrap($wraphandle)

Restores the original routine after it has been wrapped with wrap. While the signature allows any type to be passed, only the Routine::WrapHandle type returned from wrap can usefully be.

method is-wrapped

method is-wrapped()

Returns True or False, depending on the whether or not the Routine is wrapped.

method yada

method yada(Routine:D: --> Bool:D)

Returns True if the routine is a stub

say (sub f() { ... }).yada;      # OUTPUT: «True␤» 
say (sub g() { 1;  }).yada;      # OUTPUT: «False␤»

trait is cached

multi sub trait_mod:<is>(Routine $r, :$cached!)

Causes the return value of a routine to be stored, so that when subsequent calls with the same list of arguments are made, the stored value can be returned immediately instead of re-running the routine.^[1]

Useful when storing and returning the computed value is much faster than re-computing it every time, and when the time saved trumps the cost of the use of more memory.

Even if the arguments passed to the routine are "reference types" (such as objects or arrays), then for the purpose of caching they will only be compared based on their contents. Thus the second invocation will hit the cache in this case:

say foo( [1, 2, 3] );   # runs foo 
say foo( [1, 2, 3] );   # doesn't run foo, uses cached value 

Since it's still at the experimental stage, you will have to insert the use experimental :cached; statement in any module or script that uses it.

use experimental :cached;
 
sub nth-prime(Int:D $x where * > 0) is cached {
    say "Calculating {$x}th prime";
    return (2..*).grep(*.is-prime)[$x - 1];
}
 
say nth-prime(43);
say nth-prime(43);
say nth-prime(43);

produces this output:

Calculating 43th prime
191
191
191

Marks something as deprecated

trait is pure

multi sub trait_mod:<is>(Routine $r, :$pure!)

Marks a subroutine as pure, that is, it asserts that for the same input, it will always produce the same output without any additional side effects.

The is pure trait is a promise by the programmer to the compiler that it can constant-fold calls to such functions when the arguments are known at compile time.

sub syllables() is pure {
    say "Generating syllables";
    my @vowels = <a e i o u>;
    return  @vowels.append: <k m n sh d r t y> X~ @vowels;
}

You can mark function as pure even if they throw exceptions in edge cases or if they modify temporary objects; hence the is pure trait can cover cases that the compiler cannot deduce on its own. On the other hand, you might not want to constant-fold functions that produce a large return value (such as the string or list repetition operators, infix x and xx) even if they are pure, to avoid large precompilation files.

To see it an action with a particular compiler you can try this example:

BEGIN { say ‘Begin’ }
say ‘Start’;
say (^100).map: { syllables().pick(4).join("") };
 
 
# Example output: 
# Begin 
# Generating syllables 
# Start 
# (matiroi yeterani shoriyuru... 

Essentially this allows the compiler to perform some operations at compile time. The benefits of constant-folding may include better performance, especially in cases when the folded code is precompiled.

In addition, using a pure function or operator in sink context (that is, where the result is discarded) may lead to a warning. The code

sub double($x) is pure { 2 * $x };
double(21);
say "anything";
# WARNING: «Useless use of "double(21)" in expression "double(21)" in sink context (line 2)»

If you want to apply this trait to a multi, you need to apply it to the proto; it will not work otherwise, at least in versions 2018.08 and below.

trait is rw

multi sub trait_mod:<is>(Routine $r, :$rw!)

When a routine is modified with this trait, its return value will be writable. This is useful when returning variables or writable elements of hashes or arrays, for example:

sub walk(\thing, *@keys) is rw {
    my $current := thing;
    for @keys -> $k {
        if $k ~~ Int {
            $current := $current[$k];
        }
        else {
            $current := $current{$k};
        }
    }
    $current;
}
 
my %hash;
walk(%hash, 'some', 'key', 1, 2) = 'autovivified';
 
say %hash.raku;

produces

("some" => {"key" => [Any, [Any, Any, "autovivified"]]}).hash

Note that return marks return values as read only; if you need an early exit from an is rw routine, you have to use return-rw instead.

trait is export

multi sub trait_mod:<is>(Routine $r, :$export!)

Marks a routine as exported to the rest of the world

module Foo {
    sub double($x) <strong>is export</strong> {
        2 * $x
    }
}
 
import Foo;         # makes sub double available 
say double 21;      # 42 

From inside another file you'd say use Foo; to load a module and import the exported functions.

See Exporting and Selective Importing Modules for more details.

trait is DEPRECATED

multi sub trait_mod:<is>(Routine:D $r, :$DEPRECATED!)

Marks a Routine as deprecated; that is, it should no longer be used going forward, and will eventually be removed. An optional message specifying the replacement functionality can be specified

By having both the original (deprecated) and new Routine available simultaneously, you can avoid a breaking change in a single release, by allowing users time and instructions on how to update their code. Remove the deprecated version only after at least one release that includes both the warning and the new Routine.

This code

sub f() is DEPRECATED('the literal 42') { 42 }
say f();

produces this output:

42
Saw 1 occurrence of deprecated code.
================================================================================
Sub f (from GLOBAL) seen at:
  deprecated.p6, line 2
Please use the literal 42 instead.
--------------------------------------------------------------------------------
Please contact the author to have these occurrences of deprecated code
adapted, so that this message will disappear!

trait is hidden-from-backtrace

multi sub trait_mod:<is>(Routine:D, :$hidden-from-backtrace!)

Hides a routine from showing up in a default backtrace. For example

sub inner { die "OH NOEZ" };
sub outer { inner() };
outer();

produces the error message and backtrace

OH NOEZ
  in sub inner at bt.p6:1
  in sub outer at bt.p6:2
  in block <unit> at bt.p6:3

but if inner is marked with hidden-from-backtrace

sub inner is hidden-from-backtrace { die "OH NOEZ" };
sub outer { inner() };
outer();

the error backtrace does not show it:

OH NOEZ
  in sub outer at bt.p6:2
  in block <unit> at bt.p6:3

trait is default

multi sub trait_mod:<is>(Routine:D $r, :$default!)

There is a special trait for Routines called is default. This trait is designed as a way to disambiguate multi calls that would normally throw an error because the compiler would not know which one to use. This means that given the following two Routines, the one with the is default trait will be called.

multi sub f() is default { say "Hello there" }
multi sub f() { say "Hello friend" }
f();   # OUTPUT: «"Hello there"␤»

The is default trait can become very useful for debugging and other uses but keep in mind that it will only resolve an ambiguous dispatch between two Routines of the same precedence. If one of the Routines is narrower than another, then that one will be called. For example:

multi sub f() is default { say "Hello there" }
multi sub f(:$greet) { say "Hello " ~ $greet }
f();   # "Use of uninitialized value $greet..." 

In this example, the multi without is default was called because it was actually narrower than the Sub with it.

trait is raw

multi sub trait_mod:<is>(Routine:D $r, :$raw!)

Gives total access to the data structure returned by the routine.

my @zipi = <zape zapatilla>;
sub þor() is raw {
    return @zipi
};
þor()[1] = 'pantuflo';
say @zipi;  # OUTPUT: «[zape pantuflo]␤» 

trait is test-assertion

multi sub trait_mod:<is>(Routine:D, :$test-assertion!)

Declares that a routine generates test output (aka TAP). When failures are reported, the calling routine's location is used instead of this routine. For example:

use Test;
sub foo-test($value) is test-assertion {
    is $value, 42, "is the value 42?";
}
foo-test(666);    # <-- error is reported on this line