Syntax
Raku borrows many concepts from human language. Which is not surprising, considering it was designed by a linguist.
It reuses common elements in different contexts; it has the notion of nouns (terms) and verbs (operators); it is context-sensitive (in the every day sense, not necessarily in the Computer Science interpretation), so a symbol can have a different meaning depending on whether a noun or a verb is expected.
It is also self-clocking, so that the parser can detect most of the common errors and give good error messages.
Lexical conventions
Raku code is Unicode text. Current implementations support UTF-8 as the input encoding.
See also Unicode versus ASCII symbols.
Free form
Raku code is also free-form, in the sense that you are mostly free to chose the amount of whitespace you use, though in some cases, the presence or absence of whitespace carries meaning.
So you can write
if True {
say "Hello";
}
or
if True {
say "Hello"; # Bad indentation intended
}
or
if True { say "Hello" }
or even
if True {say "Hello"}
though you can't leave out any more whitespace in this last example.
Unspace
In places where the compiler would not allow a space you can use any amount of whitespace, as long as it is quoted with a backslash. Unspaces in tokens, however, are not supported. Newlines that are unspaced still count when the compiler produces line numbers. Use cases for unspace are separation of postfix operators and routine argument lists.
sub alignment(+@l) { +@l };
sub long-name-alignment(+@l) { +@l };
alignment\ (1,2,3,4).say;
long-name-alignment(3,5)\ .say;
say Inf+Inf\i;
In this case, our intention was to make the . of both statements, as
well as the parentheses, align, so we precede the whitespace used for
padding with a \.
Separating statements with semicolons
A Raku program is a list of statements, separated by semicolons ;.
say "Hello";
say "world";
A semicolon after the final statement (or after the final statement inside a block) is optional.
say "Hello";
say "world"
if True {
say "Hello"
}
say "world"
Implied separator rule (for statements ending in blocks)
Complete statements ending in bare blocks can omit the trailing
semicolon, if no additional statements on the same line follow the
block's closing curly brace }. This is called the "implied separator
rule." For example, you don't need to write a semicolon after an if
statement block as seen above, and below.
if True { say "Hello" }
say "world";
However, semicolons are required to separate a block from trailing statements in the same line.
if True { say "Hello" }; say "world";
# ^^^ this ; is required
This implied statement separator rule applies in other ways, besides
control statements, that could end with a bare block. For example, in
combination with the colon : syntax for method calls.
my @names = <Foo Bar Baz>;
my @upper-case-names = @names.map: { .uc } # OUTPUT: [FOO BAR BAZ]
For a series of blocks that are part of the same if/elsif/else
(or similar) construct, the implied separator rule only applies at the
end of the last block of that series. These three are equivalent:
if True { say "Hello" } else { say "Goodbye" }; say "world";
# ^^^ this ; is required
if True { say "Hello" } else { say "Goodbye" } # <- implied statement separator
say "world";
if True { say "Hello" } # still in the middle of an if/else statement
else { say "Goodbye" } # <- no semicolon required because it ends in a block
# without trailing statements in the same line
say "world";
Comments
Comments are parts of the program text which are only intended for human readers; the Raku compilers do not evaluate them as program text. They are part of the non-ambient code that includes Pod6 text.
Comments count as whitespace in places where the absence or presence of whitespace disambiguates possible parses.
Single-line comments
The most common form of comments in Raku starts with a single hash character
# and goes until the end of the line.
if $age > 250 { # catch obvious outliers
# this is another comment!
die "That doesn't look right"
}
Multi-line / embedded comments
Multi-line and embedded comments start with a hash character, followed by a backtick, and then a Unicode Open Punctuation character, and end with the matching Close Punctuation character. Whitespace is not permitted between the backtick and the punctuation character; it will be treated as a single-line comment. The content can not only span multiple lines, but can also be embedded inline.
if #`( why would I ever write an inline comment here? ) True {
say "something stupid";
}
These comments can extend multiple lines
#`[
And this is how a multi would work.
That says why we do what we do below.
]
say "No more";
Curly braces inside the comment can be nested, so in #`{ a { b } c }, the
comment goes until the very end of the string; this is why if the opening
bracketing character also occurs in the body of the comment, e.g. #`[ This is a
box [ of stuff ] ], it must have a paired closing character, as shown. You may
also use multiple curly braces, such as #`{{ double-curly-brace }}, which
might help disambiguate from nested delimiters. You can embed these comments in
expressions, as long as you don't insert them in the middle of keywords or
identifiers.
Pod comments
Pod syntax can be used for multi-line comments
say "this is code";
=begin comment
Here are several
lines
of comment
=end comment
say 'code again';
Identifiers
Identifiers are grammatical building blocks that may be used to give a name to entities/objects such as constants, variables (e.g. Scalars) and routines (e.g. Subs and Methods). In a variable name, any sigil (and twigil) precedes the identifier and does not form a part thereof.
constant c = 299792458; # identifier "c" names an Int
my $a = 123; # identifier "a" in the name "$a" of a Scalar
sub hello { say "Hello!" }; # identifier "hello" names a Sub
Identifiers come in different forms: ordinary, extended, and compound identifiers.
Ordinary identifiers
An ordinary identifier is composed of a leading alphabetic character
which may be followed by one or more alphanumeric characters. It may also
contain isolated, embedded apostrophes ' and/or hyphens -, provided
that the next character is each time alphabetic.
The definitions of "alphabetic" and "alphanumeric" include appropriate Unicode
characters. Which characters are "appropriate" depends on the implementation.
In the Rakudo/MoarVM Raku implementation alphabetic characters include
characters with the Unicode General Category value Letter (L), and the
underscore _. Alphanumeric characters additionally include characters with
the Unicode General Category value Number, Decimal Digit (Nd).
# valid ordinary identifiers:
x
_snake_oil
something-longer
with-numbers1234
don't-do-that
piece_of_π
駱駝道 # "Rakuda-dō", Japanese for "Way of the camel"
# invalid ordinary identifiers:
42 # identifier does not start with alphabetic character
with-numbers1234-5 # embedded hyphen not followed by alphabetic character
is-prime? # question mark is not alphanumeric
x² # superscript 2 is not alphanumeric (explained above)
Extended identifiers
It is often convenient to have names that contain characters that are not
allowed in ordinary identifiers. Use cases include situations where a set of
entities shares a common "short" name, but still needs for each of its elements
to be identifiable individually. For example, you might use a module whose short
name is Dog, while its long name includes its naming authority and version:
Dog:auth<Somebody>:ver<1.0> # long module names including author and version
Dog:auth<Somebody>:ver<2.0>
use Dog:auth<Somebody>:ver<2.0>;
# Selection of second module causes its full name to be aliased to the
# short name for the rest of # the lexical scope, allowing a declaration
# like this.
my Dog $spot .= new("woof");
Similarly, sets of operators work together in various syntactic categories with
names like prefix, infix and postfix. The official names of these
operators often contain characters that are excluded from ordinary identifiers.
The long name is what constitutes the extended identifier, and includes this
syntactic category; the short name will be included in quotes in the definition:
infix:<+> # the official name of the operator in $a + $b
infix:<*> # the official name of the operator in $a * $b
infix:«<=» # the official name of the operator in $a <= $b
For all such uses, you can append one or more colon-separated strings to an ordinary identifier to create a so-called extended identifier. When appended to an identifier (that is, in postfix position), this colon-separated string generates unique variants of that identifier.
These strings have the form :key<value>, wherein key or value are
optional; that is, after the colon that separates it from a regular identifier,
there will be a key and/or a quoting bracketing construct such as < >, «
» or [' '] which quotes one or more arbitrary characters value.[1]
# exemplary valid extended identifiers:
postfix:<²> # the official long name of the operator in $x²
WOW:That'sAwesome
WOW:That's<<🆒>>
party:sweet<16>
# exemplary invalid extended identifiers:
party:16<sweet> # 16 is not an ordinary identifier
party:16sweet
party:!a # ...and neither is !a
party:$a # ...nor $a
In an extended identifier, the postfix string is considered an integral part of
the name, so infix:<+> and infix:<-> are two different operators. The
bracketing characters used, however, do not count as part of it; only the quoted
data matters. So these are all the same name:
infix:<+>
infix:<<+>>
infix:«+»
infix:['+']
infix:('+')
Similarly, all of this works:
my $foo:bar<baz> = 'quux';
say $foo:bar«baz»; # OUTPUT: «quux»
my $take-me:<home> = 'Where the glory has no end';
say $take-me:['home']; # OUTPUT: «Where [...]»
my $foo:bar<2> = 5;
say $foo:bar(1+1); # OUTPUT: «5»
Where an extended identifier comprises two or more colon pairs, their order is generally significant:
my $a:b<c>:d<e> = 100;
my $a:d<e>:b<c> = 200;
say $a:b<c>:d<e>; # OUTPUT: «100», NOT: «200»
An exception to this rule is module versioning; so these identifiers effectively name the same module:
use ThatModule:auth<Somebody>:ver<2.7.18.28.18>
use ThatModule:ver<2.7.18.28.18>:auth<Somebody>
Furthermore, extended identifiers support compile-time interpolation; this requires the use of constants for the interpolation values:
constant $c = 42; # Constant binds to Int; $-sigil enables interpolation
my $a:foo<42> = "answer";
say $a:foo«$c»; # OUTPUT: «answer»
Although quoting bracketing constructs are generally interchangeable in the
context of identifiers, they are not identical. In particular, angle brackets
< > (which mimic single quote interpolation characteristics) cannot be used
for the interpolation of constant names.
constant $what = 'are';
my @we:<are>= <the champions>;
say @we:«$what»; # OUTPUT: «[the champions]»
say @we:<$what>;
# Compilation error: Variable '@we:<$what>' is not declared
Compound identifiers
A compound identifier is an identifier that is composed of two or more
ordinary and/or extended identifiers that are separated from one another by a
double colon ::.
The double colon :: is known as the namespace separator or the
package delimiter, which clarifies its semantic function in a name: to force
the preceding portion of the name to be considered a
package/namespace through which the subsequent portion of
the name is to be located:
module MyModule { # declare a module package
our $var = "Hello"; # declare package-scoped variable
}
say $MyModule::var # OUTPUT: «Hello»
In the example above, MyModule::var is a compound identifier, composed
of the package name identifier MyModule and the identifier part of the
variable name var. Altogether $MyModule::var is often referred
to as a package-qualified name.
Separating identifiers with double colons causes the rightmost name to be inserted into existing (see above example) or automatically created packages:
my $foo::bar = 1;
say OUR::.keys; # OUTPUT: «(foo)»
say OUR::foo.HOW # OUTPUT: «Perl6::Metamodel::PackageHOW.new»
The last lines shows how the foo package was created automatically, as a
deposit for variables in that namespace.
The double colon syntax enables runtime
interpolation of a string into a
package or variable name using ::($expr) where you'd ordinarily put a package
or variable name:
my $buz = "quux";
my $bur::quux = 7;
say $bur::($buz); # OUTPUT: «7»
term term:<>
You can use term:<> to introduce new terms, which is handy for introducing
constants that defy the rules of normal identifiers:
use Test; plan 1; constant &term:<👍> = &ok.assuming(True);
👍
# OUTPUT: «1..1ok 1 - »
But terms don't have to be constant: you can also use them for functions that don't take any arguments, and force the parser to expect an operator after them. For instance:
sub term:<dice> { (1..6).pick };
say dice + dice;
can print any number between 2 and 12.
If instead we had declared dice as a regular
sub dice() {(1...6).pick }
, the expression dice + dice would be parsed as
dice(+(dice())), resulting in an error since sub dice expects zero
arguments.
Statements and expressions
Raku programs are made of lists of statements. A special case of a statement
is an expression, which returns a value. For example if True { say 42 }
is syntactically a statement, but not an expression, whereas 1 + 2 is an
expression (and thus also a statement).
The do prefix turns statements into expressions. So while
my $x = if True { 42 }; # Syntax error!
is an error,
my $x = do if True { 42 };
assigns the return value of the if statement (here 42) to the variable
$x.
Terms
Terms are the basic nouns that, optionally together with operators, can
form expressions. Examples for terms are variables ($x), barewords
such as type names (Int), literals (42), declarations (sub f() { })
and calls (f()).
For example, in the expression 2 * $salary, 2 and $salary are two
terms (an Int literal and a variable).
Variables
Variables typically start with a special character called the sigil, and are followed by an identifier. Variables must be declared before you can use them.
# declaration:
my $number = 21;
# usage:
say $number * 2;
See the documentation on variables for more details.
Barewords (constants, type names)
Pre-declared identifiers can be terms on their own. Those are typically type
names or constants, but also the term self which refers to an object that
a method was called on (see objects), and sigilless
variables:
say Int; # OUTPUT: «(Int)»
# ^^^ type name (built in)
constant answer = 42;
say answer;
# ^^^^^^ constant
class Foo {
method type-name {
self.^name;
# ^^^^ built-in term 'self'
}
}
say Foo.type-name; # OUTPUT: «Foo»
# ^^^ type name
Packages and qualified names
Named entities, such as variables, constants, classes, modules or subs, are part
of a namespace. Nested parts of a name use :: to separate the hierarchy. Some
examples:
$foo # simple identifiers
$Foo::Bar::baz # compound identifiers separated by ::
$Foo::($bar)::baz # compound identifiers that perform interpolations
Foo::Bar::bob(23) # function invocation given qualified name
See the documentation on packages for more details.
Literals
A literal is a representation of a constant value in source code. Raku has literals for several built-in types, like Str, several numeric types, Pairs and more.
String literals
String literals are surrounded by quotes:
say 'a string literal';
say "a string literal\nthat interprets escape sequences";
See quoting for many more options, including
interpolation quoting qq. Raku uses
the standard escape characters in literals: \0 \a \b \t \n \f \r \e,
with the same meaning as the ASCII escape codes, specified in
the design document.
say "🔔\a"; # OUTPUT: «🔔␇»
Number literals
Number literals are generally specified in base ten (which can be specified
literally, if needed, via the prefix 0d), unless a prefix like 0x
(hexadecimal, base 16), 0o (octal, base 8) or 0b (binary, base 2)
or an explicit base in adverbial notation like :16<A0> specifies it
otherwise. Unlike other programming languages, leading zeros do not indicate
base 8; instead a compile-time warning is issued.
In all literal formats, you can use underscores to group digits, although they don't carry any semantic information; the following literals all evaluate to the same number:
1000000
1_000_000
10_00000
100_00_00
Int literals
Integers default to signed base-10, but you can use other bases. For details, see Int.
# not a single literal, but unary - operator applied to numeric literal 2
-2
12345
0xBEEF # base 16
0o755 # base 8
:3<1201> # arbitrary base, here base 3
Rat literals
Rat literals (rationals) are very common, and take the place of decimals or floats in many other languages. Integer division also results in a Rat.
1. # Error: A number must have at least one digit after the radix point
1.0
3.14159
-2.5 # Not actually a literal, but still a Rat
:3<21.0012> # Base 3 rational
⅔
2/3 # Not actually a literal, but still a Rat
Num literals
Scientific notation with an integer exponent to base ten after an e produces
a Num (a floating point value):
1.e0 # error: A number must have at least one digit after the radix point
1e0
6.022e23
1e-9
-2e48
2e2.5 # error
Complex literals
Complex numbers are written either as an imaginary number
(which is just a rational number with postfix i appended), or as a sum of
a real and an imaginary number:
1.+2i # error: A number must have at least one digit after the radix point
1+2.i # error: A number must have at least one digit after the radix point
1+2i
6.123e5i # note that this is 6.123e5 * i, not 6.123 * 10 ** (5i)
Pair literals
Pairs are made of a key and a value, and there are two
basic forms for constructing them: key => 'value' and
:key('value').
Arrow pairs
Arrow pairs can have an expression, a string literal or a "bare identifier", which is a string with ordinary-identifier syntax that does not need quotes on the left-hand side:
like-an-identifier-ain't-it => 42
"key" => 42
('a' ~ 'b') => 1
Adverbial pairs (colon pairs)
Short forms without explicit values:
my $thing = 42;
:$thing # same as thing => $thing
:thing # same as thing => True
:!thing # same as thing => False
The variable form also works with other sigils, like :&callback or
:@elements. If the value is a number literal, it can also be
expressed in this short form:
:42thing # same as thing => 42
:٤٢thing # same as thing => 42
This order is inverted if you use another alphabet
:٤٢ث # same as ث => ٤٢
the thaa letter precedes the number.
Long forms with explicit values:
:thing($value) # same as thing => $value
:thing<quoted list> # same as thing => <quoted list>
:thing['some', 'values'] # same as thing => ['some', 'values']
:thing{a => 'b'} # same as thing => { a => 'b' }
Boolean literals
True and False are Boolean literals; they will always have initial capital
letter.
Array literals
A pair of square brackets can surround an expression to form an itemized Array literal; typically there is a comma-delimited list inside:
say ['a', 'b', 42].join(' '); # OUTPUT: «a b 42»
# ^^^^^^^^^^^^^^ Array constructor
If the constructor is given a single Iterable, it'll clone and flatten it. If you want an Array with just 1 element that is an Iterable, ensure to use a comma after it:
my @a = 1, 2;
say [@a].raku; # OUTPUT: «[1, 2]»
say [@a,].raku; # OUTPUT: «[[1, 2],]»
The Array constructor does not flatten other types of contents. Use
the Slip prefix operator (|) to flatten the needed
items:
my @a = 1, 2;
say [@a, 3, 4].raku; # OUTPUT: «[[1, 2], 3, 4]»
say [|@a, 3, 4].raku; # OUTPUT: «[1, 2, 3, 4]»
List type can be explicitly created from an array literal declaration without a coercion from Array, using is trait on declaration.
my @a is List = 1, 2; # a List, not an Array
# wrong: creates an Array of Lists
my List @a;
Hash literals
A leading associative sigil and pair of parenthesis %( ) can surround
a List of Pairs to form a Hash literal; typically
there is a comma-delimited List of Pairs inside. If a non-pair is
used, it is assumed to be a key and the next element is the value. Most
often this is used with simple arrow pairs.
say %( a => 3, b => 23, :foo, :dog<cat>, "french", "fries" );
# OUTPUT: «a => 3, b => 23, dog => cat, foo => True, french => fries»
say %(a => 73, foo => "fish").keys.join(" "); # OUTPUT: «a foo»
# ^^^^^^^^^^^^^^^^^^^^^^^^^ Hash constructor
When assigning to a %-sigiled variable on the left-hand side, the
sigil and parenthesis surrounding the right-hand side Pair are
optional.
my %ages = fred => 23, jean => 87, ann => 4;
By default, keys in %( ) are forced to strings. To compose a hash with
non-string keys, use curly brace delimiters with a colon prefix :{ } :
my $when = :{ (now) => "Instant", (DateTime.now) => "DateTime" };
Note that with objects as keys, you cannot access non-string keys as strings:
say :{ -1 => 41, 0 => 42, 1 => 43 }<0>; # OUTPUT: «(Any)»
say :{ -1 => 41, 0 => 42, 1 => 43 }{0}; # OUTPUT: «42»
Particular types that implement Associative role, Map (including Hash and Stash subclasses) and QuantHash (and its subclasses), can be explicitly created from a hash literal without a coercion, using is trait on declaration:
my %hash; # Hash
my %hash is Hash; # explicit Hash
my %map is Map; # Map
my %stash is Stash; # Stash
my %quant-hash is QuantHash; # QuantHash
my %setty is Setty; # Setty
my %set is Set; # Set
my %set-hash is SetHash; # SetHash
my %baggy is Baggy; # Baggy
my %bag is Bag; # Bag
my %bag-hash is BagHash; # BagHash
my %mixy is Mixy; # Mixy
my %mix is Mix; # Mix
my %mix-hash is MixHash; # MixHash
Note that using a usual type declaration with a hash sigil creates a typed Hash, not a particular type:
# This is wrong: creates a Hash of Mixes, not Mix:
my Mix %mix;
# Works with $ sigil:
my Mix $mix;
# Can be typed:
my Mix[Int] $mix-of-ints;
Regex literals
A Regex is declared with slashes like /foo/. Note that
this // syntax is shorthand for the full rx// syntax.
/foo/ # Short version
rx/foo/ # Longer version
Q :regex /foo/ # Even longer version
my $r = /foo/; # Regexes can be assigned to variables
Signature literals
Signatures can be used standalone for pattern matching, in addition to the typical usage in sub and block declarations. A standalone signature is declared starting with a colon:
say "match!" if 5, "fish" ~~ :(Int, Str); # OUTPUT: «match!»
my $sig = :(Int $a, Str);
say "match!" if (5, "fish") ~~ $sig; # OUTPUT: «match!»
given "foo", 42 {
when :(Str, Str) { "This won't match" }
when :(Str, Int $n where $n > 20) { "This will!" }
}
See the signatures documentation for more information.
Declarations
Variable declaration
my $x; # simple lexical variable
my $x = 7; # initialize the variable
my Int $x = 7; # declare the type
my Int:D $x = 7; # specify that the value must be defined (not undef)
my Int $x where { $_ > 3 } = 7; # constrain the value based on a function
my Int $x where * > 3 = 7; # same constraint, but using Whatever shorthand
See Variable Declarators and
Scope
for more details on other scopes (our, has).
Callable declarations
Raku provides syntax for multiple Callable code objects (that is, code objects that can be invoked, such as subroutines). Specifically, Raku provides syntax for subroutines (both single- and multiple-dispatch), code blocks, and methods (again, both single- and multiple-dispatch).
Subroutine declaration
Subroutines are created with the keyword sub followed by an optional
name, an optional signature and a code block. Subroutines are lexically
scoped, so if a name is specified at the declaration time, the same name
can be used in the lexical scope to invoke the subroutine. A subroutine
is an instance of type Sub and can be assigned to any
container.
sub {}
# The minimal legal subroutine declaration
sub say-hello1 { say "Hello!" }
# A subroutine with a name
sub say-hello2(Str $to-whom) { say "Hello $to-whom!" }
# A subroutine with a name and a signature
You can assign subroutines to variables.
my &greet0 = sub { say "Hello!" } # Unnamed sub assigned to &greet0
my &greet1 = sub say-hello1 { say "Hello!" } # Named sub assigned to &greet1
Multiple-dispatch subroutine declaration
Subroutines can be declared as a multi – that is, as a subroutine that with
multiple candidates, each with a different signature. A multiple dispatch
subroutine is created almost exactly like a single-dispatch subroutine: with the
keyword multi, optionally followed by the keyword sub, followed by an
optional name, an optional signature, and a code block. See
Multi-dispatch for details.
The two single-dispatch subroutines greet0 and greet1 (defined above)
could be combined into a single greet multi as follows:
multi greet { say "Hello!" }
# ^^^ optional
multi greet($name) { say "Hello $name!" }
You may optionally precede multi declarations with a
proto declarations; protos declare a signature
that any call must conform to before being dispatched to candidates. For
example, this proto requires that all candidates take at least one positional
parameter:
proto at-least1($, |) {*}
multi at-least1($first) { note $first }
multi at-least1($first, $second) { note "got two" }
multi at-least1($first, :$named) { note "got named" }
# The following is legal to *declare* but can never be called
multi at-least1 { note "Got none"}
Block declaration
Blocks are invocable code objects similar to subroutines but
intended for small-scale code reuse. Blocks are created by a code block inside
of { } curly braces, optionally preceded by -> followed by a signature (or
<-> followed by a signature for rw (aka,
read-write) blocks). If no signature is provided,
Blocks can accept parameters implicitly by using placeholder variables with the
^ twigil and the : twigil.
Blocks with neither an explicit nor implicit signature have a default signature of $_.
my &greet0 = { say "Hello!" } # Block stored in &greet0
my &greet1 = { say "Hello $_!" } # Block with default signature
my &greet2 = -> $to-whom { say "Hello $to-whom!" } # Block with explicit signature
my &greet3 = { say "Hello $^to-whom!" } # Block with implicit positional
my &greet4 = { say "Hello $:to-whom!" } # Block with implicit named
my &add1-in-place = <-> $a { ++$a } # Block with rw signature
Although doing so is less idiomatic, Blocks may also be stored in scalar variables
my $greet = { say "Hello!" }
Method declaration
Methods are a callable code object invoked against a specific
object (called the method's "invocant"). Outside of a class declaration,
methods are declared by the method keyword, followed by a signature, followed
by a code block. Within the method signature, the method's invocant is followed
by a : instead of the , that normally separates arguments. Methods
declared outside of a class must be stored in a variable to be used.
my &m = method ($invocant: $arg1, $arg2) { note }
This syntax is unusual – typically, methods are declared inside a class. In
this context, method declaration more closely resembles subroutine declaration:
methods are declared by the keyword method followed by a name, followed by an
optional signature, followed by a code block. Methods defined inside a class,
have that class as their default invocant but can override that default (for
example, to constrain the invocant's
definiteness).
class Greeter {
method greet($to-whom) { say "Hello $to-whom!" }
method defined-greet(Greeter:D:) { say "Hello!" }
}
For more details on methods, see Methods.
Multiple-dispatch method declaration
Inside of a class, you can declare multiple-dispatch methods with syntax that's
very similar to the syntax for multiple-dispatch subroutines. As with multi
subroutines, you can also declare a proto for multiple-dispatch methods.
class Greeter {
multi method greet { say "Hello!" }
multi method greet($to-whom) { say "Hello $to-whom!" }
proto method at-least1($, |) {*}
multi method at-least1($first) { note $first }
# The following is legal to *declare* but can never be called
multi method at-least1 { note "Got none"}
}
package, module, class, role, and grammar declaration|Syntax,unit;Syntax,module;Syntax,package
There are several types of package, each declared with a keyword, a name, some optional traits, and a body of subroutines, methods, or rules.
package P { }
module M { }
class C { }
role R { }
grammar G { }
Several packages may be declared in a single file. However, you can declare
a unit package at the start of the file (preceded only by comments or use
statements), and the rest of the file will be taken as being the body of the
package. In this case, the curly braces are not required.
unit module M;
# ... stuff goes here instead of in {}'s
Invoking code objects
Raku provides standard syntax for invoking subroutines/blocks and for invoking methods. Additionally, Raku provides alternate syntax to invoke subroutines with method-like syntax and to invoke methods with subroutine-like syntax
Invoking subroutines or blocks
Lexically declared subroutines and subroutine/blocks assigned to &-sigiled
variables can be invoked by their name followed by their arguments (optionally
enclosed in (…)). Alternatively, their name may be preceded by the &
sigil but, in that case, the (…) surrounding the function's arguments is
mandatory but may be preceded by an optional .. (An &-sigiled variable
without (…) refers to the function as an object – that is, without invoking
it). Thus, all of the following call the function foo with no arguments:
foo;
foo();
&foo();
&foo.();
If a subroutine or block is assigned to a $-sigiled variable, then it can
only be invoked using parentheses:
my $foo = sub { note } $foo(); $foo.();
For more information on invoking subroutines/blocks, see functions.
Invoking methods
A method defined in a class is invoked on an instance of that class by
referring to the class followed by a . followed by the method name. Invoking
the method without arguments doesn't require parentheses. To supply arguments,
you must either surround them in (…) or follow the method name with a :.
The following code shows the syntax described above:
class Person {
has $.age = 0;
has Str $.name = "Anon";
multi method greet { say "Hello, $.name()!" }
multi method greet($name) { say "Hello, $name!" }
};
my $person = Person.new(:name<Jane>, :age(98));
$person.greet; # Calls greet method with 0 args
$person.greet(); # Calls greet method with 0 args
$person.greet('Nushi'); # Calls greet method with 1 arg
$person.greet: 'Nushi'; # Calls greet method with 1 arg
Invoking methods with : (precedence drop)
Note that the syntax in the final line results in the method treating the
remainder of the statement as its argument list. (Or, said differently, it
drops the precedence of the method call; for that reason, the : used here is
sometimes called the "precedence drop"). The following code illustrates the
consequences of that change:
my $band = 'Foo Fighters';
say $band.substr( 0, 3 ).uc; # OUTPUT: «FOO»
say $band.substr: 0, 3 .uc; # OUTPUT: «Foo»
The final line isn't equivalent to the one above it; instead, it's the same as
$band.substr(0, 3.uc) – likely not what was intended here. That's because in
the second method call the uc method is called on "3" and not to the result
of the leftmost substr; in other words, the substr method yields
precedence to the uc method.
Invoking methods on the topic
If a method is called without an invocant (that is, with nothing to the left of
the .), then it will use the current topic
variable as its invocant. For example:
given 'Foo Fighters' {
say .substr: 0, 3; # OUTPUT: «Foo»
}
Method-like function calls & function-like method calls
Subroutines can be invoked with method-like syntax – that is, you can call a
function on an object followed by a dot in much the same way that you can call a
method (including by optionally using a :). The only limitation is that you
must precede the function name with an &. For example:
sub greet($name, :$excited = True) {
say "Hello $name" ~ ($excited ?? '!' !! '.')
}
greet "Maria"; # OUTPUT: «Hello Maria!»
"Maria".&greet; # OUTPUT: «Hello Maria!»
"Maria".&greet(); # OUTPUT: «Hello Maria!»
given "Maria" { .&greet } # OUTPUT: «Hello Maria!»
"Maria".&greet(:!excited); # OUTPUT: «Hello Maria.»
"Maria".&greet: :!excited; # OUTPUT: «Hello Maria.»
Similarly, methods may be invoked with function-like syntax – that is, you can
call a method by providing the method name first followed by the method's
arguments (including its invocant). To do so, simply supply the invocant as the
first argument followed by a :.
class Person {
has Str $.name;
multi method greet { say "Hello, $.name()!" }
multi method greet($name) { say "Hello, $name!" }
};
my $person = Person.new(:name<Ruòxī>, :age(11));
greet $person:; # same as $person.greet;
greet Person: 'Yasmin'; # same as Person.greet('Yasmin');
Due to the presence of method-like function syntax and function-like method syntax, it is especially important to be clear on whether a given syntactic form calls a method or a subroutine. This detail can be easy to neglect, especially because in many cases Raku provides a method and a subroutine of the same name.
For instance, the following simple example produces the same output for both function and method calls.
say 42; # Subroutine call
42.&say; # Still a subroutine call
42.say; # Method call
say 42:; # Also a method call
However, even when both a subroutine and a method exist, calling the correct one
can make a large difference. To see this in action, consider map, which
exists as both a subroutine and a method but where the method expects its
arguments in a different order:
my @list = 1..9;
sub add1($a) { $a + 1 }
map &add1, @list; # Sub call; expects list last
map @list: &add1; # Method call; expects function last
@list.map(&add1); # Method call; expects function last
&add1.&map(@list); # Sub call; expects list last
Operators
See Operators for lots of details.
Operators are functions with a more symbol heavy and composable syntax. Like other functions, operators can be multi-dispatch to allow for context-specific usage.
There are five types (arrangements) for operators, each taking either one or two arguments.
++$x # prefix, operator comes before single input
5 + 3 # infix, operator is between two inputs
$x++ # postfix, operator is after single input
<the blue sky> # circumfix, operator surrounds single input
%foo<bar> # postcircumfix, operator comes after first input and surrounds second
Metaoperators
Operators can be composed. A common example of this is combining an infix (binary) operator with assignment. You can combine assignment with any binary operator.
$x += 5 # Adds 5 to $x, same as $x = $x + 5
$x min= 3 # Sets $x to the smaller of $x and 3, same as $x = $x min 3
$x .= child # Equivalent to $x = $x.child
Wrap an infix operator in [ ] to create a new reduction operator that works
on a single list of inputs, resulting in a single value.
say [+] <1 2 3 4 5>; # OUTPUT: «15»
(((1 + 2) + 3) + 4) + 5 # equivalent expanded version
Wrap an infix operator in « » (or the ASCII equivalent << >>) to
create a new hyper operator that works pairwise on two lists.
say <1 2 3> «+» <4 5 6> # OUTPUT: «(5 7 9)»
The direction of the arrows indicates what to do when the lists are not the same size.
@a «+« @b # Result is the size of @b, elements from @a will be re-used
@a »+» @b # Result is the size of @a, elements from @b will be re-used
@a «+» @b # Result is the size of the biggest input, the smaller one is re-used
@a »+« @b # Exception if @a and @b are different sizes
You can also wrap a unary operator with a hyper operator.
say -« <1 2 3> # OUTPUT: «(-1 -2 -3)»
[1]Starting
with Raku language version 6.d, colon pairs with sym as the key (e.g.
:sym<foo>) are reserved for possible future use.