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 @uppercase-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§
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)»