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| category | language | filename | contributors | ||||||
|---|---|---|---|---|---|---|---|---|---|
| language | Raku | learnraku.raku |
|
Raku (formerly Perl 6) is a highly capable, feature-rich programming language made for at least the next hundred years.
The primary Raku compiler is called Rakudo, which runs on the JVM and the MoarVM.
Meta-note:
- Although the pound sign (
#) is used for sentences and notes, Pod-styled comments (more below about them) are used whenever it's convenient. # OUTPUT:is used to represent the output of a command to any standard stream. If the output has a newline, it's represented by thesymbol. The output is always enclosed by angle brackets («and»).#=>represents the value of an expression, return value of a sub, etc. In some cases, the value is accompanied by a comment.- Backticks are used to distinguish and highlight the language constructs from the text.
####################################################
# 0. Comments
####################################################
# Single line comments start with a pound sign.
#`( Multiline comments use #` and a quoting construct.
(), [], {}, 「」, etc, will work.
)
=for comment
Use the same syntax for multiline comments to embed comments.
for #`(each element in) @array {
put #`(or print element) $_ #`(with newline);
}
# You can also use Pod-styled comments. For example:
=comment This is a comment that extends until an empty
newline is found.
=comment
The comment doesn't need to start in the same line as the directive.
=begin comment
This comment is multiline.
Empty newlines can exist here too!
=end comment
####################################################
# 1. Variables
####################################################
# In Raku, you declare a lexical variable using the `my` keyword:
my $variable;
# Raku has 3 basic types of variables: scalars, arrays, and hashes.
#
# 1.1 Scalars
#
# Scalars represent a single value. They start with the `$` sigil:
my $str = 'String';
# Double quotes allow for interpolation (which we'll see later):
my $str2 = "$str";
# Variable names can contain but not end with simple quotes and dashes,
# and can contain (and end with) underscores:
my $person's-belongings = 'towel'; # this works!
my $bool = True; # `True` and `False` are Raku's boolean values.
my $inverse = !$bool; # Invert a bool with the prefix `!` operator.
my $forced-bool = so $str; # And you can use the prefix `so` operator
$forced-bool = ?$str; # to turn its operand into a Bool. Or use `?`.
#
# 1.2 Arrays and Lists
#
# Arrays represent multiple values. An array variable starts with the `@`
# sigil. Unlike lists, from which arrays inherit, arrays are mutable.
my @array = 'a', 'b', 'c';
# equivalent to:
my @letters = <a b c>;
# In the previous statement, we use the quote-words (`<>`) term for array
# of words, delimited by space. Similar to perl's qw, or Ruby's %w.
@array = 1, 2, 4;
# Array indices start at 0. Here the third element is being accessed.
say @array[2]; # OUTPUT: «4»
say "Interpolate an array using []: @array[]";
# OUTPUT: «Interpolate an array using []: 1 2 3»
@array[0] = -1; # Assigning a new value to an array index
@array[0, 1] = 5, 6; # Assigning multiple values
my @keys = 0, 2;
@array[@keys] = @letters; # Assignment using an array containing index values
say @array; # OUTPUT: «a 6 b»
#
# 1.3 Hashes, or key-value Pairs.
#
# Hashes are pairs of keys and values. You can construct a `Pair` object
# using the syntax `key => value`. Hash tables are very fast for lookup,
# and are stored unordered. Keep in mind that keys get "flattened" in hash
# context, and any duplicated keys are deduplicated.
my %hash = 'a' => 1, 'b' => 2;
# Keys get auto-quoted when the fat comman (`=>`) is used. Trailing commas are
# okay.
%hash = a => 1, b => 2, ;
# Even though hashes are internally stored differently than arrays,
# Raku allows you to easily create a hash from an even numbered array:
%hash = <key1 value1 key2 value2>; # Or:
%hash = "key1", "value1", "key2", "value2";
%hash = key1 => 'value1', key2 => 'value2'; # same result as above
# You can also use the "colon pair" syntax. This syntax is especially
# handy for named parameters that you'll see later.
%hash = :n(2), # equivalent to `n => 2`
:is-even, # equivalent to `:is-even(True)` or `is-even => True`
:!is-odd, # equivalent to `:is-odd(False)` or `is-odd => False`
;
# The `:` (as in `:is-even`) and `:!` (as `:!is-odd`) constructs are known
# as the `True` and `False` shortcuts respectively.
# As demonstrated in the example below, you can use {} to get the value from a key.
# If it's a string without spaces, you can actually use the quote-words operator
# (`<>`). Since Raku doesn't have barewords, as Perl does, `{key1}` doesn't work
# though.
say %hash{'n'}; # OUTPUT: «2», gets value associated to key 'n'
say %hash<is-even>; # OUTPUT: «True», gets value associated to key 'is-even'
####################################################
# 2. Subroutines
####################################################
# Subroutines, or functions as most other languages call them, are
# created with the `sub` keyword.
sub say-hello { say "Hello, world" }
# You can provide (typed) arguments. If specified, the type will be checked
# at compile-time if possible, otherwise at runtime.
sub say-hello-to( Str $name ) {
say "Hello, $name !";
}
# A sub returns the last value of the block. Similarly, the semicolon in
# the last expression can be omitted.
sub return-value { 5 }
say return-value; # OUTPUT: «5»
sub return-empty { }
say return-empty; # OUTPUT: «Nil»
# Some control flow structures produce a value, for instance `if`:
sub return-if {
if True { "Truthy" }
}
say return-if; # OUTPUT: «Truthy»
# Some don't, like `for`:
sub return-for {
for 1, 2, 3 { 'Hi' }
}
say return-for; # OUTPUT: «Nil»
# Positional arguments are required by default. To make them optional, use
# the `?` after the parameters' names.
# In the following example, the sub `with-optional` returns `(Any)` (Perl's
# null-like value) if no argument is passed. Otherwise, it returns its argument.
sub with-optional( $arg? ) {
$arg;
}
with-optional; # returns Any
with-optional(); # returns Any
with-optional(1); # returns 1
# You can also give provide a default value when they're not passed. Doing
# this make said parameter optional. Required parameters must come before
# optional ones.
# In the sub `greeting`, the parameter `$type` is optional.
sub greeting( $name, $type = "Hello" ) {
say "$type, $name!";
}
greeting("Althea"); # OUTPUT: «Hello, Althea!»
greeting("Arthur", "Good morning"); # OUTPUT: «Good morning, Arthur!»
# You can also, by using a syntax akin to the one of hashes (yay unified syntax!),
# declared named parameters and thus pass named arguments to a subroutine.
# By default, named parameter are optional and will default to `Any`.
sub with-named( $normal-arg, :$named ) {
say $normal-arg + $named;
}
with-named(1, named => 6); # OUTPUT: «7»
# There's one gotcha to be aware of, here: If you quote your key, Raku
# won't be able to see it at compile time, and you'll have a single `Pair`
# object as a positional parameter, which means the function subroutine
# `with-named(1, 'named' => 6);` fails.
with-named(2, :named(5)); # OUTPUT: «7»
# Similar to positional parameters, you can provide your named arguments with
# default values.
sub named-def( :$def = 5 ) {
say $def;
}
named-def; # OUTPUT: «5»
named-def(def => 15); # OUTPUT: «15»
# In order to make a named parameter mandatory, you can append `!` to the
# parameter. This is the inverse of `?`, which makes a required parameter
# optional.
sub with-mandatory-named( :$str! ) {
say "$str!";
}
with-mandatory-named(str => "My String"); # OUTPUT: «My String!»
# with-mandatory-named; # runtime error: "Required named parameter not passed"
# with-mandatory-named(3);# runtime error: "Too many positional parameters passed"
# If a sub takes a named boolean argument, you can use the same "short boolean"
# hash syntax we discussed earlier.
sub takes-a-bool( $name, :$bool ) {
say "$name takes $bool";
}
takes-a-bool('config', :bool); # OUTPUT: «config takes True»
takes-a-bool('config', :!bool); # OUTPUT: «config takes False»
# Since parenthesis can be omitted when calling a subroutine, you need to use
# `&` in order to distinguish between a call to a sub with no arguments and
# the code object.
# For instance, in this example we must use `&` to store the sub `say-hello`
# (i.e., the sub's code object) in a variable, not a subroutine call.
my &s = &say-hello;
my &other-s = sub { say "Anonymous function!" }
# A sub can have a "slurpy" parameter, or what one'd call a
# "doesn't-matter-how-many" parameter. This is Raku's way of supporting variadic
# functions. For this, you must use `*@` (slurpy) which will "take everything
# else". You can have as many parameters *before* a slurpy one, but not *after*.
sub as-many($head, *@rest) {
@rest.join(' / ') ~ " !";
}
say as-many('Happy', 'Happy', 'Birthday'); # OUTPUT: «Happy / Birthday !»
say as-many('Happy', ['Happy', 'Birthday'], 'Day'); # OUTPUT: «Happy / Birthday / Day !»
# Note that the splat (the *) did not consume the parameter before it.
# There are other two variations of slurpy parameters in Raku. The previous one
# (namely, `*@`), known as flattened slurpy, flattens passed arguments. The other
# two are `**@` and `+@` known as unflattened slurpy and "single argument rule"
# slurpy respectively. The unflattened slurpy doesn't flatten its listy
# arguments (or Iterable ones).
sub b(**@arr) { @arr.perl.say };
b(['a', 'b', 'c']); # OUTPUT: «[["a", "b", "c"],]»
b(1, $('d', 'e', 'f'), [2, 3]); # OUTPUT: «[1, ("d", "e", "f"), [2, 3]]»
b(1, [1, 2], ([3, 4], 5)); # OUTPUT: «[1, [1, 2], ([3, 4], 5)]»
# On the other hand, the "single argument rule" slurpy follows the "single argument
# rule" which dictates how to handle the slurpy argument based upon context and
# roughly states that if only a single argument is passed and that argument is
# Iterable, that argument is used to fill the slurpy parameter array. In any
# other case, `+@` works like `**@`.
sub c(+@arr) { @arr.perl.say };
c(['a', 'b', 'c']); # OUTPUT: «["a", "b", "c"]»
c(1, $('d', 'e', 'f'), [2, 3]); # OUTPUT: «[1, ("d", "e", "f"), [2, 3]]»
c(1, [1, 2], ([3, 4], 5)); # OUTPUT: «[1, [1, 2], ([3, 4], 5)]»
# You can call a function with an array using the "argument list flattening"
# operator `|` (it's not actually the only role of this operator,
# but it's one of them).
sub concat3($a, $b, $c) {
say "$a, $b, $c";
}
concat3(|@array); # OUTPUT: «a, b, c»
# `@array` got "flattened" as a part of the argument list
####################################################
# 3. Containers
####################################################
# In Raku, values are actually stored in "containers". The assignment
# operator asks the container on the left to store the value on its right.
# When passed around, containers are marked as immutable which means that,
# in a function, you'll get an error if you try to mutate one of your
# arguments. If you really need to, you can ask for a mutable container by
# using the `is rw` trait.
sub mutate( $n is rw ) {
$n++; # postfix ++ operator increments its argument but returns its old value
}
my $m = 42;
mutate $m; #=> 42, the value is incremented but the old value is returned
say $m; # OUTPUT: «43»
# This works because we are passing the container $m to the `mutate` sub.
# If we try to just pass a number instead of passing a variable, it won't work
# because there is no container being passed and integers are immutable by
# themselves:
# mutate 42; # Parameter '$n' expected a writable container, but got Int value
# Similar error would be obtained, if a bound variable is passed to
# to the subroutine. In Raku, you bind a value to a variable using the binding
# operator `:=`.
my $v := 50; # binding 50 to the variable $v
# mutate $v; # Parameter '$n' expected a writable container, but got Int value
# If what you want is a copy instead, use the `is copy` trait which will
# cause the argument to be copied and allow you to modify the argument
# inside the routine without modifying the passed argument.
# A sub itself returns a container, which means it can be marked as `rw`.
# Alternatively, you can explicitly mark the returned container as mutable
# by using `return-rw` instead of `return`.
my $x = 42;
my $y = 45;
sub x-store is rw { $x }
sub y-store { return-rw $y }
# In this case, the parentheses are mandatory or else Raku thinks that
# `x-store` and `y-store` are identifiers.
x-store() = 52;
y-store() *= 2;
say $x; # OUTPUT: «52»
say $y; # OUTPUT: «90»
####################################################
# 4.Control Flow Structures
####################################################
#
# 4.1 if/if-else/if-elsif-else/unless
#
# Before talking about `if`, we need to know which values are "truthy"
# (represent `True`), and which are "falsey" (represent `False`). Only these
# values are falsey: 0, (), {}, "", Nil, a type (like `Str`, `Int`, etc.) and
# of course, `False` itself. Any other value is truthy.
my $number = 5;
if $number < 5 {
say "Number is less than 5"
}
elsif $number == 5 {
say "Number is equal to 5"
}
else {
say "Number is greater than 5"
}
unless False {
say "It's not false!";
}
# `unless` is the equivalent of `if not (X)` which inverts the sense of a
# conditional statement. However, you cannot use `else` or `elsif` with it.
# As you can see, you don't need parentheses around conditions. However, you
# do need the curly braces around the "body" block. For example,
# `if (True) say 'It's true';` doesn't work.
# You can also use their statement modifier (postfix) versions:
say "Quite truthy" if True; # OUTPUT: «Quite truthy»
say "Quite falsey" unless False; # OUTPUT: «Quite falsey»
# The ternary operator (`??..!!`) is structured as follows `condition ??
# expression1 !! expression2` and it returns expression1 if the condition is
# true. Otherwise, it returns expression2.
my $age = 30;
say $age > 18 ?? "You are an adult" !! "You are under 18";
# OUTPUT: «You are an adult»
#
# 4.2 with/with-else/with-orwith-else/without
#
# The `with` statement is like `if`, but it tests for definedness rather than
# truth, and it topicalizes on the condition, much like `given` which will
# be discussed later.
my $s = "raku";
with $s.index("r") { say "Found a at $_" }
orwith $s.index("k") { say "Found c at $_" }
else { say "Didn't find r or k" }
# Similar to `unless` that checks un-truthiness, you can use `without` to
# check for undefined-ness.
my $input01;
without $input01 {
say "No input given."
}
# OUTPUT: «No input given.»
# There are also statement modifier versions for both `with` and `without`.
my $input02 = 'Hello';
say $input02 with $input02; # OUTPUT: «Hello»
say "No input given." without $input02;
#
# 4.3 given/when, or Raku's switch construct
#
=begin comment
`given...when` looks like other languages' `switch`, but is much more
powerful thanks to smart matching and Raku's "topic variable", `$_`.
The topic variable `$_ `contains the default argument of a block, a loop's
current iteration (unless explicitly named), etc.
`given` simply puts its argument into `$_` (like a block would do),
and `when` compares it using the "smart matching" (`~~`) operator.
Since other Raku constructs use this variable (as said before, like `for`,
blocks, `with` statement etc), this means the powerful `when` is not only
applicable along with a `given`, but instead anywhere a `$_` exists.
=end comment
given "foo bar" {
say $_; # OUTPUT: «foo bar»
# Don't worry about smart matching yet. Just know `when` uses it. This is
# equivalent to `if $_ ~~ /foo/`.
when /foo/ {
say "Yay !";
}
# smart matching anything with `True` is `True`, i.e. (`$a ~~ True`)
# so you can also put "normal" conditionals. For example, this `when` is
# equivalent to this `if`: `if $_ ~~ ($_.chars > 50) {...}`
# which means: `if $_.chars > 50 {...}`
when $_.chars > 50 {
say "Quite a long string !";
}
# same as `when *` (using the Whatever Star)
default {
say "Something else"
}
}
#
# 4.4 Looping constructs
#
# The `loop` construct is an infinite loop if you don't pass it arguments, but
# can also be a C-style `for` loop:
loop {
say "This is an infinite loop !";
last;
}
# In the previous example, `last` breaks out of the loop very much
# like the `break` keyword in other languages.
# The `next` keyword skips to the next iteration, like `continue` in other
# languages. Note that you can also use postfix conditionals, loops, etc.
loop (my $i = 0; $i < 5; $i++) {
next if $i == 3;
say "This is a C-style for loop!";
}
# The `for` constructs iterates over a list of elements.
my @odd-array = 1, 3, 5, 7, 9;
# Accessing the array's elements with the topic variable $_.
for @odd-array {
say "I've got $_ !";
}
# Accessing the array's elements with a "pointy block", `->`.
# Here each element is read-only.
for @odd-array -> $variable {
say "I've got $variable !";
}
# Accessing the array's elements with a "doubly pointy block", `<->`.
# Here each element is read-write so mutating `$variable` mutates
# that element in the array.
for @odd-array <-> $variable {
say "I've got $variable !";
}
# As we saw with `given`, a `for` loop's default "current iteration" variable
# is `$_`. That means you can use `when` in a `for`loop just like you were
# able to in a `given`.
for @odd-array {
say "I've got $_";
# This is also allowed. A dot call with no "topic" (receiver) is sent to
# `$_` (topic variable) by default.
.say;
# This is equivalent to the above statement.
$_.say;
}
for @odd-array {
# You can...
next if $_ == 3; # Skip to the next iteration (`continue` in C-like lang.)
redo if $_ == 4; # Re-do iteration, keeping the same topic variable (`$_`)
last if $_ == 5; # Or break out of loop (like `break` in C-like lang.)
}
# The "pointy block" syntax isn't specific to the `for` loop. It's just a way
# to express a block in Raku.
sub long-computation { "Finding factors of large primes" }
if long-computation() -> $result {
say "The result is $result.";
}
####################################################
# 5. Operators
####################################################
=begin comment
Since Perl languages are very much operator-based languages, Raku
operators are actually just funny-looking subroutines, in syntactic
categories, like infix:<+> (addition) or prefix:<!> (bool not).
The categories are:
- "prefix": before (like `!` in `!True`).
- "postfix": after (like `++` in `$a++`).
- "infix": in between (like `*` in `4 * 3`).
- "circumfix": around (like `[`-`]` in `[1, 2]`).
- "post-circumfix": around, after another term (like `{`-`}` in
`%hash{'key'}`)
The associativity and precedence list are explained below.
Alright, you're set to go!
=end comment
#
# 5.1 Equality Checking
#
# `==` is numeric comparison
say 3 == 4; # OUTPUT: «False»
say 3 != 4; # OUTPUT: «True»
# `eq` is string comparison
say 'a' eq 'b'; # OUTPUT: «False»
say 'a' ne 'b'; # OUTPUT: «True», not equal
say 'a' !eq 'b'; # OUTPUT: «True», same as above
# `eqv` is canonical equivalence (or "deep equality")
say (1, 2) eqv (1, 3); # OUTPUT: «False»
say (1, 2) eqv (1, 2); # OUTPUT: «True»
say Int === Int; # OUTPUT: «True»
# `~~` is the smart match operator which aliases the left hand side to $_ and
# then evaluates the right hand side.
# Here are some common comparison semantics:
# String or numeric equality
say 'Foo' ~~ 'Foo'; # OUTPUT: «True», if strings are equal.
say 12.5 ~~ 12.50; # OUTPUT: «True», if numbers are equal.
# Regex - For matching a regular expression against the left side.
# Returns a `Match` object, which evaluates as True if regexp matches.
my $obj = 'abc' ~~ /a/;
say $obj; # OUTPUT: «｢a｣»
say $obj.WHAT; # OUTPUT: «(Match)»
# Hashes
say 'key' ~~ %hash; # OUTPUT: «True», if key exists in hash.
# Type - Checks if left side "is of type" (can check superclasses and roles).
say 1 ~~ Int; # OUTPUT: «True»
# Smart-matching against a boolean always returns that boolean (and will warn).
say 1 ~~ True; # OUTPUT: «True», smartmatch against True always matches
say False.so ~~ True; # OUTPUT: «True», use .so for truthiness
# General syntax is `$arg ~~ &bool-returning-function;`. For a complete list
# of combinations, refer to the table at:
# https://docs.raku.org/language/operators#index-entry-smartmatch_operator
# Of course, you also use `<`, `<=`, `>`, `>=` for numeric comparison.
# Their string equivalent are also available: `lt`, `le`, `gt`, `ge`.
say 3 > 4; # OUTPUT: «False»
say 3 >= 4; # OUTPUT: «False»
say 3 < 4; # OUTPUT: «True»
say 3 <= 4; # OUTPUT: «True»
say 'a' gt 'b'; # OUTPUT: «False»
say 'a' ge 'b'; # OUTPUT: «False»
say 'a' lt 'b'; # OUTPUT: «True»
say 'a' le 'b'; # OUTPUT: «True»
#
# 5.2 Range constructor
#
say 3 .. 7; # OUTPUT: «3..7», both included.
say 3 ..^ 7; # OUTPUT: «3..^7», exclude right endpoint.
say 3 ^.. 7; # OUTPUT: «3^..7», exclude left endpoint.
say 3 ^..^ 7; # OUTPUT: «3^..^7», exclude both endpoints.
# The range 3 ^.. 7 is similar like 4 .. 7 when we only consider integers.
# But when we consider decimals:
say 3.5 ~~ 4 .. 7; # OUTPUT: «False»
say 3.5 ~~ 3 ^.. 7; # OUTPUT: «True»,
# This is because the range `3 ^.. 7` only excludes anything strictly
# equal to 3. Hence, it contains decimals greater than 3. This could
# mathematically be described as 3.5 ∈ (3,7] or in set notation,
# 3.5 ∈ { x | 3 < x ≤ 7 }.
say 3 ^.. 7 ~~ 4 .. 7; # OUTPUT: «False»
# This also works as a shortcut for `0..^N`:
say ^10; # OUTPUT: «^10», which means 0..^10
# This also allows us to demonstrate that Raku has lazy/infinite arrays,
# using the Whatever Star:
my @natural = 1..*; # 1 to Infinite! Equivalent to `1..Inf`.
# You can pass ranges as subscripts and it'll return an array of results.
say @natural[^10]; # OUTPUT: «1 2 3 4 5 6 7 8 9 10», doesn't run out of memory!
# NOTE: when reading an infinite list, Raku will "reify" the elements
# it needs, then keep them in memory. They won't be calculated more than once.
# It also will never calculate more elements than that are needed.
# An array subscript can also be a closure. It'll be called with the array's
# length as the argument. The following two examples are equivalent:
say join(' ', @array[15..*]); # OUTPUT: «15 16 17 18 19»
say join(' ', @array[-> $n { 15..$n }]); # OUTPUT: «15 16 17 18 19»
# NOTE: if you try to do either of those with an infinite array, you'll
# trigger an infinite loop (your program won't finish).
# You can use that in most places you'd expect, even when assigning to an array:
my @numbers = ^20;
# Here the numbers increase by 6, like an arithmetic sequence; more on the
# sequence (`...`) operator later.
my @seq = 3, 9 ... * > 95; # 3 9 15 21 27 [...] 81 87 93 99;
# In this example, even though the sequence is infinite, only the 15
# needed values will be calculated.
@numbers[5..*] = 3, 9 ... *;
say @numbers; # OUTPUT: «0 1 2 3 4 3 9 15 21 [...] 81 87», only 20 values
#
# 5.3 and (&&), or (||)
#
# Here `and` calls `.Bool` on both 3 and 4 and gets `True` so it returns
# 4 since both are `True`.
say (3 and 4); # OUTPUT: «4», which is truthy.
say (3 and 0); # OUTPUT: «0»
say (0 and 4); # OUTPUT: «0»
# Here `or` calls `.Bool` on `0` and `False` which are both `False`
# so it returns `False` since both are `False`.
say (0 or False); # OUTPUT: «False».
# Both `and` and `or` have tighter versions which also shortcut circuits.
# They're `&&` and `||` respectively.
# `&&` returns the first operand that evaluates to `False`. Otherwise,
# it returns the last operand.
my ($a, $b, $c, $d, $e) = 1, 0, False, True, 'pi';
say $a && $b && $c; # OUTPUT: «0», the first falsey value
say $a && $b && $c; # OUTPUT: «False», the first falsey value
say $a && $d && $e; # OUTPUT: «pi», last operand since everthing before is truthy
# `||` returns the first argument that evaluates to `True`.
say $b || $a || $d; # OUTPUT: «1»
say $e || $d || $a; # OUTPUT: «pi»
# And because you're going to want them, you also have compound assignment
# operators:
$a *= 2; # multiply and assignment. Equivalent to $a = $a * 2;
$b %%= 5; # divisible by and assignment. Equivalent to $b = $b %% 2;
$c div= 3; # return divisor and assignment. Equivalent to $c = $c div 3;
$d mod= 4; # return remainder and assignment. Equivalent to $d = $d mod 4;
@array .= sort; # calls the `sort` method and assigns the result back
####################################################
# 6. More on subs!
####################################################
# As we said before, Raku has *really* powerful subs. We're going
# to see a few more key concepts that make them better than in any
# other language :-).
#
# 6.1 Unpacking!
#
# Unpacking is the ability to "extract" arrays and keys
# (AKA "destructuring"). It'll work in `my`s and in parameter lists.
my ($f, $g) = 1, 2;
say $f; # OUTPUT: «1»
my ($, $, $h) = 1, 2, 3; # keep the non-interesting values anonymous (`$`)
say $h; # OUTPUT: «3»
my ($head, *@tail) = 1, 2, 3; # Yes, it's the same as with "slurpy subs"
my (*@small) = 1;
sub unpack_array( @array [$fst, $snd] ) {
say "My first is $fst, my second is $snd! All in all, I'm @array[].";
# (^ remember the `[]` to interpolate the array)
}
unpack_array(@tail);
# OUTPUT: «My first is 2, my second is 3! All in all, I'm 2 3.»
# If you're not using the array itself, you can also keep it anonymous,
# much like a scalar:
sub first-of-array( @ [$fst] ) { $fst }
first-of-array(@small); #=> 1
# However calling `first-of-array(@tail);` will throw an error ("Too many
# positional parameters passed"), which means the `@tail` has too many
# elements.
# You can also use a slurpy parameter. You could keep `*@rest` anonymous
# Here, `@rest` is `(3,)`, since `$fst` holds the `2`. This results
# since the length (.elems) of `@rest` is 1.
sub slurp-in-array(@ [$fst, *@rest]) {
say $fst + @rest.elems;
}
slurp-in-array(@tail); # OUTPUT: «3»
# You could even extract on a slurpy (but it's pretty useless ;-).)
sub fst(*@ [$fst]) { # or simply: `sub fst($fst) { ... }`
say $fst;
}
fst(1); # OUTPUT: «1»
# Calling `fst(1, 2);` will throw an error ("Too many positional parameters
# passed") though. After all, the `fst` sub declares only a single positional
# parameter.
# You can also destructure hashes (and classes, which you'll learn about later).
# The syntax is basically the same as
# `%hash-name (:key($variable-to-store-value-in))`.
# The hash can stay anonymous if you only need the values you extracted.
# In order to call the function, you must supply a hash wither created with
# curly braces or with `%()` (recommended). Alternatively, you can pass
# a variable that contains a hash.
sub key-of( % (:value($val), :qua($qua)) ) {
say "Got value $val, $qua time" ~~
$qua == 1 ?? '' !! 's';
}
my %foo-once = %(value => 'foo', qua => 1);
key-of({value => 'foo', qua => 2}); # OUTPUT: «Got val foo, 2 times.»
key-of(%(value => 'foo', qua => 0)); # OUTPUT: «Got val foo, 0 times.»
key-of(%foo-once); # OUTPUT: «Got val foo, 1 time.»
# The last expression of a sub is returned automatically (though you may
# indicate explicitly by using the `return` keyword, of course):
sub next-index( $n ) {
$n + 1;
}
my $new-n = next-index(3); # $new-n is now 4
# This is true for everything, except for the looping constructs (due to
# performance reasons): there's no reason to build a list if we're just going to
# discard all the results. If you still want to build one, you can use the
# `do` statement prefix or the `gather` prefix, which we'll see later:
sub list-of( $n ) {
do for ^$n { $_ }
}
my @list3 = list-of(3); #=> (0, 1, 2)
#
# 6.2 Lambdas (or anonymous subroutines)
#
# You can create a lambda by using a pointy block (`-> {}`), a
# block (`{}`) or creating a `sub` without a name.
my &lambda1 = -> $argument {
"The argument passed to this lambda is $argument"
}
my &lambda2 = {
"The argument passed to this lambda is $_"
}
my &lambda3 = sub ($argument) {
"The argument passed to this lambda is $argument"
}
# Both pointy blocks and blocks are pretty much the same thing, except that
# the former can take arguments, and that the latter can be mistaken as
# a hash by the parser. That being said, blocks can declare what's known
# as placeholders parameters through the twigils `$^` (for positional
# parameters) and `$:` (for named parameters). More on them later on.
my &mult = { $^numbers * $:times }
say mult 4, :times(6); #=> «24»
# Both pointy blocks and blocks are quite versatile when working with functions
# that accepts other functions such as `map`, `grep`, etc. For example,
# we add 3 to each value of an array using the `map` function with a lambda:
my @nums = 1..4;
my @res1 = map -> $v { $v + 3 }, @nums; # pointy block, explicit parameter
my @res2 = map { $_ + 3 }, @nums; # block using an implicit parameter
my @res3 = map { $^val + 3 }, @nums; # block with placeholder parameter
# A sub (`sub {}`) has different semantics than a block (`{}` or `-> {}`):
# A block doesn't have a "function context" (though it can have arguments),
# which means that if you return from it, you're going to return from the
# parent function.
# Compare:
sub is-in( @array, $elem ) {
say map({ return True if $_ == $elem }, @array);
say 'Hi';
}
# with:
sub truthy-array( @array ) {
say map sub ($i) { $i ?? return True !! return False }, @array;
say 'Hi';
}
# In the `is-in` sub, the block will `return` out of the `is-in` sub once the
# condition evaluates to `True`, the loop won't be run anymore and the
# following statement won't be executed. The last statement is only executed
# if the block never returns.
# On the contrary, the `truthy-array` sub will produce an array of `True` and
# `False`, which will printed, and always execute the last execute statement.
# Thus, the `return` only returns from the anonymous `sub`
# The `anon` declarator can be used to create an anonymous sub from a
# regular subroutine. The regular sub knows its name but its symbol is
# prevented from getting installed in the lexical scope, the method table
# and everywhere else.
my $anon-sum = anon sub summation(*@a) { [+] @a }
say $anon-sum.name; # OUTPUT: «summation»
say $anon-sum(2, 3, 5); # OUTPUT: «10»
#say summation; # Error: Undeclared routine: ...
# You can also use the Whatever Star to create an anonymous subroutine.
# (it'll stop at the furthest operator in the current expression).
# The following is the same as `{$_ + 3 }`, `-> { $a + 3 }`,
# `sub ($a) { $a + 3 }`, or even `{$^a + 3}` (more on this later).
my @arrayplus3v0 = map * + 3, @nums;
# The following is the same as `-> $a, $b { $a + $b + 3 }`,
# `sub ($a, $b) { $a + $b + 3 }`, or `{ $^a + $^b + 3 }` (more on this later).
my @arrayplus3v1 = map * + * + 3, @nums;
say (*/2)(4); # OUTPUT: «2», immediately execute the Whatever function created.
say ((*+3)/5)(5); # OUTPUT: «1.6», it works even in parens!
# But if you need to have more than one argument (`$_`) in a block (without
# wanting to resort to `-> {}`), you can also either `$^` and `$:` which
# declared placeholder parameters or self-declared positional/named parameters.
say map { $^a + $^b + 3 }, @nums;
# which is equivalent to the following which uses a `sub`:
map sub ($a, $b) { $a + $b + 3 }, @nums;
# Placeholder parameters are sorted lexicographically so the following two
# statements are equivalent:
say sort { $^b <=> $^a }, @nums;
say sort -> $a, $b { $b <=> $a }, @nums;
#
# 6.3 Multiple Dispatch
#
# Raku can decide which variant of a `sub` to call based on the type of the
# arguments, or on arbitrary preconditions, like with a type or `where`:
# with types:
multi sub sayit( Int $n ) { # note the `multi` keyword here
say "Number: $n";
}
multi sayit( Str $s ) { # a multi is a `sub` by default
say "String: $s";
}
sayit "foo"; # OUTPUT: «String: foo»
sayit 25; # OUTPUT: «Number: 25»
sayit True; # fails at *compile time* with "calling 'sayit' will never
# work with arguments of types ..."
# with arbitrary preconditions (remember subsets?):
multi is-big(Int $n where * > 50) { "Yes!" } # using a closure
multi is-big(Int $n where {$_ > 50}) { "Yes!" } # similar to above
multi is-big(Int $ where 10..50) { "Quite." } # Using smart-matching
multi is-big(Int $) { "No" }
subset Even of Int where * %% 2;
multi odd-or-even(Even) { "Even" } # Using the type. We don't name the argument.
multi odd-or-even($) { "Odd" } # "everything else" hence the $ variable
# You can even dispatch based on the presence of positional and named arguments:
multi with-or-without-you($with) {
say "I wish I could but I can't";
}
multi with-or-without-you(:$with) {
say "I can live! Actually, I can't.";
}
multi with-or-without-you {
say "Definitely can't live.";
}
# This is very, very useful for many purposes, like `MAIN` subs (covered
# later), and even the language itself uses it in several places.
# For example, the `is` trait is actually a `multi sub` named `trait_mod:<is>`,
# and it works off that. Thus, `is rw`, is simply a dispatch to a function with
# this signature `sub trait_mod:<is>(Routine $r, :$rw!) {}`
####################################################
# 7. About types...
####################################################
# Raku is gradually typed. This means you can specify the type of your
# variables/arguments/return types, or you can omit the type annotations in
# in which case they'll default to `Any`. Obviously you get access to a few
# base types, like `Int` and `Str`. The constructs for declaring types are
# `subset`, `class`, `role`, etc. which you'll see later.
# For now, let us examine `subset` which is a "sub-type" with additional
# checks. For example, "a very big integer is an `Int` that's greater than 500".
# You can specify the type you're subtyping (by default, `Any`), and add
# additional checks with the `where` clause.
subset VeryBigInteger of Int where * > 500;
# Or the set of the whole numbers:
subset WholeNumber of Int where * >= 0;
my WholeNumber $whole-six = 6; # OK
#my WholeNumber $nonwhole-one = -1; # Error: type check failed...
# Or the set of Positive Even Numbers whose Mod 5 is 1. Notice we're
# using the previously defined WholeNumber subset.
subset PENFO of WholeNumber where { $_ %% 2 and $_ mod 5 == 1 };
my PENFO $yes-penfo = 36; # OK
#my PENFO $no-penfo = 2; # Error: type check failed...
####################################################
# 8. Scoping
####################################################
# In Raku, unlike many scripting languages, (such as Python, Ruby, PHP),
# you must declare your variables before using them. The `my` declarator
# we've used so far uses "lexical scoping". There are a few other declarators,
# (`our`, `state`, ..., ) which we'll see later. This is called
# "lexical scoping", where in inner blocks, you can access variables from
# outer blocks.
my $file_scoped = 'Foo';
sub outer {
my $outer_scoped = 'Bar';
sub inner {
say "$file_scoped $outer_scoped";
}
&inner; # return the function
}
outer()(); # OUTPUT: «Foo Bar»
# As you can see, `$file_scoped` and `$outer_scoped` were captured.
# But if we were to try and use `$outer_scoped` outside the `outer` sub,
# the variable would be undefined (and you'd get a compile time error).
####################################################
# 9. Twigils
####################################################
# There are many special `twigils` (composed sigils) in Raku. Twigils
# define a variable's scope.
# The `*` and `?` twigils work on standard variables:
# * for dynamic variables
# ? for compile-time variables
#
# The `!` and the `.` twigils are used with Raku's objects:
# ! for attributes (instance attribute)
# . for methods (not really a variable)
#
# `*` twigil: Dynamic Scope
#
# These variables use the `*` twigil to mark dynamically-scoped variables.
# Dynamically-scoped variables are looked up through the caller, not through
# the outer scope.
my $*dyn_scoped_1 = 1;
my $*dyn_scoped_2 = 10;
sub say_dyn {
say "$*dyn_scoped_1 $*dyn_scoped_2";
}
sub call_say_dyn {
# Defines $*dyn_scoped_1 only for this sub.
my $*dyn_scoped_1 = 25;
# Will change the value of the file scoped variable.
$*dyn_scoped_2 = 100;
# $*dyn_scoped 1 and 2 will be looked for in the call.
say_dyn(); # OUTPUT: «25 100»
# The call to `say_dyn` uses the value of $*dyn_scoped_1 from inside
# this sub's lexical scope even though the blocks aren't nested (they're
# call-nested).
}
say_dyn(); # OUTPUT: «1 10»
# Uses $*dyn_scoped_1 as defined in `call_say_dyn` even though we are calling it
# from outside.
call_say_dyn(); # OUTPUT: «25 100»
# We changed the value of $*dyn_scoped_2 in `call_say_dyn` so now its
# value has changed.
say_dyn(); # OUTPUT: «1 100»
# TODO: Add information about remaining twigils
####################################################
# 10. Object Model
####################################################
# To call a method on an object, add a dot followed by the method name:
# `$object.method`
# Classes are declared with the `class` keyword. Attributes are declared
# with the `has` keyword, and methods declared with the `method` keyword.
# Every attribute that is private uses the `!` twigil. For example: `$!attr`.
# Immutable public attributes use the `.` twigil which creates a read-only
# method named after the attribute. In fact, declaring an attribute with `.`
# is equivalent to declaring the same attribute with `!` and then creating
# a read-only method with the attribute's name. However, this is done for us
# by Raku automatically. The easiest way to remember the `$.` twigil is
# by comparing it to how methods are called.
# Raku's object model ("SixModel") is very flexible, and allows you to
# dynamically add methods, change semantics, etc... Unfortunately, these will
# not all be covered here, and you should refer to:
# https://docs.raku.org/language/objects.html.
class Human {
has Str $.name; # `$.name` is immutable but with an accessor method.
has Str $.bcountry; # Use `$!bcountry` to modify it inside the class.
has Str $.ccountry is rw; # This attribute can be modified from outside.
has Int $!age = 0; # A private attribute with default value.
method birthday {
$!age += 1; # Add a year to human's age
}
method get-age {
return $!age;
}
# This method is private to the class. Note the `!` before the
# method's name.
method !do-decoration {
return "$!name born in $!bcountry and now lives in $!ccountry."
}
# This method is public, just like `birthday` and `get-age`.
method get-info {
# Invoking a method on `self` inside the class.
# Use `self!priv-method` for private method.
say self!do-decoration;
# Use `self.public-method` for public method.
say "Age: ", self.get-age;
}
};
# Create a new instance of Human class.
# NOTE: Only attributes declared with the `.` twigil can be set via the
# default constructor (more later on). This constructor only accepts named
# arguments.
my $person1 = Human.new(
name => "Jord",
bcountry => "Togo",
ccountry => "Togo"
);
# Make human 10 years old.
$person1.birthday for 1..10;
say $person1.name; # OUTPUT: «Jord»
say $person1.bcountry; # OUTPUT: «Togo»
say $person1.ccountry; # OUTPUT: «Togo»
say $person1.get-age; # OUTPUT: «10»
# This fails, because the `has $.bcountry`is immutable. Jord can't change
# his birthplace.
# $person1.bcountry = "Mali";
# This works because the `$.ccountry` is mutable (`is rw`). Now Jord's
# current country is France.
$person1.ccountry = "France";
# Calling methods on the instance objects.
$person1.birthday; #=> 1
$person1.get-info; #=> Jord born in Togo and now lives in France. Age: 10
# $person1.do-decoration; # This fails since the method `do-decoration` is private.
#
# 10.1 Object Inheritance
#
# Raku also has inheritance (along with multiple inheritance). While
# methods are inherited, submethods are not. Submethods are useful for
# object construction and destruction tasks, such as `BUILD`, or methods that
# must be overridden by subtypes. We will learn about `BUILD` later on.
class Parent {
has $.age;
has $.name;
# This submethod won't be inherited by the Child class.
submethod favorite-color {
say "My favorite color is Blue";
}
# This method is inherited
method talk { say "Hi, my name is $!name" }
}
# Inheritance uses the `is` keyword
class Child is Parent {
method talk { say "Goo goo ga ga" }
# This shadows Parent's `talk` method.
# This child hasn't learned to speak yet!
}
my Parent $Richard .= new(age => 40, name => 'Richard');
$Richard.favorite-color; # OUTPUT: «My favorite color is Blue»
$Richard.talk; # OUTPUT: «Hi, my name is Richard»
# $Richard is able to access the submethod and he knows how to say his name.
my Child $Madison .= new(age => 1, name => 'Madison');
$Madison.talk; # OUTPUT: «Goo goo ga ga», due to the overridden method.
# $Madison.favorite-color # does not work since it is not inherited.
# When you use `my T $var`, `$var` starts off with `T` itself in it, so you can
# call `new` on it. (`.=` is just the dot-call and the assignment operator).
# Thus, `$a .= b` is the same as `$a = $a.b`. Also note that `BUILD` (the method
# called inside `new`) will set parent's properties too, so you can pass `val =>
# 5`.
#
# 10.2 Roles, or Mixins
#
# Roles are supported too (which are called Mixins in other languages)
role PrintableVal {
has $!counter = 0;
method print {
say $.val;
}
}
# you "apply" a role (or mixin) with the `does` keyword:
class Item does PrintableVal {
has $.val;
=begin comment
When `does`-ed, a `role` literally "mixes in" the class:
the methods and attributes are put together, which means a class
can access the private attributes/methods of its roles (but
not the inverse!):
=end comment
method access {
say $!counter++;
}
=begin comment
However, this: method print {} is ONLY valid when `print` isn't a `multi`
with the same dispatch. This means a parent class can shadow a child class's
`multi print() {}`, but it's an error if a role does)
NOTE: You can use a role as a class (with `is ROLE`). In this case,
methods will be shadowed, since the compiler will consider `ROLE`
to be a class.
=end comment
}
####################################################
# 11. Exceptions
####################################################
# Exceptions are built on top of classes, in the package `X` (like `X::IO`).
# In Raku, exceptions are automatically 'thrown':
# open 'foo'; # OUTPUT: «Failed to open file foo: no such file or directory»
# It will also print out what line the error was thrown at
# and other error info.
# You can throw an exception using `die`. Here it's been commented out to
# avoid stopping the program's execution:
# die 'Error!'; # OUTPUT: «Error!»
# Or more explicitly (commented out too):
# X::AdHoc.new(payload => 'Error!').throw; # OUTPUT: «Error!»
# In Raku, `orelse` is similar to the `or` operator, except it only matches
# undefined variables instead of anything evaluating as `False`.
# Undefined values include: `Nil`, `Mu` and `Failure` as well as `Int`, `Str`
# and other types that have not been initialized to any value yet.
# You can check if something is defined or not using the defined method:
my $uninitialized;
say $uninitialized.defined; # OUTPUT: «False»
# When using `orelse` it will disarm the exception and alias $_ to that
# failure. This will prevent it to being automatically handled and printing
# lots of scary error messages to the screen. We can use the `exception`
# method on the `$_` variable to access the exception
open 'foo' orelse say "Something happened {.exception}";
# This also works:
open 'foo' orelse say "Something happened $_";
# OUTPUT: «Something happened Failed to open file foo: no such file or directory»
# Both of those above work but in case we get an object from the left side
# that is not a failure we will probably get a warning. We see below how we
# can use try` and `CATCH` to be more specific with the exceptions we catch.
#
# 11.1 Using `try` and `CATCH`
#
# By using `try` and `CATCH` you can contain and handle exceptions without
# disrupting the rest of the program. The `try` block will set the last
# exception to the special variable `$!` (known as the error variable).
# NOTE: This has no relation to $!variables seen inside class definitions.
try open 'foo';
say "Well, I tried! $!" if defined $!;
# OUTPUT: «Well, I tried! Failed to open file foo: no such file or directory»
# Now, what if we want more control over handling the exception?
# Unlike many other languages, in Raku, you put the `CATCH` block *within*
# the block to `try`. Similar to how the `$_` variable was set when we
# 'disarmed' the exception with `orelse`, we also use `$_` in the CATCH block.
# NOTE: The `$!` variable is only set *after* the `try` block has caught an
# exception. By default, a `try` block has a `CATCH` block of its own that
# catches any exception (`CATCH { default {} }`).
try {
my $a = (0 %% 0);
CATCH {
default { say "Something happened: $_" }
}
}
# OUTPUT: «Something happened: Attempt to divide by zero using infix:<%%>»
# You can redefine it using `when`s (and `default`) to handle the exceptions
# you want to catch explicitly:
try {
open 'foo';
CATCH {
# In the `CATCH` block, the exception is set to the $_ variable.
when X::AdHoc {
say "Error: $_"
}
when X::Numeric::DivideByZero {
say "Error: $_";
}
=begin comment
Any other exceptions will be re-raised, since we don't have a `default`.
Basically, if a `when` matches (or there's a `default`), the
exception is marked as "handled" so as to prevent its re-throw
from the `CATCH` block. You still can re-throw the exception
(see below) by hand.
=end comment
default {
say "Any other error: $_"
}
}
}
# OUTPUT: «Failed to open file /dir/foo: no such file or directory»
# There are also some subtleties to exceptions. Some Raku subs return a
# `Failure`, which is a wrapper around an `Exception` object which is
# "unthrown". They're not thrown until you try to use the variables containing
# them unless you call `.Bool`/`.defined` on them - then they're handled.
# (the `.handled` method is `rw`, so you can mark it as `False` back yourself)
# You can throw a `Failure` using `fail`. Note that if the pragma `use fatal`
# is on, `fail` will throw an exception (like `die`).
my $value = 0/0; # We're not trying to access the value, so no problem.
try {
say 'Value: ', $value; # Trying to use the value
CATCH {
default {
say "It threw because we tried to get the fail's value!"
}
}
}
# There is also another kind of exception: Control exceptions. Those are "good"
# exceptions, which happen when you change your program's flow, using operators
# like `return`, `next` or `last`. You can "catch" those with `CONTROL` (not 100%
# working in Rakudo yet).
####################################################
# 12. Packages
####################################################
# Packages are a way to reuse code. Packages are like "namespaces", and any
# element of the six model (`module`, `role`, `class`, `grammar`, `subset` and
# `enum`) are actually packages. (Packages are the lowest common denominator)
# Packages are important - especially as Perl is well-known for CPAN,
# the Comprehensive Perl Archive Network.
# You can use a module (bring its declarations into scope) with `use`:
use JSON::Tiny; # if you installed Rakudo* or Panda, you'll have this module
say from-json('[1]').perl; # OUTPUT: «[1]»
# You should not declare packages using the `package` keyword (unlike Perl).
# Instead, use `class Package::Name::Here;` to declare a class, or if you only
# want to export variables/subs, you can use `module` instead.
# If `Hello` doesn't exist yet, it'll just be a "stub", that can be redeclared
# as something else later.
module Hello::World { # bracketed form
# declarations here
}
# The file-scoped form which extends until the end of the file. For
# instance, `unit module Parse::Text;` will extend until of the file.
# A grammar is a package, which you could `use`. You will learn more about
# grammars in the regex section.
grammar Parse::Text::Grammar {
}
# As said before, any part of the six model is also a package.
# Since `JSON::Tiny` uses its own `JSON::Tiny::Actions` class, you can use it:
my $actions = JSON::Tiny::Actions.new;
# We'll see how to export variables and subs in the next part.
####################################################
# 13. Declarators
####################################################
# In Raku, you get different behaviors based on how you declare a variable.
# You've already seen `my` and `has`, we'll now explore the others.
# `our` - these declarations happen at `INIT` time -- (see "Phasers" below).
# It's like `my`, but it also creates a package variable. All packagish
# things such as `class`, `role`, etc. are `our` by default.
module Var::Increment {
# NOTE: `our`-declared variables cannot be typed.
our $our-var = 1;
my $my-var = 22;
our sub Inc {
our sub available { # If you try to make inner `sub`s `our`...
# ... Better know what you're doing (Don't !).
say "Don't do that. Seriously. You'll get burned.";
}
my sub unavailable { # `sub`s are `my`-declared by default
say "Can't access me from outside, I'm 'my'!";
}
say ++$our-var; # Increment the package variable and output its value
}
}
say $Var::Increment::our-var; # OUTPUT: «1», this works!
say $Var::Increment::my-var; # OUTPUT: «(Any)», this will not work!
say Var::Increment::Inc; # OUTPUT: «2»
say Var::Increment::Inc; # OUTPUT: «3», notice how the value of $our-var was retained.
# Var::Increment::unavailable; # OUTPUT: «Could not find symbol '&unavailable'»
# `constant` - these declarations happen at `BEGIN` time. You can use
# the `constant` keyword to declare a compile-time variable/symbol:
constant Pi = 3.14;
constant $var = 1;
# And if you're wondering, yes, it can also contain infinite lists.
constant why-not = 5, 15 ... *;
say why-not[^5]; # OUTPUT: «5 15 25 35 45»
# `state` - these declarations happen at run time, but only once. State
# variables are only initialized one time. In other languages such as C
# they exist as `static` variables.
sub fixed-rand {
state $val = rand;
say $val;
}
fixed-rand for ^10; # will print the same number 10 times
# Note, however, that they exist separately in different enclosing contexts.
# If you declare a function with a `state` within a loop, it'll re-create the
# variable for each iteration of the loop. See:
for ^5 -> $a {
sub foo {
# This will be a different value for every value of `$a`
state $val = rand;
}
for ^5 -> $b {
# This will print the same value 5 times, but only 5. Next iteration
# will re-run `rand`.
say foo;
}
}
####################################################
# 14. Phasers
####################################################
# Phasers in Raku are blocks that happen at determined points of time in
# your program. They are called phasers because they mark a change in the
# phase of a program. For example, when the program is compiled, a for loop
# runs, you leave a block, or an exception gets thrown (The `CATCH` block is
# actually a phaser!). Some of them can be used for their return values,
# some of them can't (those that can have a "[*]" in the beginning of their
# explanation text). Let's have a look!
#
# 14.1 Compile-time phasers
#
BEGIN { say "[*] Runs at compile time, as soon as possible, only once" }
CHECK { say "[*] Runs at compile time, as late as possible, only once" }
#
# 14.2 Run-time phasers
#
INIT { say "[*] Runs at run time, as soon as possible, only once" }
END { say "Runs at run time, as late as possible, only once" }
#
# 14.3 Block phasers
#
ENTER { say "[*] Runs every time you enter a block, repeats on loop blocks" }
LEAVE {
say "Runs every time you leave a block, even when an exception
happened. Repeats on loop blocks."
}
PRE {
say "Asserts a precondition at every block entry,
before ENTER (especially useful for loops)";
say "If this block doesn't return a truthy value,
an exception of type X::Phaser::PrePost is thrown.";
}
# Example (commented out):
for 0..2 {
# PRE { $_ > 1 } # OUTPUT: «Precondition '{ $_ > 1 }' failed
}
POST {
say "Asserts a postcondition at every block exit,
after LEAVE (especially useful for loops)";
say "If this block doesn't return a truthy value,
an exception of type X::Phaser::PrePost is thrown, like PRE.";
}
# Example (commented out):
for 0..2 {
# POST { $_ < 1 } # OUTPUT: «Postcondition '{ $_ < 1 }' failed
}
#
# 14.4 Block/exceptions phasers
#
{
KEEP { say "Runs when you exit a block successfully
(without throwing an exception)" }
UNDO { say "Runs when you exit a block unsuccessfully
(by throwing an exception)" }
}
#
# 14.5 Loop phasers
#
for ^5 {
FIRST { say "[*] The first time the loop is run, before ENTER" }
NEXT { say "At loop continuation time, before LEAVE" }
LAST { say "At loop termination time, after LEAVE" }
}
#
# 14.6 Role/class phasers
#
COMPOSE {
say "When a role is composed into a class. /!\ NOT YET IMPLEMENTED"
}
# They allow for cute tricks or clever code...:
say "This code took " ~ (time - CHECK time) ~ "s to compile";
# ... or clever organization:
class DB {
method start-transaction { say "Starting transaction!" }
method commit { say "Committing transaction..." }
method rollback { say "Something went wrong. Rolling back!" }
}
sub do-db-stuff {
my DB $db .= new;
$db.start-transaction; # start a new transaction
KEEP $db.commit; # commit the transaction if all went well
UNDO $db.rollback; # or rollback if all hell broke loose
}
do-db-stuff();
####################################################
# 15. Statement prefixes
####################################################
# Those act a bit like phasers: they affect the behavior of the following
# code. Though, they run in-line with the executable code, so they're in
# lowercase. (`try` and `start` are theoretically in that list, but explained
# elsewhere) NOTE: all of these (except start) don't need explicit curly
# braces `{` and `}`.
#
# 15.1 `do` - It runs a block or a statement as a term.
#
# Normally you cannot use a statement as a value (or "term"). `do` helps
# us do it. With `do`, an `if`, for example, becomes a term returning a value.
=for comment :reason<this fails since `if` is a statement>
my $value = if True { 1 }
# this works!
my $get-five = do if True { 5 }
#
# 15.1 `once` - makes sure a piece of code only runs once.
#
for ^5 {
once say 1
};
# OUTPUT: «1», only prints ... once
# Similar to `state`, they're cloned per-scope.
for ^5 {
sub { once say 1 }()
};
# OUTPUT: «1 1 1 1 1», prints once per lexical scope.
#
# 15.2 `gather` - co-routine thread.
#
# The `gather` constructs allows us to `take` several values from an array/list,
# much like `do`.
say gather for ^5 {
take $_ * 3 - 1;
take $_ * 3 + 1;
}
# OUTPUT: «-1 1 2 4 5 7 8 10 11 13»
say join ',', gather if False {
take 1;
take 2;
take 3;
}
# Doesn't print anything.
#
# 15.3 `eager` - evaluates a statement eagerly (forces eager context).
# Don't try this at home. This will probably hang for a while (and might crash)
# so commented out.
# eager 1..*;
# But consider, this version which doesn't print anything
constant thricev0 = gather for ^3 { say take $_ };
# to:
constant thricev1 = eager gather for ^3 { say take $_ }; # OUTPUT: «0 1 2»
####################################################
# 16. Iterables
####################################################
# Iterables are objects that can be iterated over for things such as
# the `for` construct.
#
# 16.1 `flat` - flattens iterables.
#
say (1, 10, (20, 10) ); # OUTPUT: «(1 10 (20 10))», notice how nested
# lists are preserved
say (1, 10, (20, 10) ).flat; # OUTPUT: «(1 10 20 10)», now the iterable is flat
#
# 16.2 `lazy` - defers actual evaluation until value is fetched by forcing lazy context.
#
my @lazy-array = (1..100).lazy;
say @lazy-array.is-lazy; # OUTPUT: «True», check for laziness with the `is-lazy` method.
say @lazy-array; # OUTPUT: «[...]», List has not been iterated on!
# This works and will only do as much work as is needed.
for @lazy-array { .print };
# (**TODO** explain that gather/take and map are all lazy)
#
# 16.3 `sink` - an `eager` that discards the results by forcing sink context.
#
constant nilthingie = sink for ^3 { .say } #=> 0 1 2
say nilthingie.perl; # OUTPUT: «Nil»
#
# 16.4 `quietly` - suppresses warnings in blocks.
#
quietly { warn 'This is a warning!' }; # No output
####################################################
# 17. More operators thingies!
####################################################
# Everybody loves operators! Let's get more of them.
# The precedence list can be found here:
# https://docs.raku.org/language/operators#Operator_Precedence
# But first, we need a little explanation about associativity:
#
# 17.1 Binary operators
#
my ($p, $q, $r) = (1, 2, 3);
# Given some binary operator § (not a Raku-supported operator), then:
# $p § $q § $r; # with a left-associative §, this is ($p § $q) § $r
# $p § $q § $r; # with a right-associative §, this is $p § ($q § $r)
# $p § $q § $r; # with a non-associative §, this is illegal
# $p § $q § $r; # with a chain-associative §, this is ($p § $q) and ($q § $r)§
# $p § $q § $r; # with a list-associative §, this is `infix:<>`
#
# 17.2 Unary operators
#
# Given some unary operator § (not a Raku-supported operator), then:
# §$p§ # with left-associative §, this is (§$p)§
# §$p§ # with right-associative §, this is §($p§)
# §$p§ # with non-associative §, this is illegal
#
# 17.3 Create your own operators!
#
# Okay, you've been reading all of that, so you might want to try something
# more exciting?! I'll tell you a little secret (or not-so-secret):
# In Raku, all operators are actually just funny-looking subroutines.
# You can declare an operator just like you declare a sub. In the following
# example, `prefix` refers to the operator categories (prefix, infix, postfix,
# circumfix, and post-circumfix).
sub prefix:<win>( $winner ) {
say "$winner Won!";
}
win "The King"; # OUTPUT: «The King Won!»
# you can still call the sub with its "full name":
say prefix:<!>(True); # OUTPUT: «False»
prefix:<win>("The Queen"); # OUTPUT: «The Queen Won!»
sub postfix:<!>( Int $n ) {
[*] 2..$n; # using the reduce meta-operator... See below ;-)!
}
say 5!; # OUTPUT: «120»
# Postfix operators ('after') have to come *directly* after the term.
# No whitespace. You can use parentheses to disambiguate, i.e. `(5!)!`
sub infix:<times>( Int $n, Block $r ) { # infix ('between')
for ^$n {
# You need the explicit parentheses to call the function in `$r`,
# else you'd be referring at the code object itself, like with `&r`.
$r();
}
}
3 times -> { say "hello" }; # OUTPUT: «hellohellohello»
# It's recommended to put spaces around your infix operator calls.
# For circumfix and post-circumfix ones
multi circumfix:<[ ]>( Int $n ) {
$n ** $n
}
say [5]; # OUTPUT: «3125»
# Circumfix means 'around'. Again, no whitespace.
multi postcircumfix:<{ }>( Str $s, Int $idx ) {
$s.substr($idx, 1);
}
say "abc"{1}; # OUTPUT: «b», after the term `"abc"`, and around the index (1)
# Post-circumfix is 'after a term, around something'
# This really means a lot -- because everything in Raku uses this.
# For example, to delete a key from a hash, you use the `:delete` adverb
# (a simple named argument underneath). For instance, the following statements
# are equivalent.
my %person-stans =
'Giorno Giovanna' => 'Gold Experience',
'Bruno Bucciarati' => 'Sticky Fingers';
my $key = 'Bruno Bucciarati';
%person-stans{$key}:delete;
postcircumfix:<{ }>( %person-stans, 'Giorno Giovanna', :delete );
# (you can call operators like this)
# It's *all* using the same building blocks! Syntactic categories
# (prefix infix ...), named arguments (adverbs), ..., etc. used to build
# the language - are available to you. Obviously, you're advised against
# making an operator out of *everything* -- with great power comes great
# responsibility.
#
# 17.4 Meta operators!
#
# Oh boy, get ready!. Get ready, because we're delving deep into the rabbit's
# hole, and you probably won't want to go back to other languages after
# reading this. (I'm guessing you don't want to go back at this point but
# let's continue, for the journey is long and enjoyable!).
# Meta-operators, as their name suggests, are *composed* operators. Basically,
# they're operators that act on another operators.
# The reduce meta-operator is a prefix meta-operator that takes a binary
# function and one or many lists. If it doesn't get passed any argument,
# it either returns a "default value" for this operator (a meaningless value)
# or `Any` if there's none (examples below). Otherwise, it pops an element
# from the list(s) one at a time, and applies the binary function to the last
# result (or the first element of a list) and the popped element.
# To sum a list, you could use the reduce meta-operator with `+`, i.e.:
say [+] 1, 2, 3; # OUTPUT: «6», equivalent to (1+2)+3.
# To multiply a list
say [*] 1..5; # OUTPUT: «120», equivalent to ((((1*2)*3)*4)*5).
# You can reduce with any operator, not just with mathematical ones.
# For example, you could reduce with `//` to get first defined element
# of a list:
say [//] Nil, Any, False, 1, 5; # OUTPUT: «False»
# (Falsey, but still defined)
# Or with relational operators, i.e., `>` to check elements of a list
# are ordered accordingly:
say [>] 234, 156, 6, 3, -20; # OUTPUT: «True»
# Default value examples:
say [*] (); # OUTPUT: «1», empty product
say [+] (); # OUTPUT: «0», empty sum
say [//]; # OUTPUT: «(Any)»
# There's no "default value" for `//`.
# You can also use it with a function you made up,
# You can also surround using double brackets:
sub add($a, $b) { $a + $b }
say [[&add]] 1, 2, 3; # OUTPUT: «6»
# The zip meta-operator is an infix meta-operator that also can be used as a
# "normal" operator. It takes an optional binary function (by default, it
# just creates a pair), and will pop one value off of each array and call
# its binary function on these until it runs out of elements. It returns an
# array with all of these new elements.
say (1, 2) Z (3, 4); # OUTPUT: «((1, 3), (2, 4))»
say 1..3 Z+ 4..6; # OUTPUT: «(5, 7, 9)»
# Since `Z` is list-associative (see the list above), you can use it on more
# than one list.
(True, False) Z|| (False, False) Z|| (False, False); # (True, False)
# And, as it turns out, you can also use the reduce meta-operator with it:
[Z||] (True, False), (False, False), (False, False); # (True, False)
# And to end the operator list:
# The sequence operator (`...`) is one of Raku's most powerful features:
# It's composed by the list (which might include a closure) you want Raku to
# deduce from on the left and a value (or either a predicate or a Whatever Star
# for a lazy infinite list) on the right that states when to stop.
# Basic arithmetic sequence
my @listv0 = 1, 2, 3...10;
# This dies because Raku can't figure out the end
# my @list = 1, 3, 6...10;
# As with ranges, you can exclude the last element (the iteration ends when
# the predicate matches).
my @listv1 = 1, 2, 3...^10;
# You can use a predicate (with the Whatever Star).
my @listv2 = 1, 3, 9...* > 30;
# Equivalent to the example above but using a block here.
my @listv3 = 1, 3, 9 ... { $_ > 30 };
# Lazy infinite list of fibonacci sequence, computed using a closure!
my @fibv0 = 1, 1, *+* ... *;
# Equivalent to the above example but using a pointy block.
my @fibv1 = 1, 1, -> $a, $b { $a + $b } ... *;
# Equivalent to the above example but using a block with placeholder parameters.
my @fibv2 = 1, 1, { $^a + $^b } ... *;
# In the examples with explicit parameters (i.e., $a and $b), $a and $b
# will always take the previous values, meaning that for the Fibonacci sequence,
# they'll start with $a = 1 and $b = 1 (values we set by hand), then $a = 1
# and $b = 2 (result from previous $a + $b), and so on.
# In the example we use a range as an index to access the sequence. However,
# it's worth noting that for ranges, once reified, elements aren't re-calculated.
# That's why, for instance, `@primes[^100]` will take a long time the first
# time you print it but then it will be instantaneous.
say @fibv0[^10]; # OUTPUT: «1 1 2 3 5 8 13 21 34 55»
####################################################
# 18. Regular Expressions
####################################################
# I'm sure a lot of you have been waiting for this one. Well, now that you know
# a good deal of Raku already, we can get started. First off, you'll have to
# forget about "PCRE regexps" (perl-compatible regexps).
# IMPORTANT: Don't skip them because you know PCRE. They're different. Some
# things are the same (like `?`, `+`, and `*`), but sometimes the semantics
# change (`|`). Make sure you read carefully, because you might trip over a
# new behavior.
# Raku has many features related to RegExps. After all, Rakudo parses itself.
# We're first going to look at the syntax itself, then talk about grammars
# (PEG-like), differences between `token`, `regex` and `rule` declarators,
# and some more. Side note: you still have access to PCRE regexps using the
# `:P5` modifier which we won't be discussing this in this tutorial, though.
# In essence, Raku natively implements PEG ("Parsing Expression Grammars").
# The pecking order for ambiguous parses is determined by a multi-level
# tie-breaking test:
# - Longest token matching: `foo\s+` beats `foo` (by 2 or more positions)
# - Longest literal prefix: `food\w*` beats `foo\w*` (by 1)
# - Declaration from most-derived to less derived grammars
# (grammars are actually classes)
# - Earliest declaration wins
say so 'a' ~~ /a/; # OUTPUT: «True»
say so 'a' ~~ / a /; # OUTPUT: «True», more readable with some spaces!
# In all our examples, we're going to use the smart-matching operator against
# a regexp. We're converting the result using `so` to a Boolean value because,
# in fact, it's returning a `Match` object. They know how to respond to list
# indexing, hash indexing, and return the matched string. The results of the
# match are available in the `$/` variable (implicitly lexically-scoped). You
# can also use the capture variables which start at 0: `$0`, `$1', `$2`...
# You can also note that `~~` does not perform start/end checking, meaning
# the regexp can be matched with just one character of the string. We'll
# explain later how you can do it.
# In Raku, you can have any alphanumeric as a literal, everything else has
# to be escaped by using a backslash or quotes.
say so 'a|b' ~~ / a '|' b /; # OUTPUT: «True», it wouldn't mean the same
# thing if `|` wasn't escaped.
say so 'a|b' ~~ / a \| b /; # OUTPUT: «True», another way to escape it.
# The whitespace in a regex is actually not significant, unless you use the
# `:s` (`:sigspace`, significant space) adverb.
say so 'a b c' ~~ / a b c /; #=> `False`, space is not significant here!
say so 'a b c' ~~ /:s a b c /; #=> `True`, we added the modifier `:s` here.
# If we use only one space between strings in a regex, Raku will warn us
# about space being not signicant in the regex:
say so 'a b c' ~~ / a b c /; # OUTPUT: «False»
say so 'a b c' ~~ / a b c /; # OUTPUT: «False»
# NOTE: Please use quotes or `:s` (`:sigspace`) modifier (or, to suppress this
# warning, omit the space, or otherwise change the spacing). To fix this and make
# the spaces less ambiguous, either use at least two spaces between strings
# or use the `:s` adverb.
# As we saw before, we can embed the `:s` inside the slash delimiters, but we
# can also put it outside of them if we specify `m` for 'match':
say so 'a b c' ~~ m:s/a b c/; # OUTPUT: «True»
# By using `m` to specify 'match', we can also use other delimiters:
say so 'abc' ~~ m{a b c}; # OUTPUT: «True»
say so 'abc' ~~ m[a b c]; # OUTPUT: «True»
# `m/.../` is equivalent to `/.../`:
say 'raku' ~~ m/raku/; # OUTPUT: «True»
say 'raku' ~~ /raku/; # OUTPUT: «True»
# Use the `:i` adverb to specify case insensitivity:
say so 'ABC' ~~ m:i{a b c}; # OUTPUT: «True»
# However, whitespace is important as for how modifiers are applied
# (which you'll see just below) ...
#
# 18.1 Quantifiers - `?`, `+`, `*` and `**`.
#
# `?` - zero or one match
say so 'ac' ~~ / a b c /; # OUTPUT: «False»
say so 'ac' ~~ / a b? c /; # OUTPUT: «True», the "b" matched 0 times.
say so 'abc' ~~ / a b? c /; # OUTPUT: «True», the "b" matched 1 time.
# ... As you read before, whitespace is important because it determines which
# part of the regex is the target of the modifier:
say so 'def' ~~ / a b c? /; # OUTPUT: «False», only the "c" is optional
say so 'def' ~~ / a b? c /; # OUTPUT: «False», whitespace is not significant
say so 'def' ~~ / 'abc'? /; # OUTPUT: «True», the whole "abc" group is optional
# Here (and below) the quantifier applies only to the "b"
# `+` - one or more matches
say so 'ac' ~~ / a b+ c /; # OUTPUT: «False», `+` wants at least one 'b'
say so 'abc' ~~ / a b+ c /; # OUTPUT: «True», one is enough
say so 'abbbbc' ~~ / a b+ c /; # OUTPUT: «True», matched 4 "b"s
# `*` - zero or more matches
say so 'ac' ~~ / a b* c /; # OUTPUT: «True», they're all optional
say so 'abc' ~~ / a b* c /; # OUTPUT: «True»
say so 'abbbbc' ~~ / a b* c /; # OUTPUT: «True»
say so 'aec' ~~ / a b* c /; # OUTPUT: «False», "b"(s) are optional, not replaceable.
# `**` - (Unbound) Quantifier
# If you squint hard enough, you might understand why exponentation is used
# for quantity.
say so 'abc' ~~ / a b**1 c /; # OUTPUT: «True», exactly one time
say so 'abc' ~~ / a b**1..3 c /; # OUTPUT: «True», one to three times
say so 'abbbc' ~~ / a b**1..3 c /; # OUTPUT: «True»
say so 'abbbbbbc' ~~ / a b**1..3 c /; # OUTPUT: «Fals», too much
say so 'abbbbbbc' ~~ / a b**3..* c /; # OUTPUT: «True», infinite ranges are ok
#
# 18.2 `<[]>` - Character classes
#
# Character classes are the equivalent of PCRE's `[]` classes, but they use a
# more raku-ish syntax:
say 'fooa' ~~ / f <[ o a ]>+ /; # OUTPUT: «fooa»
# You can use ranges (`..`):
say 'aeiou' ~~ / a <[ e..w ]> /; # OUTPUT: «ae»
# Just like in normal regexes, if you want to use a special character, escape
# it (the last one is escaping a space which would be equivalent to using
# ' '):
say 'he-he !' ~~ / 'he-' <[ a..z \! \ ]> + /; # OUTPUT: «he-he !»
# You'll get a warning if you put duplicate names (which has the nice effect
# of catching the raw quoting):
'he he' ~~ / <[ h e ' ' ]> /;
# Warns "Repeated character (') unexpectedly found in character class"
# You can also negate character classes... (`<-[]>` equivalent to `[^]` in PCRE)
say so 'foo' ~~ / <-[ f o ]> + /; # OUTPUT: «False»
# ... and compose them:
# any letter except "f" and "o"
say so 'foo' ~~ / <[ a..z ] - [ f o ]> + /; # OUTPUT: «False»
# no letter except "f" and "o"
say so 'foo' ~~ / <-[ a..z ] + [ f o ]> + /; # OUTPUT: «True»
# the + doesn't replace the left part
say so 'foo!' ~~ / <-[ a..z ] + [ f o ]> + /; # OUTPUT: «True»
#
# 18.3 Grouping and capturing
#
# Group: you can group parts of your regexp with `[]`. Unlike PCRE's `(?:)`,
# these groups are *not* captured.
say so 'abc' ~~ / a [ b ] c /; # OUTPUT: «True», the grouping does nothing
say so 'foo012012bar' ~~ / foo [ '01' <[0..9]> ] + bar /; # OUTPUT: «True»
# The previous line returns `True`. The regex matches "012" one or more time
# (achieved by the the `+` applied to the group).
# But this does not go far enough, because we can't actually get back what
# we matched.
# Capture: The results of a regexp can be *captured* by using parentheses.
say so 'fooABCABCbar' ~~ / foo ( 'A' <[A..Z]> 'C' ) + bar /; # OUTPUT: «True»
# (using `so` here, see `$/` below)
# So, starting with the grouping explanations. As we said before, our `Match`
# object is stored inside the `$/` variable:
say $/; # Will either print the matched object or `Nil` if nothing matched.
# As we also said before, it has array indexing:
say $/[0]; # OUTPUT: «｢ABC｣ ｢ABC｣»,
# The corner brackets (｢..｣) represent (and are) `Match` objects. In the
# previous example, we have an array of them.
say $0; # The same as above.
# Our capture is `$0` because it's the first and only one capture in the
# regexp. You might be wondering why it's an array, and the answer is simple:
# Some captures (indexed using `$0`, `$/[0]` or a named one) will be an array
# if and only if they can have more than one element. Thus any capture with
# `*`, `+` and `**` (whatever the operands), but not with `?`.
# Let's use examples to see that:
# NOTE: We quoted A B C to demonstrate that the whitespace between them isn't
# significant. If we want the whitespace to *be* significant there, we can use the
# `:sigspace` modifier.
say so 'fooABCbar' ~~ / foo ( "A" "B" "C" )? bar /; # OUTPUT: «True»
say $/[0]; # OUTPUT: «｢ABC｣»
say $0.WHAT; # OUTPUT: «(Match)»
# There can't be more than one, so it's only a single match object.
say so 'foobar' ~~ / foo ( "A" "B" "C" )? bar /; # OUTPUT: «True»
say $0.WHAT; # OUTPUT: «(Any)», this capture did not match, so it's empty.
say so 'foobar' ~~ / foo ( "A" "B" "C" ) ** 0..1 bar /; #=> OUTPUT: «True»
say $0.WHAT; # OUTPUT: «(Array)», A specific quantifier will always capture
# an Array, be a range or a specific value (even 1).
# The captures are indexed per nesting. This means a group in a group will be
# nested under its parent group: `$/[0][0]`, for this code:
'hello-~-world' ~~ / ( 'hello' ( <[ \- \~ ]> + ) ) 'world' /;
say $/[0].Str; # OUTPUT: «hello~»
say $/[0][0].Str; # OUTPUT: «~»
# This stems from a very simple fact: `$/` does not contain strings, integers
# or arrays, it only contains `Match` objects. These contain the `.list`, `.hash`
# and `.Str` methods but you can also just use `match<key>` for hash access
# and `match[idx]` for array access.
# In the following example, we can see `$_` is a list of `Match` objects.
# Each of them contain a wealth of information: where the match started/ended,
# the "ast" (see actions later), etc. You'll see named capture below with
# grammars.
say $/[0].list.perl; # OUTPUT: «(Match.new(...),).list»
# Alternation - the `or` of regexes
# WARNING: They are DIFFERENT from PCRE regexps.
say so 'abc' ~~ / a [ b | y ] c /; # OUTPUT: «True», Either "b" or "y".
say so 'ayc' ~~ / a [ b | y ] c /; # OUTPUT: «True», Obviously enough...
# The difference between this `|` and the one you're used to is
# LTM ("Longest Token Matching") strategy. This means that the engine will
# always try to match as much as possible in the string.
say 'foo' ~~ / fo | foo /; # OUTPUT: «foo», instead of `fo`, because it's longer.
# To decide which part is the "longest", it first splits the regex in two parts:
#
# * The "declarative prefix" (the part that can be statically analyzed)
# which includes alternations (`|`), conjunctions (`&`), sub-rule calls (not
# yet introduced), literals, characters classes and quantifiers.
#
# * The "procedural part" includes everything else: back-references,
# code assertions, and other things that can't traditionally be represented
# by normal regexps.
# Then, all the alternatives are tried at once, and the longest wins.
# Examples:
# DECLARATIVE | PROCEDURAL
/ 'foo' \d+ [ <subrule1> || <subrule2> ] /;
# DECLARATIVE (nested groups are not a problem)
/ \s* [ \w & b ] [ c | d ] /;
# However, closures and recursion (of named regexes) are procedural.
# There are also more complicated rules, like specificity (literals win
# over character classes).
# NOTE: The alternation in which all the branches are tried in order
# until the first one matches still exists, but is now spelled `||`.
say 'foo' ~~ / fo || foo /; # OUTPUT: «fo», in this case.
####################################################
# 19. Extra: the MAIN subroutine
####################################################
# The `MAIN` subroutine is called when you run a Raku file directly. It's
# very powerful, because Raku actually parses the arguments and pass them
# as such to the sub. It also handles named argument (`--foo`) and will even
# go as far as to autogenerate a `--help` flag.
sub MAIN($name) {
say "Hello, $name!";
}
# Supposing the code above is in file named cli.raku, then running in the command
# line (e.g., $ raku cli.raku) produces:
# Usage:
# cli.raku <name>
# And since MAIN is a regular Raku sub, you can have multi-dispatch:
# (using a `Bool` for the named argument so that we can do `--replace`
# instead of `--replace=1`. The presence of `--replace` indicates truthness
# while its absence falseness). For example:
# convert to IO object to check the file exists
=begin comment
subset File of Str where *.IO.d;
multi MAIN('add', $key, $value, Bool :$replace) { ... }
multi MAIN('remove', $key) { ... }
multi MAIN('import', File, Str :$as) { ... } # omitting parameter name
=end comment
# Thus $ raku cli.raku produces:
# Usage:
# cli.raku [--replace] add <key> <value>
# cli.raku remove <key>
# cli.raku [--as=<Str>] import <File>
# As you can see, this is *very* powerful. It even went as far as to show inline
# the constants (the type is only displayed if the argument is `$`/is named).
####################################################
# 20. APPENDIX A:
####################################################
# It's assumed by now you know the Raku basics. This section is just here to
# list some common operations, but which are not in the "main part" of the
# tutorial to avoid bloating it up.
#
# 20.1 Operators
#
# Sort comparison - they return one value of the `Order` enum: `Less`, `Same`
# and `More` (which numerify to -1, 0 or +1 respectively).
say 1 <=> 4; # OUTPUT: «More», sort comparison for numerics
say 'a' leg 'b'; # OUTPUT: «Lessre», sort comparison for string
say 1 eqv 1; # OUTPUT: «Truere», sort comparison using eqv semantics
say 1 eqv 1.0; # OUTPUT: «False»
# Generic ordering
say 3 before 4; # OUTPUT: «True»
say 'b' after 'a'; # OUTPUT: «True»
# Short-circuit default operator - similar to `or` and `||`, but instead
# returns the first *defined* value:
say Any // Nil // 0 // 5; # OUTPUT: «0»
# Short-circuit exclusive or (XOR) - returns `True` if one (and only one) of
# its arguments is true
say True ^^ False; # OUTPUT: «True»
# Flip flops. These operators (`ff` and `fff`, equivalent to P5's `..`
# and `...`) are operators that take two predicates to test: They are `False`
# until their left side returns `True`, then are `True` until their right
# side returns `True`. Similar to ranges, you can exclude the iteration when
# it become `True`/`False` by using `^` on either side. Let's start with an
# example :
for <well met young hero we shall meet later> {
# by default, `ff`/`fff` smart-match (`~~`) against `$_`:
if 'met' ^ff 'meet' { # Won't enter the if for "met"
.say # (explained in details below).
}
if rand == 0 ff rand == 1 { # compare variables other than `$_`
say "This ... probably will never run ...";
}
}
# This will print "young hero we shall meet" (excluding "met"): the flip-flop
# will start returning `True` when it first encounters "met" (but will still
# return `False` for "met" itself, due to the leading `^` on `ff`), until it
# sees "meet", which is when it'll start returning `False`.
# The difference between `ff` (awk-style) and `fff` (sed-style) is that `ff`
# will test its right side right when its left side changes to `True`, and can
# get back to `False` right away (*except* it'll be `True` for the iteration
# that matched) while `fff` will wait for the next iteration to try its right
# side, once its left side changed:
# The output is due to the right-hand-side being tested directly (and returning
# `True`). "B"s are printed since it matched that time (it just went back to
# `False` right away).
.say if 'B' ff 'B' for <A B C B A>; # OUTPUT: «B B»,
# In this case the right-hand-side wasn't tested until `$_` became "C"
# (and thus did not match instantly).
.say if 'B' fff 'B' for <A B C B A>; #=> «B C B»,
# A flip-flop can change state as many times as needed:
for <test start print it stop not printing start print again stop not anymore> {
# exclude both "start" and "stop",
.say if $_ eq 'start' ^ff^ $_ eq 'stop'; # OUTPUT: «print it print again»
}
# You might also use a Whatever Star, which is equivalent to `True` for the
# left side or `False` for the right, as shown in this example.
# NOTE: the parenthesis are superfluous here (sometimes called "superstitious
# parentheses"). Once the flip-flop reaches a number greater than 50, it'll
# never go back to `False`.
for (1, 3, 60, 3, 40, 60) {
.say if $_ > 50 ff *; # OUTPUT: «6034060»
}
# You can also use this property to create an `if` that'll not go through the
# first time. In this case, the flip-flop is `True` and never goes back to
# `False`, but the `^` makes it *not run* on the first iteration
for <a b c> { .say if * ^ff *; } # OUTPUT: «bc»
# The `===` operator, which uses `.WHICH` on the objects to be compared, is
# the value identity operator whereas the `=:=` operator, which uses `VAR()` on
# the objects to compare them, is the container identity operator.
If you want to go further and learn more about Raku, you can:
-
Read the Raku Docs. This is a great resource on Raku. If you are looking for something, use the search bar. This will give you a dropdown menu of all the pages referencing your search term (Much better than using Google to find Raku documents!).
-
Read the Raku Advent Calendar. This is a great source of Raku snippets and explanations. If the docs don't describe something well enough, you may find more detailed information here. This information may be a bit older but there are many great examples and explanations. Posts stopped at the end of 2015 when the language was declared stable and
Raku v6.cwas released. -
Come along on
#rakuatirc.libera.chat. The folks here are always helpful. -
Check the source of Raku's functions and classes. Rakudo is mainly written in Raku (with a lot of NQP, "Not Quite Perl", a Raku subset easier to implement and optimize).
-
Read the language design documents. They explain Raku from an implementor point-of-view, but it's still very interesting.