iskm/learnxinyminutes-docs
1
0
Fork 0
mirror of https://github.com/adambard/learnxinyminutes-docs.git synced 2024-12-24 18:11:38 +00:00
Code Issues Actions Packages Projects Releases Wiki Activity
0731b18cb7
learnxinyminutes-docs/perl6.html.markdown
Amans Tofu f0f161981f [Perl6/en]Modify an error about the Range constructor (#3612) 
[Perl6/en] Modify an error about the Range constructor
2019-08-30 14:52:27 +05:30

1977 lines
74 KiB
Raku
Raw Blame History

This file contains ambiguous Unicode characters

This file contains Unicode characters that might be confused with other characters. If you think that this is intentional, you can safely ignore this warning. Use the Escape button to reveal them.

---
category: language
language: perl6
filename: learnperl6.p6
contributors:
- ["vendethiel", "http://github.com/vendethiel"]
- ["Samantha McVey", "https://cry.nu"]
---
Perl 6 is a highly capable, feature-rich programming language made for at
least the next hundred years.
The primary Perl 6 compiler is called [Rakudo](http://rakudo.org), which runs on
the JVM and [the MoarVM](http://moarvm.com).
Meta-note: double pound signs (`##`) are used to indicate paragraphs,
while single pound signs (`#`) indicate notes.
`#=>` represents the output of a command.
```perl6
# Single line comments start with a pound sign.
#`( Multiline comments use #` and a quoting construct.
(), [], {}, 「」, etc, will work.
)
# Use the same syntax for multiline comments to embed comments.
for #`(each element in) @array {
put #`(or print element) $_ #`(with newline);
}
```
## Variables
```perl6
## In Perl 6, you declare a lexical variable using the `my` keyword:
my $variable;
## Perl 6 has 3 basic types of variables: scalars, arrays, and hashes.
```
### Scalars
```perl6
# Scalars represent a single value. They start with the `$` sigil:
my $str = 'String';
# Double quotes allow for interpolation (which we'll see later):
my $str2 = "String";
## 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 Perl 6'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 `?`.
```
### Arrays and Lists
```perl6
## 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>; # array of words, delimited by space.
# Similar to perl5's qw, or Ruby's %w.
@array = 1, 2, 3;
say @array[2]; # Array indices start at 0. Here the third element
# is being accessed.
say "Interpolate an array using []: @array[]";
#=> 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; #=> a 6 b
```
### Hashes, or key-value Pairs.
```perl6
## 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;
%hash = a => 1, # keys get auto-quoted when => (fat comma) is used.
b => 2, # Trailing commas are okay.
;
## Even though hashes are internally stored differently than arrays,
## Perl 6 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 = :w(1), # equivalent to `w => 1`
:truey, # equivalent to `:truey(True)` or `truey => True`
:!falsey, # equivalent to `:falsey(False)` or `falsey => False`
;
## The :truey and :!falsey constructs are known as the
## `True` and `False` shortcuts respectively.
say %hash{'key1'}; # You can use {} to get the value from a key.
say %hash<key2>; # If it's a string without spaces, you can actually use
# <> (quote-words operator). `{key1}` doesn't work,
# as Perl6 doesn't have barewords.
```
## Subs
```perl6
## 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 can be omitted.
sub return-value { 5 }
say return-value; # prints 5
sub return-empty { }
say return-empty; # prints Nil
## Some control flow structures produce a value, like `if`:
sub return-if {
if True { "Truthy" }
}
say return-if; # prints Truthy
## Some don't, like `for`:
sub return-for {
for 1, 2, 3 { 'Hi' }
}
say return-for; # prints Nil
## Positional arguments are required by default. To make them optional, use
## the `?` after the parameters' names.
sub with-optional( $arg? ) {
# This sub returns `(Any)` (Perl's null-like value) if
# no argument is passed. Otherwise, it returns its argument.
$arg;
}
with-optional; # returns Any
with-optional(); # returns Any
with-optional(1); # returns 1
## You can also give them a default value when they're not passed.
## Required parameters must come before optional ones.
sub greeting( $name, $type = "Hello" ) {
say "$type, $name!";
}
greeting("Althea"); #=> Hello, Althea!
greeting("Arthur", "Good morning"); #=> Good morning, Arthur!
## You can also, by using a syntax akin to the one of hashes
## (yay unified syntax !), pass *named* arguments to a `sub`. They're
## optional, and will default to "Any".
sub with-named( $normal-arg, :$named ) {
say $normal-arg + $named;
}
with-named(1, named => 6); #=> 7
## There's one gotcha to be aware of, here: If you quote your key, Perl 6
## won't be able to see it at compile time, and you'll have a single `Pair`
## object as a positional parameter, which means
## `with-named(1, 'named' => 6);` fails.
with-named(2, :named(5)); #=> 7
## To make a named argument mandatory, you can append `!` to the parameter,
## which is the inverse of `?`:
sub with-mandatory-named( :$str! ) {
say "$str!";
}
with-mandatory-named(str => "My String"); #=> 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...
sub takes-a-bool( $name, :$bool ) {
say "$name takes $bool";
}
## ... you can use the same "short boolean" hash syntax:
takes-a-bool('config', :bool); #=> config takes True
takes-a-bool('config', :!bool); #=> config takes False
## You can also provide your named arguments with default values:
sub named-def( :$def = 5 ) {
say $def;
}
named-def; #=> 5
named-def(def => 15); #=> 15
## Since you can omit parenthesis to call a function with no arguments,
## you need `&` in the name to store `say-hello` in a variable. This means
## `&say-hello` is a code object and not a subroutine call.
my &s = &say-hello;
my &other-s = sub { say "Anonymous function!" }
## A sub can have a "slurpy" parameter, or "doesn't-matter-how-many". For
## this, you must use `*@` (slurpy) which will "take everything else". You can
## have as many parameters *before* a slurpy one, but not *after*.
sub as-many($head, *@rest) {
say @rest.join(' / ') ~ " !";
}
say as-many('Happy', 'Happy', 'Birthday');#=> Happy / Birthday !
# Note that the splat (the *) did not
# consume the parameter before it.
## 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); #=> a, b, c
# `@array` got "flattened" as a part of the argument list
```
## Containers
```perl6
## In Perl 6, 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; # the value is incremented but the old value is returned
#=> 42
say $m; #=> 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:
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.
## A sub itself returns a container, which means it can be marked as rw:
my $x = 42;
sub x-store() is rw { $x }
x-store() = 52; # in this case, the parentheses are mandatory
# (else Perl 6 thinks `x-store` is an identifier)
say $x; #=> 52
```
## Control Flow Structures
### Conditionals
```perl6
## - `if`
## 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` or `Int`) and
## of course False itself. Any other value is Truthy.
if True {
say "It's true!";
}
unless False {
say "It's not false!";
}
## 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;` doesn't work.
## You can also use their statement modifier (postfix) versions:
say "Quite truthy" if True; #=> Quite truthy
say "Quite falsey" unless False; #=> Quite falsey
## - Ternary operator, "x ?? y !! z"
## This returns $value-if-true if the condition is true and $value-if-false
## if it is false.
## my $result = condition ?? $value-if-true !! $value-if-false;
my $age = 30;
say $age > 18 ?? "You are an adult" !! "You are under 18";
#=> You are an adult
```
### given/when, or Perl 6's switch construct
```perl6
## `given...when` looks like other languages' `switch`, but is much more
## powerful thanks to smart matching and Perl 6'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 Perl 6 constructs use this variable (as said before, like `for`,
## blocks, etc), this means the powerful `when` is not only applicable along
## with a `given`, but instead anywhere a `$_` exists.
given "foo bar" {
say $_; #=> foo bar
when /foo/ { # Don't worry about smart matching yet. Just know
say "Yay !"; # `when` uses it. This is equivalent to `if $_ ~~ /foo/`.
}
when $_.chars > 50 { # smart matching anything with True is True,
# i.e. (`$a ~~ True`)
# so you can also put "normal" conditionals.
# This `when` is equivalent to this `if`:
# `if $_ ~~ ($_.chars > 50) {...}`
# which means: `if $_.chars > 50 {...}`
say "Quite a long string !";
}
default { # same as `when *` (using the Whatever Star)
say "Something else"
}
}
```
### Looping constructs
```perl6
## - `loop` 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; # last breaks out of the loop, like
# the `break` keyword in other languages
}
loop (my $i = 0; $i < 5; $i++) {
next if $i == 3; # `next` skips to the next iteration, like `continue`
# in other languages. Note that you can also use postfix
# conditionals, loops, etc.
say "This is a C-style for loop!";
}
## - `for` - Iterating through an array
my @array = 1, 2, 6, 7, 3;
## Accessing the array's elements with the topic variable $_.
for @array {
say "I've got $_ !";
}
## Accessing the array's elements with a "pointy block", `->`.
## Here each element is read-only.
for @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 @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 @array {
say "I've got $_";
.say; # This is also allowed. A dot call with no "topic" (receiver)
# is sent to `$_` by default
$_.say; # This is equivalent to the above statement.
}
for @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 Perl 6.
sub long-computation { "Finding factors of large primes" }
if long-computation() -> $result {
say "The result is $result.";
}
```
## Operators
```perl6
## Since Perl languages are very much operator-based languages, Perl 6
## operators are actually just funny-looking subroutines, in syntactic
## categories, like infix:<+> (addition) or prefix:<!> (bool not).
## The categories are:
## - "prefix": before (like `!` in `!True`).
## - "postfix": after (like `++` in `$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!
## Equality Checking
##------------------
## - `==` is numeric comparison
3 == 4; #=> False
3 != 4; #=> True
## - `eq` is string comparison
'a' eq 'b'; #=> False
'a' ne 'b'; #=> True, not equal
'a' !eq 'b'; #=> True, same as above
## - `eqv` is canonical equivalence (or "deep equality")
(1, 2) eqv (1, 3); #=> False
(1, 2) eqv (1, 2); #=> True
Int === Int #=> True
## - `~~` is the smart match operator
## Aliases the left hand side to $_ and then evaluates the right hand side.
## Here are some common comparison semantics:
## String or numeric equality
'Foo' ~~ 'Foo'; # True if strings are equal.
12.5 ~~ 12.50; # 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; #=> ｢a｣
say $obj.WHAT; #=> (Match)
## Hashes
'key' ~~ %hash; # True if key exists in hash.
## Type - Checks if left side "is of type" (can check superclasses and
## roles).
say 1 ~~ Int; #=> True
## Smart-matching against a boolean always returns that boolean
## (and will warn).
say 1 ~~ True; #=> True
say False ~~ True; #=> True
## General syntax is `$arg ~~ &bool-returning-function;`. For a complete list
## of combinations, use this table:
## http://perlcabal.org/syn/S03.html#Smart_matching
## Of course, you also use `<`, `<=`, `>`, `>=` for numeric comparison.
## Their string equivalent are also available: `lt`, `le`, `gt`, `ge`.
3 > 4; # False
3 >= 4; # False
3 < 4; # True
3 <= 4; # True
'a' gt 'b'; # False
'a' ge 'b'; # False
'a' lt 'b'; # True
'a' le 'b'; # True
## Range constructor
##------------------
3 .. 7; # 3 to 7, both included.
3 ..^ 7; # 3 to 7, exclude right endpoint.
3 ^.. 7; # 3 to 7, exclude left endpoint.
3 ^..^ 7; # 3 to 7, exclude both endpoints.
# 3 ^.. 7 almost like 4 .. 7 when we only consider integers.
# But when we consider decimals :
3.5 ~~ 4 .. 7; # False
3.5 ~~ 3 ^.. 7; # True, This Range also contains decimals greater than 3.
# We describe it like this in some math books: 3.5 ∈ (3,7]
# If you don’t want to understand the concept of interval
# for the time being. At least we should know：
3 ^.. 7 ~~ 4 .. 7; # False
## This also works as a shortcut for `0..^N`:
^10; # means 0..^10
## This also allows us to demonstrate that Perl 6 has lazy/infinite arrays,
## using the Whatever Star:
my @array = 1..*; # 1 to Infinite! Equivalent to `1..Inf`.
say @array[^10]; # You can pass ranges as subscripts and it'll return
# an array of results. This will print
# "1 2 3 4 5 6 7 8 9 10" (and not run out of memory!)
## Note: when reading an infinite list, Perl 6 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 that are needed.
## An array subscript can also be a closure. It'll be called with the length
## as the argument:
say join(' ', @array[15..*]); #=> 15 16 17 18 19
## which is equivalent to:
say join(' ', @array[-> $n { 15..$n }]);
## 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;
@numbers[5..*] = 3, 9 ... *; # even though the sequence is infinite,
# only the 15 needed values will be calculated.
say @numbers; #=> 0 1 2 3 4 3 9 15 21 [...] 81 87
# (only 20 values)
## and (&&), or (||)
##------------------
3 && 4; # 4, which is Truthy. Calls `.Bool` on both 3 and 4 and gets `True`
# so it returns 4 since both are `True`.
3 && 0; # 0
0 && 4; # 0
0 || False; # False. Calls `.Bool` on `0` and `False` which are both `False`
# so it retusns `False` since both are `False`.
## Short-circuit (and tight) versions of the above
## Return the first argument that evaluates to False, or the last argument.
my ( $a, $b, $c ) = 1, 0, 2;
$a && $b && $c; # Returns 0, the first False value
## || Returns the first argument that evaluates to True
$b || $a; # 1
## 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 = $d mod 4;
@array .= sort; # calls the `sort` method and assigns the result back
```
## More on subs!
```perl6
## As we said before, Perl 6 has *really* powerful subs. We're going
## to see a few more key concepts that make them better than in any
## other language :-).
```
### Unpacking!
```perl6
## Unpacking is the ability to "extract" arrays and keys
## (AKA "destructuring"). It'll work in `my`s and in parameter lists.
my ($f, $g) = 1, 2;
say $f; #=> 1
my ($, $, $h) = 1, 2, 3; # keep the non-interesting values anonymous (`$`)
say $h; #=> 3
my ($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 $snd! All in all, I'm @array[].";
# (^ remember the `[]` to interpolate the array)
}
unpack_array(@tail); #=> My first is 2, my second is 3! All in all, I'm 2 3.
## If you're not using the array itself, you can also keep it anonymous,
## much like a scalar:
sub first-of-array( @ [$fst] ) { $fst }
first-of-array(@small); #=> 1
first-of-array(@tail); # Error: "Too many positional parameters passed"
# (which means the array is too big).
## You can also use a slurp...
sub slurp-in-array(@ [$fst, *@rest]) { # You could keep `*@rest` anonymous
say $fst + @rest.elems; # `.elems` returns a list's length.
# Here, `@rest` is `(3,)`, since `$fst`
# holds the `2`.
}
slurp-in-array(@tail); #=> 3
## You could even extract on a slurpy (but it's pretty useless ;-).)
sub fst(*@ [$fst]) { # or simply: `sub fst($fst) { ... }`
say $fst;
}
fst(1); #=> 1
fst(1, 2); # errors with "Too many positional parameters passed"
## You can also destructure hashes (and classes, which you'll learn about
## later). The syntax is basically the same as
## `%hash-name (:key($variable-to-store-value-in))`.
## The hash can stay anonymous if you only need the values you extracted.
sub key-of( % (:value($val), :qua($qua)) ) {
say "Got val $val, $qua times.";
}
## Then call it with a hash. You need to keep the curly braces for it to be a
## hash or use `%()` instead to indicate a hash is being passed.
key-of({value => 'foo', qua => 1}); #=> Got val foo, 1 times.
key-of(%(value => 'foo', qua => 1)); #=> Got val foo, 1 times.
#key-of(%hash); # the same (for an equivalent `%hash`)
## 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 { # note the range-to prefix operator `^` (`0..^N`)
$_ # current loop iteration known as the "topic" variable
}
}
my @list3 = list-of(3); #=> (0, 1, 2)
```
### lambdas (or anonymous subroutines)
```perl6
## You can create a lambda with `-> {}` ("pointy block") ,
## `{}` ("block") or `sub {}`.
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"
}
## `-> {}` and `{}` 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.
## We can, for example, add 3 to each value of an array using the
## `map` function with a lambda:
my @arrayplus3 = map({ $_ + 3 }, @array); # $_ is the implicit argument
## 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 ) {
# this will `return` out of the `is-in` sub once the condition evaluated
## to True, the loop won't be run anymore.
map({ return True if $_ == $elem }, @array);
}
## with:
sub truthy-array( @array ) {
# this will produce an array of `True` and `False`:
# (you can also say `anon sub` for "anonymous subroutine")
map(sub ($i) { if $i { return True } else { return False } }, @array);
# ^ 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; #=> summation
say $anon-sum(2, 3, 5); #=> 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)
my @arrayplus3 = map(*+3, @array); # `*+3` is the same as `{ $_ + 3 }`
my @arrayplus3 = map(*+*+3, @array); # Same as `-> $a, $b { $a + $b + 3 }`
# also `sub ($a, $b) { $a + $b + 3 }`
say (*/2)(4); #=> 2
# Immediately execute the function Whatever created.
say ((*+3)/5)(5); #=> 1.6
# It works even in parens!
## But if you need to have more than one argument (`$_`) in a block (without
## wanting to resort to `-> {}`), you can also use the implicit argument
## syntax, `$^`:
map({ $^a + $^b + 3 }, @array);
# which is equivalent to the following which uses a `sub`:
map(sub ($a, $b) { $a + $b + 3 }, @array);
## The parameters `$^a`, `$^b`, etc. are known as placeholder parameters or
## self-declared positional parameters. They're sorted lexicographically so
## `{ $^b / $^a }` is equivalent `-> $a, $b { $b / $a }`.
```
### About types...
```perl6
## Perl 6 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;
```
### Multiple Dispatch
```perl6
## Perl 6 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"); #=> "String: foo"
sayit(25); #=> "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
# (could use a regexp, etc)
multi is-big(Int $) { "No" }
subset Even of Int where * %% 2;
multi odd-or-even(Even) { "Even" } # The main case using the type.
# We don't name the argument.
multi odd-or-even($) { "Odd" } # "everthing 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.
##
## - `is`, for example, is actually a `multi sub` named `trait_mod:<is>`,
## and it works off that.
## - `is rw`, is simply a dispatch to a function with this signature:
## sub trait_mod:<is>(Routine $r, :$rw!) {}
##
## (commented out because running this would be a terrible idea!)
```
## Scoping
```perl6
## In Perl 6, unlike many scripting languages, (such as Python, Ruby, PHP),
## you must declare your variables before using them. The `my` declarator
## you have learned 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 $outer_scoped";
}
&inner; # return the function
}
outer()(); #=> 'Foo Bar'
## As you can see, `$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).
```
## Twigils
```perl6
## There are many special `twigils` (composed sigils) in Perl 6. Twigils
## define the variables' scope.
## The * and ? twigils work on standard variables:
## * Dynamic variable
## ? Compile-time variable
## The ! and the . twigils are used with Perl 6's objects:
## ! Attribute (instance attribute)
## . Method (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 {
my $*dyn_scoped_1 = 25; # Defines $*dyn_scoped_1 only for this sub.
$*dyn_scoped_2 = 100; # Will change the value of the file scoped variable.
say_dyn(); #=> 25 100, $*dyn_scoped 1 and 2 will be looked
# for in the call.
# It 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(); #=> 1 10
call_say_dyn(); #=> 25 100
# Uses $*dyn_scoped_1 as defined in call_say_dyn even though
# we are calling it from outside.
say_dyn(); #=> 1 100 We changed the value of $*dyn_scoped_2 in
# call_say_dyn so now its value has changed.
```
## Object Model
```perl6
## To call a method on an object, add a dot followed by the method name:
## `$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: `$!attr`.
## Immutable public attributes use the `.` twigil which creates a read-only
## method named after the attribute. In fact, declaring an attribute with `.`
## is equivalent to declaring the same attribute with `!` and then creating
## a read-only method with the attribute's name. However, this is done for us
## by Perl 6 automatically. The easiest way to remember the `$.` twigil is
## by comparing it to how methods are called.
## Perl 6'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.perl6.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 was born in $!bcountry and now lives in $!ccountry."
}
# This method is public, just like `birthday` and `get-age`.
method get-info {
self.do-decoration; # Invoking a method on `self` inside the class.
# Use `self!priv-method` for private method.
# Use `self.publ-method` for public method.
}
};
## Create a new instance of Human class.
## Note: you can't set private-attribute from here (more later on).
my $person1 = Human.new(
name => "Jord",
bcountry = "Togo",
ccountry => "Togo"
);
say $person1.name; #=> Jord
say $person1.bcountry; #=> Togo
say $person1.ccountry; #=> Togo
# $person1.bcountry = "Mali"; # This fails, because the `has $.bcountry`
# is immutable. Jord can't change his birthplace.
$person1.ccountry = "France"; # This works because the `$.ccountry` is mutable
# (`is rw`). Now Jord's current country is France.
# Calling methods on the instance objects.
$person1.birthday; #=> 1
$person1.get-info; #=> Jord was born in Togo and now lives in France.
$person1.do-decoration; # This fails since the method `do-decoration` is
# private.
```
### Object Inheritance
```perl6
## Perl 6 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; #=> "My favorite color is Blue"
$Richard.talk; #=> "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; #=> "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:
## `$a .= b` is the same as `$a = $a.b`)
## Also note that `BUILD` (the method called inside `new`)
## will set parent's properties too, so you can pass `val => 5`.
```
### Roles, or Mixins
```perl6
## 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 `does` keyword:
class Item does PrintableVal {
has $.val;
## 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!):
method access {
say $!counter++;
}
## 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.
}
```
## Exceptions
```perl6
## Exceptions are built on top of classes, in the package `X` (like `X::IO`).
## In Perl6 exceptions are automatically 'thrown':
open 'foo'; #=> 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`:
die 'Error!'; #=> Error!
## Or more explicitly:
X::AdHoc.new(payload => 'Error!').throw; #=> Error!
## In Perl 6, `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 $uninitiazilzed.defined; #=> 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 $_"; #=> Something happened
#=> Failed to open file foo: no such file or directory
## Both of those above work but in case we get an object from the left side
## that is not a failure we will probably get a warning. We see below how we
## can use try` and `CATCH` to be more specific with the exceptions we catch.
```
### Using `try` and `CATCH`
```perl6
## By using `try` and `CATCH` you can contain and handle exceptions without
## disrupting the rest of the program. The `try` block will set the last
## exception to the special variable `$!` (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 $!;
#=> Well, I tried! Failed to open file foo: no such file or directory
## Now, what if we want more control over handling the exception?
## Unlike many other languages, in Perl 6, you put the `CATCH` block *within*
## the block to `try`. Similar to how the `$_` 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 {
say "Something happened: $_"
}
}
#=> 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: $_";
}
## 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.
}
}
#=>Error: Failed to open file /dir/foo: no such file or directory
## There are also some subtleties to exceptions. Some Perl 6 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`).
fail "foo"; # We're not trying to access the value, so no problem.
try {
fail "foo";
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).
```
## Packages
```perl6
## 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
## the `use` keyword:
use JSON::Tiny; # if you installed Rakudo* or Panda, you'll have this module
say from-json('[1]').perl; #=> [1]
## You should not declare packages using the `package` keyword (unlike Perl 5).
## Instead, use `class Package::Name::Here;` to declare a class, or if you only
## want to export variables/subs, you can use `module` instead.
module Hello::World { # bracketed form
# If `Hello` doesn't exist yet, it'll just be a "stub",
# that can be redeclared as something else later.
# ... declarations here ...
}
unit module Parse::Text; # file-scoped form which extends until
# the end of the file
grammar Parse::Text::Grammar {
# A grammar is a package, which you could `use`.
# You will learn more about grammars in the regex section
}
## 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.
```
## Declarators
```perl6
## In Perl 6, 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 {
our $our-var = 1; # Note: `our`-declared variables cannot be typed.
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; #=> 1, this works!
say $Var::Increment::my-var; #=> (Any), this will not work!
Var::Increment::Inc; #=> 2
Var::Increment::Inc; #=> 3 , notice how the value of $our-var was
# retained.
Var::Increment::unavailable; #=> 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]; #=> 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 {
state $val = rand; # This will be a different value for
# every value of `$a`
}
for ^5 -> $b {
say foo; # This will print the same value 5 times,
# but only 5. Next iteration will re-run `rand`.
}
}
```
## Phasers
```perl6
## Phasers in Perl 6 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!
## 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" }
## 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" }
## Block phasers
ENTER { say "[*] Runs everytime you enter a block, repeats on loop blocks" }
LEAVE {
say "Runs everytime 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:
for 0..2 {
PRE { $_ > 1 } # This is going to blow up with "Precondition 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.";
}
for 0..2 {
POST { $_ < 2 } # This is going to blow up with "Postcondition failed"
}
## Block/exceptions phasers
sub {
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)" }
}
## 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" }
}
## Role/class phasers
COMPOSE { "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:
sub do-db-stuff {
$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
}
```
## Statement prefixes
```perl6
## 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 `}`.
## `do` - (which you already saw) 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.
# my $value = if True { 1 } # this fails since `if` is a statement
my $a = do if True { 5 } # with `do`, `if` is now a term returning a value
## `once` - makes sure a piece of code only runs once.
for ^5 {
once say 1
}; #=> 1, only prints ... once
## Similar to `state`, they're cloned per-scope.
for ^5 {
sub { once say 1 }()
}; #=> 1 1 1 1 1, prints once per lexical scope.
## `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;
}
#=> -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.
## `eager` - evaluates a statement eagerly (forces eager context)
## Don't try this at home:
# eager 1..*; # this will probably hang for a while (and might crash ...).
## But consider:
constant thrice = gather for ^3 { say take $_ }; # Doesn't print anything
## versus:
constant thrice = eager gather for ^3 { say take $_ }; #=> 0 1 2
```
## Iterables
```perl6
## Iterables are objects that can be iterated over which are
## are similar to the `for` construct.
## `flat` - flattens iterables.
say (1, 10, (20, 10) ); #=> (1 10 (20 10)), notice how neste lists are
# preserved
say (1, 10, (20, 10) ).flat; #=> (1 10 20 10), now the iterable is flat
## - `lazy` - defers actual evaluation until value is fetched by forcing
## lazy context.
my @lazy-array = (1..100).lazy;
say @lazy-array.is-lazy; #=> True, check for laziness with the `is-lazy` method.
say @lazy-array; #=> [...] List has not been iterated on!
my @lazy-array { .print }; # This works and will only do as much work as
# is needed.
# ( **TODO** explain that gather/take and map are all lazy)
## `sink` - an `eager` that discards the results by forcing sink context.
constant nilthingie = sink for ^3 { .say } #=> 0 1 2
say nilthingie.perl; #=> Nil
## `quietly` - suppresses warnings in blocks.
quietly { warn 'This is a warning!' }; #=> No output
## `contend` - attempts side effects under STM
## Not yet implemented!
```
## More operators thingies!
```perl6
## Everybody loves operators! Let's get more of them.
## The precedence list can be found here:
## https://docs.perl6.org/language/operators#Operator_Precedence
## But first, we need a little explanation about associativity:
## Binary operators:
$a ! $b ! $c; # with a left-associative `!`, this is `($a ! $b) ! $c`
$a ! $b ! $c; # with a right-associative `!`, this is `$a ! ($b ! $c)`
$a ! $b ! $c; # with a non-associative `!`, this is illegal
$a ! $b ! $c; # with a chain-associative `!`, this is `($a ! $b) and ($b ! $c)`
$a ! $b ! $c; # with a list-associative `!`, this is `infix:<>`
## Unary operators:
!$a! # with left-associative `!`, this is `(!$a)!`
!$a! # with right-associative `!`, this is `!($a!)`
!$a! # with non-associative `!`, this is illegal
```
### Create your own operators!
```perl6
## 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 Perl 6, all operators are actually just funny-looking subroutines.
## You can declare an operator just like you declare a sub:
# prefix refers to the operator categories (prefix, infix, postfix, etc).
sub prefix:<win>( $winner ) {
say "$winner Won!";
}
win "The King"; #=> The King Won!
# (prefix means 'before')
## you can still call the sub with its "full name":
say prefix:<!>(True); #=> False
prefix:<win>("The Queen"); #=> The Queen Won!
sub postfix:<!>( Int $n ) {
[*] 2..$n; # using the reduce meta-operator... See below ;-)!
}
say 5!; #=> 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 {
$r(); # You need the explicit parentheses to call the function in `$r`,
# else you'd be referring at the variable itself, like with `&r`.
}
}
3 times -> { say "hello" }; #=> hello
#=> hello
#=> hello
## It's recommended to put spaces around your
## infix operator calls.
## For circumfix and post-circumfix ones
sub circumfix:<[ ]>( Int $n ) {
$n ** $n
}
say [5]; #=> 3125
# circumfix means 'around'. Again, no whitespace.
sub postcircumfix:<{ }>( Str $s, Int $idx ) {
## post-circumfix is 'after a term, around something'
$s.substr($idx, 1);
}
say "abc"{1}; #=> b
# after the term `"abc"`, and around the index (1)
## This really means a lot -- because everything in Perl 6 uses this.
## For example, to delete a key from a hash, you use the `:delete` adverb
## (a simple named argument underneath):
%h{$key}:delete;
## equivalent to:
postcircumfix:<{ }>( %h, $key, :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.
```
### Meta operators!
```perl6
## 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 list's first element) and the popped element.
## To sum a list, you could use the reduce meta-operator with `+`, i.e.:
say [+] 1, 2, 3; #=> 6, equivalent to (1+2)+3.
## To multiply a list
say [*] 1..5; #=> 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; #=> False
# (Falsey, but still defined)
## Or with relational operators, i.e., `>` to check elements of a list
## are ordered accordingly:
say say [>] 234, 156, 6, 3, -20; #=> True
## Default value examples:
say [*] (); #=> 1
say [+] (); #=> 0
# meaningless values, since N*1=N and N+0=N.
say [//]; #=> (Any)
# There's no "default value" for `//`.
## You can also call it with a function you made up, using double brackets:
sub add($a, $b) { $a + $b }
say [[&add]] 1, 2, 3; #=> 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); #=> ((1, 3), (2, 4)), since by default the function
# makes an array.
say 1..3 Z+ 4..6; #=> (5, 7, 9), using the custom infix:<+> function
## 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 Perl 6's most powerful features:
## it's composed of first, on the left, the list you want Perl 6 to deduce from
## (and might include a closure), and on the right, a value or the predicate
## that says when to stop (or a Whatever Star for a lazy infinite list).
my @list = 1, 2, 3...10; # basic arithmetic sequence
# my @list = 1, 3, 6...10; # this dies because Perl 6 can't figure out the end
my @list = 1, 2, 3...^10; # as with ranges, you can exclude the last element
# (the iteration ends when the predicate matches).
my @list = 1, 3, 9...* > 30; # you can use a predicate (with the Whatever Star).
my @list = 1, 3, 9 ... { $_ > 30 }; # (equivalent to the above
# using a block here).
my @fib = 1, 1, *+* ... *; # lazy infinite list of fibonacci sequence,
# computed using a closure!
my @fib = 1, 1, -> $a, $b { $a + $b } ... *; # (equivalent to the above)
my @fib = 1, 1, { $^a + $^b } ... *; # (also equivalent to the above)
## $a and $b will always take the previous values, meaning here
## 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.
say @fib[^10]; #=> 1 1 2 3 5 8 13 21 34 55
# (using a range as the index)
## Note: as for ranges, once reified, elements aren't re-calculated.
## That's why `@primes[^100]` will take a long time the first time you print
## it, then will be instateneous.
```
## Regular Expressions
```perl6
## I'm sure a lot of you have been waiting for this one. Well, now that you know
## a good deal of Perl 6 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.
##
## Perl 6 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, Perl 6 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/; #=> True
say so 'a' ~~ / a /; #=> 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 Perl 6, 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 /; #=> `True`, it wouldn't mean the same thing if
# `|` wasn't escaped.
say so 'a|b' ~~ / a \| b /; #=> `True`, another way to escape it.
## The whitespace in a regexp 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, Perl 6 will warn us:
say so 'a b c' ~~ / a b c /; #=> `False`, with warning about space
say so 'a b c' ~~ / a b c /; #=> `False`
## 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/; #=> `True`
## By using `m` to specify 'match', we can also use delimiters other than
## slashes:
say so 'abc' ~~ m{a b c}; #=> `True`
say so 'abc' ~~ m[a b c]; #=> `True`
# m/.../ is equivalent to /.../
## Use the :i adverb to specify case insensitivity:
say so 'ABC' ~~ m:i{a b c}; #=> `True`
## However, whitespace is important as for how modifiers are applied (
## (which you'll see just below) ...
## Quantifying - `?`, `+`, `*` and `**`.
## `?` - zero or one match
so 'ac' ~~ / a b c /; #=> `False`
so 'ac' ~~ / a b? c /; #=> `True`, the "b" matched 0 times.
so 'abc' ~~ / a b? c /; #=> `True`, the "b" matched 1 time.
## ...As you read before, whitespace is important because it determines which
## part of the regexp is the target of the modifier:
so 'def' ~~ / a b c? /; #=> `False`, only the `c` is optional
so 'def' ~~ / a b? c /; #=> `False`, whitespace is not significant
so 'def' ~~ / 'abc'? /; #=> `True`, the whole "abc" group is optional
## Here (and below) the quantifier applies only to the `b`
## `+` - one or more matches
so 'ac' ~~ / a b+ c /; #=> `False`, `+` wants at least one matching
so 'abc' ~~ / a b+ c /; #=> `True`, one is enough
so 'abbbbc' ~~ / a b+ c /; #=> `True`, matched 4 "b"s
## `*` - zero or more matches
so 'ac' ~~ / a b* c /; #=> `True`, they're all optional.
so 'abc' ~~ / a b* c /; #=> `True`
so 'abbbbc' ~~ / a b* c /; #=> `True`
so 'aec' ~~ / a b* c /; #=> `False`. "b"(s) are optional, not replaceable.
## `**` - (Unbound) Quantifier
## If you squint hard enough, you might understand why exponentation is used
## for quantity.
so 'abc' ~~ / a b**1 c /; #=> `True`, (exactly one time)
so 'abc' ~~ / a b**1..3 c /; #=> `True`, (one to three times)
so 'abbbc' ~~ / a b**1..3 c /; #=> `True`
so 'abbbbbbc' ~~ / a b**1..3 c /; #=> `False, (too much)
so 'abbbbbbc' ~~ / a b**3..* c /; #=> `True`, (infinite ranges are okay)
## `<[]>` - Character classes
## Character classes are the equivalent of PCRE's `[]` classes, but they use a
## more perl6-ish syntax:
say 'fooa' ~~ / f <[ o a ]>+ /; #=> 'fooa'
## You can use ranges:
say 'aeiou' ~~ / a <[ e..w ]> /; #=> '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 \! \ ]> + /; #=> '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)
so 'foo' ~~ / <-[ f o ]> + /; #=> False
## ... and compose them:
so 'foo' ~~ / <[ a..z ] - [ f o ]> + /; #=> `False`, (any letter except f and o)
so 'foo' ~~ / <-[ a..z ] + [ f o ]> + /; #=> `True`, (no letter except f and o)
so 'foo!' ~~ / <-[ a..z ] + [ f o ]> + /; #=> `True`, (the + doesn't replace the
# left part)
```
### Grouping and capturing
```perl6
## Group: you can group parts of your regexp with `[]`. Unlike PCRE's `(?:)`,
## these groups are *not* captured.
so 'abc' ~~ / a [ b ] c /; # `True`. The grouping does pretty much nothing
so 'foo012012bar' ~~ / foo [ '01' <[0..9]> ] + bar /;
## The previous line returns `True`. The regex matches "012" 1 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.
so 'fooABCABCbar' ~~ / foo ( 'A' <[A..Z]> 'C' ) + bar /; # `True`. (using `so`
# here, `$/` below)
## So, starting with the grouping explanations.
## As we said before, our `Match` object is stored inside the `$/` variable:
say $/; # Will either print some weird stuff or `Nil` if nothing matched.
## As we also said before, it has array indexing:
say $/[0]; #=> ｢ABC｣ ｢ABC｣
# These corner brackets are `Match` objects.
# Here, we have an array of these.
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 /; #=> `True`
say $/[0]; #=> ｢ABC｣
say $0.WHAT; #=> (Match)
# There can't be more than one, so it's only a single match object.
say so 'foobar' ~~ / foo ( "A" "B" "C" )? bar /; #=> True
say $0.WHAT; #=> (Any)
# This capture did not match, so it's empty
so 'foobar' ~~ / foo ( "A" "B" "C" ) ** 0..1 bar /; #=> `True`
say $0.WHAT; #=> (Array)
# A specific quantifier will always capture an Array,
# be a range or a specific value (even 1).
## The captures are indexed per nesting. This means a group in a group will be
## nested under its parent group: `$/[0][0]`, for this code:
'hello-~-world' ~~ / ( 'hello' ( <[ \- \~ ]> + ) ) 'world' /;
say $/[0].Str; #=> hello~
say $/[0][0].Str; #=> ~
## This stems from a very simple fact: `$/` 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.
say $/[0].list.perl; #=> (Match.new(...),).list
# We can see it's a list of Match objects. These contain
# a bunch of info: where the match started/ended,
# the "ast" (see actions later), etc.
# You'll see named capture below with grammars.
## Alternation - the `or` of regexps
## WARNING: They are DIFFERENT from PCRE regexps.
say so 'abc' ~~ / a [ b | y ] c /; #=> `True`. Either "b" or "y".
say so 'ayc' ~~ / a [ b | y ] c /; #=> `True`. Obviously enough...
## The difference between this `|` and the one you're used to is
## LTM ("Longest Token Matching"). This means that the engine will always
## try to match as much as possible in the string.
say 'foo' ~~ / fo | foo /; #=> `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)
## and the procedural parts:
## - The declarative prefixes include alternations (`|`), conjunctions (`&`),
## sub-rule calls (not yet introduced), literals, characters classes and
## quantifiers.
## - The procedural part include everything else: back-references,
## code assertions, and other things that can't traditionnaly 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 regexps) are procedural.
## There are also more complicated rules, like specificity (literals win over
## character classes).
## Note: the first-matching `or` still exists, but is now spelled `||`
say 'foo' ~~ / fo || foo /; #=> `fo` now.
```
## Extra: the MAIN subroutine
```perl6
## The `MAIN` subroutine is called when you run a Perl 6 file directly. It's
## very powerful, because Perl 6 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!";
}
## This produces:
## $ perl6 cli.pl
## Usage:
## t.pl <name>
## And since it's a regular Perl 6 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).
subset File of Str where *.IO.d; # convert to IO object to check the file exists
multi MAIN('add', $key, $value, Bool :$replace) { ... }
multi MAIN('remove', $key) { ... }
multi MAIN('import', File, Str :$as) { ... } # omitting parameter name
## This produces:
## $ perl6 cli.pl
## Usage:
## cli.p6 [--replace] add <key> <value>
## cli.p6 remove <key>
## cli.p6 [--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).
```
## APPENDIX A:
### List of things
```perl6
## It's assumed by now you know the Perl6 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.
## Operators
## Sort comparison - they return one value of the `Order` enum: `Less`, `Same`
## and `More` (which numerify to -1, 0 or +1 respectively).
1 <=> 4; # sort comparison for numerics
'a' leg 'b'; # sort comparison for string
$obj eqv $obj2; # sort comparison using eqv semantics
## Generic ordering
3 before 4; # True
'b' after 'a'; # True
## Short-circuit default operator - similar to `or` and `||`, but instead
## returns the first *defined* value:
say Any // Nil // 0 // 5; #=> 0
## Short-circuit exclusive or (XOR) - returns `True` if one (and only one) of
## its arguments is true
say True ^^ False; #=> True
## Flip flops - these operators (`ff` and `fff`, equivalent to P5's `..`
## and `...`) are operators that take two predicates to test: They are `False`
## until their left side returns `True`, then are `True` until their right
## side returns `True`. Similar to ranges, you can exclude the iteration when
## it become `True`/`False` by using `^` on either side. Let's start with an
## example :
for <well met young hero we shall meet later> {
# by default, `ff`/`fff` smart-match (`~~`) against `$_`:
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 `$_`
say "This ... probably will never run ...";
}
}
## This will print "young hero we shall meet" (excluding "met"): the flip-flop
## will start returning `True` when it first encounters "met" (but will still
## return `False` for "met" itself, due to the leading `^` on `ff`), until it
## sees "meet", which is when it'll start returning `False`.
## The difference between `ff` (awk-style) and `fff` (sed-style) is that `ff`
## will test its right side right when its left side changes to `True`, and can
## get back to `False` right away (*except* it'll be `True` for the iteration
## that matched) while `fff` will wait for the next iteration to try its right
## side, once its left side changed:
.say if 'B' ff 'B' for <A B C B A>; #=> B B
# because the right-hand-side was tested
# directly (and returned `True`).
# "B"s are printed since it matched that
# time (it just went back to `False`
# right away).
.say if 'B' fff 'B' for <A B C B A>; #=> B C B
# The right-hand-side wasn't tested until
# `$_` became "C"
# (and thus did not match instantly).
## 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> {
.say if $_ eq 'start' ^ff^ $_ eq 'stop'; # exclude both "start" and "stop",
#=> "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:
for (1, 3, 60, 3, 40, 60) { # Note: the parenthesis are superfluous here
# (sometimes called "superstitious parentheses")
.say if $_ > 50 ff *; # Once the flip-flop reaches a number greater
# than 50, it'll never go back to `False`
#=> 60 3 40 60
}
## You can also use this property to create an `if` that'll not go through the
## first time:
for <a b c> {
.say if * ^ff *; # the flip-flop is `True` and never goes back to `False`,
# but the `^` makes it *not run* on the first iteration
#=> b c
}
## The `===` operator is the value identity operator and uses `.WHICH` on the
## objects to compare them while `=:=` is the container identity operator
## and uses `VAR()` on the objects to compare them.
```
If you want to go further, you can:
- Read the [Perl 6 Docs](https://docs.perl6.org/). This is a great
resource on Perl6. 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 Perl 6 documents!).
- Read the [Perl 6 Advent Calendar](http://perl6advent.wordpress.com/). This
is a great source of Perl 6 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 Perl 6.c was released.
- Come along on `#perl6` at `irc.freenode.net`. The folks here are
always helpful.
- Check the [source of Perl 6's functions and
classes](https://github.com/rakudo/rakudo/tree/nom/src/core). Rakudo is
mainly written in Perl 6 (with a lot of NQP, "Not Quite Perl", a Perl 6 subset
easier to implement and optimize).
- Read [the language design documents](http://design.perl6.org). They explain
P6 from an implementor point-of-view, but it's still very interesting.
Reference in New Issue View Git Blame Copy Permalink