mirror of
https://github.com/adambard/learnxinyminutes-docs.git
synced 2024-12-23 23:33:00 +03:00
39 KiB
39 KiB
| name | category | language | filename | contributors | |||
|---|---|---|---|---|---|---|---|
| perl6 | language | perl6 | learnperl6.pl |
|
Perl 6 is a highly capable, feature-rich programming language made for the upcoming hundred years.
Perl 6 runs on the Parrot VM, the JVM and the MoarVM.
Meta-note : the triple pound signs are here to denote headlines, double paragraphs, single notes.
#=> represents the output of a command.
# Single line comment start with a pound
#`(
Multiline comments use #` and a quoting construct. (), [], {}, 「」, etc, will work.
)
### Variables
# In Perl 6, you declare a lexical variable using `my`
a
# Perl 6 has 4 variable types :
## * Scalars. They represent a single value. They start with a `$`
my $str = 'String';
my $str2 = "String"; # double quotes allow for interpolation
# variable names can contain but not end with simple quotes and dashes, and can contain (and end with) underscores :
# my $weird'variable-name_ = 5; # works !
my $bool = True; # `True` and `False` are Perl 6's boolean
my $inverse = !$bool; # You can invert a bool with the prefix `!` operator
my $forced-bool = so $str; # And you can use the prefix `so` operator which turns its operand into a Bool
## * Arrays. They represent multiple values. Their name start with `@`.
my @array = 1, 2, 3;
my @array = 'a', 'b', 'c';
# equivalent to :
my @array = <a b c>; # array of words, delimited by space. similar to perl5's qw, or Ruby's %w
say @array[2]; # Array indices start at 0 -- This is the third element
say "Interpolate an array using [] : @array[]"; #=> Interpolate an array using [] : a b c
## * Hashes. Key-Value Pairs.
# Hashes are actually arrays of Pairs (`Key => Value`),
# except they get "flattened", removing duplicated keys.
my %hash = 1 => 2,
3 => 4;
my %hash = autoquoted => "key", # keys *can* get auto-quoted
"some other" => "value", # trailing commas are okay
;
my %hash = <key1 value1 key2 value2>; # you can also create a hash from an even-numbered array
my %hash = key1 => 'value1', key2 => 'value2'; # same as this
# You can also use the "colon pair" syntax: (especially handy for named parameters that you'll see later)
my %hash = :w(1), # equivalent to `w => 1`
# this is useful for the `True` shortcut:
:truey, # equivalent to `:truey(True)`, or `truey => True`
# and for the `False` one:
:!falsey, # equivalent to `:falsey(False)`, or `falsey => False`
;
say %hash{'key1'}; # You can use {} to get the value from a key
say %hash<key2>; # if it's a string, you can actually use <>
## * Subs (subroutines, or functions in most other languages). Stored in variable, they use `&`
sub say-hello { say "Hello, world" }
sub say-hello-to(Str $name) { # you can provide the type of an argument
# and it'll be checked at compile-time
say "Hello, $name !";
}
# since you can omit parenthesis to call a function with no arguments,
# you need "&" in the name to capture `say-hello`
my &s = &say-hello;
my &other-s = sub { say "anonymous function !" }
# A sub can have a "slurpy" parameter, or "doesn't-matter-how-many"
sub as-many($head, *@rest) { # the `*@` slurpy will basically "take everything else".
# Note: you can have parameters *before* (like here) a slurpy one,
# but not *after*.
say @rest.join(' / ') ~ " !";
}
say as-many('Happy', 'Happy', 'Birthday'); #=> Happy Birthday !
# Note that the splat did not consume the parameter before.
## You can call a function with an array using the "argument list flattening" operator `|`
# (it's not actually the only feature of the operator, but it's one of them)
sub concat3($a, $b, $c) {
say "$a, $b, $c";
}
concat3(|@array); #=> a, b, c
# `@array` got "flattened" as a part of the argument list
## It can also have optional arguments:
sub with-optional($arg?) { # the "?" marks the argument optional
say "I might return `(Any)` if I don't have an argument passed, or I'll return my argument";
$arg;
}
with-optional; # returns Any
with-optional(); # returns Any
with-optional(1); # returns 1
## You can also give them a default value when they're not passed:
sub hello-to($name = "World") {
say "Hello, $name !";
}
hello-to; #=> Hello, World !
hello-to(); #=> Hello, World !
hello-to('You'); #=> Hello, You !
## You can also, by using a syntax akin to the one of hashes (yay unification !),
## pass *named* arguments to a `sub`.
sub with-named($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 as compile time,
# and you'll have a single Pair object as a positional paramater.
with-named(2, :named(5)); #=> 7
with-named(3, :4named); #=> 7
# (special colon pair syntax for numbers, mainly useful for `:2nd` etc)
with-named(3); # warns, because we tried to use the undefined $named in a `+`:
# by default, named arguments are *optional*
# To make a named argument mandatory, you can use `?`'s inverse, `!`
sub with-mandatory-named(:$str!) {
say "$named !";
}
with-mandatory-named(str => "My String"); #=> My String !
with-mandatory-named; # run time error: "Required named parameter not passed"
with-mandatory-named(3); # run time error: "Too many positional parameters passed"
## If a sub takes a named boolean argument ...
sub takes-a-bool($name, :$bool) {
say "$name takes $bool";
}
# ... you can use the same "short boolean" hash syntax:
takes-a-bool('config', :bool); # config takes True
takes-a-bool('config', :!bool); # config takes False
# or you can use the "adverb" form:
takes-a-bool('config'):bool; #=> config takes True
takes-a-bool('config'):!bool; #=> config takes False
# You'll learn to love (or maybe hate, eh) that syntax later.
## You can also provide your named arguments with defaults:
sub named-def(:$def = 5) {
say $def;
}
named-def; #=> 5
named-def(:10def); #=> 10
named-def(def => 15); #=> 15
# -- Note: we're going to learn *more* on subs really soon,
# but we need to grasp a few more things to understand their real power. Ready?
### Containers
# In Perl 6, values are actually stored in "containers".
# the assignment operator asks the container on the left to store the value on its right
# When passed around, containers are marked as immutable. Which means that, in a function,
# you'll get an error if you try to mutate one of your argument.
# If you really need to, you can ask for a mutable container using `is rw` :
sub mutate($n is rw) {
$n++;
say "\$n is now $n !";
}
# If what you want is a copy instead, use `is copy`.
# A sub itself returns a container, which means it can be marked as rw :
my $x = 42;
sub mod() is rw { $x }
mod() = 52; # in this case, the parentheses are mandatory (else Perl 6 thinks it's a "term")
say $x; #=> 52
### Control Flow Structures
# You don't need to put parenthesis around the condition,
# but that also means you always have to use brackets (`{ }`) for their body :
## Conditionals
# - `if`
# Before talking about `if`, we need to know which values are "Truthy" (represent True),
# and which are "Falsey" (or "Falsy") -- meaning they represent False.
# Only these values are Falsey: (), 0, "0", Nil, A type, and of course False itself.
# Every other value is Truthy.
if True {
say "It's true !";
}
unless False {
say "It's not false !";
}
# You can also use their postfix versions, with the keyword after:
say "Quite truthy" if True;
# if (true) say; # This doesn't work !
# - Ternary conditional, "?? !!" (like `x ? y : z` in some other languages)
my $a = $condition ?? $value-if-true !! $value-if-false;
# - `given`-`when` looks like other languages `switch`, but it's much more powerful thanks to smart matching,
# and thanks to Perl 6's "topic variable", $_.
# This variable contains the default argument of a block,
# a loop's current iteration (unless explicitly named), etc.
# Given simply puts its argument into `$_` (like a block would do),
# and `when` uses 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" {
when /foo/ { # you'll read about the smart-matching operator below -- just know `when` uses it
# this is equivalent to `if $_ ~~ /foo/`
say "Yay !";
}
when $_.chars > 50 { # smart matching anything with True (`$a ~~ True`) is True,
# so you can also put "normal" conditionals.
say "Quite a long string !";
}
default { # same as `when *` (using the Whatever Star)
say "Something else"
}
}
## Looping constructs
# - `loop` is an infinite loop if you don't pass it arguments, but can also be a c-style `for` :
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.
# Notice that you can also use postfix conditionals, loops, etc.
say "This is a C-style for loop !";
}
# - `for` - Passes through an array
for @array -> $variable {
say "I've found $variable !";
}
# As we saw with given, for's default "current iteration" variable is `$_`.
# That means you can use `when` in a `for` just like you were in a when.
for @array {
say "I've got $_";
.say; # This is also allowed.
# A dot call with no "topic" (receiver) is sent to `$_` by default
$_.say; # the above and this are equivalent.
}
for @array {
next if $_ == 3; # you can skip to the next iteration (like `continue` in C-like languages)
redo if $_ == 4; # you can re-do the iteration, keeping the same topic variable (`$_`)
last if $_ == 5; # you can also break out of a loop (like `break` in C-like languages)
}
# Note - the "lambda" `->` syntax isn't reserved to `for` :
if long-computation() -> $result {
say "The result is $result";
}
### Operators
## Since Perl languages are very much operator-based languages
## Perl 6 operators are actually just funny-looking subroutines, in syntactic categories,
## like infix:<+> (addition) or prefix:<!> (bool not)
## The categories are :
# - "prefix" : before (like `!` in `!True`).
# - "postfix" : after (like `++` in `$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';
'a' ne 'b'; # not equal
'a' !eq 'b'; # same as above
# - `eqv` is canonical equivalence (or "deep equality")
(1, 2) eqv (1, 3);
# - `~~` is smart matching
# for a complete combinations list, use this table : http://perlcabal.org/syn/S03.html#Smart_matching
'a' ~~ /a/; # true if matches regexp
'key' ~~ %hash; # true if key exists in hash
$arg ~~ &bool-returning-function; # true if the function, passed `$arg` as an argument, returns True
1 ~~ Int; # "is of type"
1 ~~ True; # smart-matching against a boolean always returns that boolean (and will warn).
# - `===` is value identity and uses `.WHICH` on the objects to compare them
# - `=:=` is container identity and uses `VAR()` on the objects to compare them
# You also, of course, have `<`, `<=`, `>`, `>=`.
# Their string equivalent are also avaiable : `lt`, `le`, `gt`, `ge`.
3 > 4;
## * Range constructors
3 .. 7; # 3 to 7, both included
# `^` on either side them exclusive on that side :
3 ^..^ 7; # 3 to 7, not included (basically `4 .. 6`)
# this also works as a shortcut for `0..^N`
^10; # means 0..^10
# This also allows us to demonstrate that Perl 6 has lazy arrays, using the Whatever Star :
my @array = 1..*; # 1 to Infinite !
say @array[^10]; # you can pass arrays 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.
# Warning, though: if you try this example in the REPL and juste put `1..*`,
# Perl 6 will be forced to try and evaluate the whole array (to print it),
# so you'll end with an infinite loop.
## * And, Or
3 && 4; # 4, which is Truthy. Calls `.Bool` on `4` and gets `True`.
0 || False; # False. Calls `.Bool` on `0`
## * Short-circuit (and tight) versions of the above
$a && $b && $c; # returns the first argument that evaluates to False, or the last argument
$a || $b;
# And because you're going to want them, you also have composed assignment operators:
$a *= 2; # multiply and assignment
$b %%= 5; # divisible by and assignment
$c .= say; # method call and assignment
### More on subs !
# As we said before, Perl 6 has *really* powerful subs.
# We're going to see a few more key concepts that make them better than in any other language :-).
## Unpacking ! It's the ability to "extract" arrays and keys. It'll work in `my`s and parameters.
my ($a, $b) = 1, 2;
say $a; #=> 1
my ($, $, $c) = 1, 2, 3; # keep the non-interesting anonymous
say $c; #=> 3
my ($head, *@tail) = 1, 2, 3; # Yes, it's the same as with "slurpy subs"
my (*@small) = 1;
sub foo(@array [$fst, $snd]) {
say "My first is $fst, my second is $snd ! All in all, I'm @array[].";
# (^ remember the `[]` to interpolate the array)
}
foo(@tail); #=> My first is 2, my second is 3 ! All in all, I'm 1 2
# If you're not using the array itself, you can also keep it anonymous, much like a scalar:
sub first-of-array(@ [$fst]) { $fst }
first-of-array(@small); #=> 1
first-of-array(@tail); # errors with "Too many positional parameters passed" (the array is too big)
# You can also use a slurp ...
sub slurp-in-array(@ [$fst, *@rest]) { # you could decide to keep `*@rest` anonymous
say $fst + @rest.elems;
}
slurp-in-array(@tail); #=> 3
# You could even extract on a slurpy (but it's pretty useless ;-).)
sub fst(*@ [$fst]) { # or simply : `sub fst($fst) { ... }`
say $fst;
}
fst(1); #=> 1
fst(1, 2); # errors with "Too many positional parameters passed"
# You can also destructure hashes (and classes, which you'll learn about later !)
# The syntax is basically `%hash-name (:key($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, $qua times.";
}
# Then call it with a hash: (you need to keep the brackets for it to be a hash)
key-of({value => 1});
#key-of(%hash); # the same (for an equivalent `%hash`)
## The last expression of a sub is returned automatically (though you may use the `return` keyword, of course):
sub next-index($n) {
$n + 1;
}
my $new-n = next-index(3); # $new-n is now 4
# This is true for everything, except for the looping constructs (due to performance reasons):
# there's no purpose in building a list if we're just going to discard all the results.
# If you still want to build one, you can use the `do` prefix: (or the `gather` prefix, which we'll see later)
sub list-of($n) {
do for ^$n { # note the use of the range-to prefix operator `^` (`0..^N`)
$_ # current loop iteration
}
}
my @list3 = list-of(3); #=> (0, 1, 2)
## You can create a lambda with `-> {}` ("pointy block") or `{}` ("block")
my &lambda = -> $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 map:
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);
}
sub truthy-array(@array) {
# this will produce an array of `True` and `False`:
# (you can also say `anon sub` for "anonymous subroutine")
map(sub { if $_ { return True } else { return False } }, @array);
# ^ the `return` only returns from the anonymous `sub`
}
# You can also use the "whatever star" to create an anonymous function
# (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); # also works. Same as `-> $a, $b { $a + $b + 3 }`
say (*/2)(4); #=> 2
# Immediatly execute the function Whatever created.
say ((*+3)/5)(5); #=> 1.6
# works even in parens !
# but if you need to have more than one argument (`$_`) in a block (without wanting to resort to `-> {}`),
# you can also use the implicit argument syntax, `$^` :
map({ $^a + $^b + 3 }, @array); # same as the above
# Note : those are sorted lexicographically. `{ $^b / $^a }` is like `-> $a, $b { $b / $a }`
## Multiple Dispatch
# 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 a `where`:
# with types
multi sub sayit(Int $n) { # note the `multi` keyword here
say "Number: $n";
}
multi sayit(Str $s) } # the `sub` is the default
say "String: $s";
}
sayit("foo"); # prints "String: foo"
sayit(True); # fails at *compile time* with "calling 'sayit' will never work with arguments of types ..."
# with arbitrary precondition:
multi is-big(Int $n where * > 50) { "Yes !" } # using a closure
multi is-big(Int $ where 10..50) { "Quite." } # this uses smart-matching (could use a regexp, etc)
multi is-big(Int $) { "No" }
# you can also name these checks, by creating "subsets":
subset Even of Int where * %% 2;
multi odd-or-even(Even) { "Even" } # the main case using the type. We don't name the argument
multi odd-or-even($) { "Odd" } # "else"
# You can even dispatch based on a positional's argument presence !
multi with-or-without-you(:$with!) { # make it mandatory to be able to dispatch against it
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 is using it in several places.
# `is`, for example, is actually a `multi sub` named `trait_mod:<is>`, and it works off that.
# `is rw`, for example, is a dispatch to a function with this signature:
# sub trait_mod:<is>(Routine $r, :$rw!) {}
# (commented because running this would probably lead to some very surprising side-effects !)
### Scoping
# In Perl 6, contrarily to many scripting languages (Python, Ruby, PHP, for example),
# you are to declare your variables before using them. You already saw it, with `my`.
# (there are other declarator keywords, like `our`, `has` and `state`, but we'll talk about them later)
# This is called "lexical scoping", where in inner blocks, you can access variables from outer blocks.
my $foo = 'Foo';
sub foo {
my $bar = 'Bar';
sub bar {
say "$foo $bar";
}
&bar; # return the function
}
foo()(); #=> 'Foo Bar'
# As you can see, `$foo` and `$bar` were captured.
# But if we were to try and use `$bar` outside of `foo`, the variable would be undefined.
# (and you'd get a compile time error)
# Perl 6 has another kind of scope : dynamic scope.
# They use the twigil (composed sigil) `*` to mark dynamically-scoped variables:
my $*a = 1;
# Dyamically-scoped variables depend on the current call stack, instead of the current block stack.
sub foo {
my $*foo = 1;
bar(); # call `bar` in-place
}
sub bar {
say $*foo; # Perl 6 will look into the call stack instead, and find `foo`'s `$*a`,
# even though the blocks aren't nested (they're call-nested).
#=> 1
}
### Object Model
## Perl 6 has a quite comprehensive object model
## You declare a class with the keyword `class`, fields with `has`, methods with `method`.
## In Perl 6, every field is private, and named `$!attr`, but if you declare it with `$.`,
## you get a public (immutable) accessor along with it.
# (Perl 6's object model ("SixModel") is very flexible, and allows you to dynamically add methods,
# change semantics, etc -- This will not be covered here, and you should refer to the Synopsis)
class A {
has $.field; # `$.field` is immutable. Use `$!field` from inside the class to modify it.
has $.other-field is rw; # You can, however, mark a public field as being read/write.
has Int $!private-field = 10;
method get-value {
$.field + $!private-field + $n;
}
method set-value($n) {
# $.field = $n; # As stated before, you can't use the `$.` immutable version.
$!field = $n; # This works, because `$!` is always mutable.
$.other-field = 5; # This works, because `$.other-field` was declared `rw` (mutable).
}
method !private-method {
say "This method is private to the class !";
}
};
# Create a new instance of A with $.field set to 5 :
# note : you can't set private-field from here (more later on)
my $a = A.new(field => 5);
$a.get-value; #=> 18
#$a.field = 5; # This fails, because the `has $.field` is immutable
$a.other-field = 10; # This, however, works, because the public field is mutable (`rw`).
## Perl 6 also has inheritance (along with multiple inheritance ... Considered a misfeature by many)
class A {
has $.val;
submethod not-inherited {
say "This method won't be available on B.";
say "This is most useful for BUILD, which we'll see later";
}
method bar { $.val * 5 }
}
class B is A { # inheritance uses `is`
method foo {
say $.val;
}
method bar { $.val * 10 } # this shadows A's `bar`
}
my B $b .= new(val => 5); # When you use `my T $var`, `$var` starts off with `T` itself in it,
# so you can call `new` on it.
# (`.=` is just the compound operator composed of the dot-call and of the assignment operator
# `$a .= b` is the same as `$a = $a.b`)
# Also note that `BUILD` (the method called inside `new`) will set parent properties too,
# so you can pass `val => 5`
# $b.not-inherited; # This won't work, for reasons explained above
$b.foo; # prints 5
$b.bar; #=> 50, since it calls B's `bar`
## Roles are supported too (also called Mixins in other languages)
role PrintableVal {
has $!counter = 0;
method print {
say $.val;
}
}
# you "use" a mixin with "does" :
class Item does PrintableVal {
has $.val;
# When `does`-ed, a `role` literally "mixes in" the class :
# the methods and fields are put together, which means a class can access
# the private fields/methods of its roles (but not the inverse !) :
method access {
say $!counter++;
}
# However, this :
# method print {}
# is an error, since the compiler wouldn't know which `print` to use :
# contrarily to inheritance, methods mixed in can't be shadowed - they're put at the same "level"
# 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
# Exceptions are built on top of classes, usually in the package `X` (like `X::IO`).
# Unlike many other languages, in Perl 6, you put the `CATCH` block *within* the block to `try`.
# By default, a `try` has a `CATCH` block that catches any exception (`CATCH { default {} }`).
# You can redefine it using `when`s (and `default`) to handle the exceptions you want:
try {
open 'foo';
CATCH {
when X::AdHoc { say "unable to open file !" }
# any other exception will be re-raised, since we don't have a `default`
}
}
# You can throw an exception using `die`:
die X::AdHoc.new(payload => 'Error !');
# TODO warn
# TODO fail
# TODO CONTROL
### 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.
# (you can say that packages are the lowest common denomitor between them)
# Packages play a big part in a language, as Perl is well-known for CPAN,
# the Comprehensive Perl Archive Network.
# You usually don't use packages directly : you use `class Package::Name::Here;`, or if you
# only want to export variables/subs, you can use `module`:
module Hello::World { # bracketed form
# if `Hello` doesn't exist yet, it'll just be created as an "empty package stub"
# that can be redeclared as something else later.
# declarations here
}
module Parse::Text; # file-scoped form
grammar Parse::Text::Grammar { # A grammar is a fine package, which you could `use`
}
# NOTE for Perl 5 users: even though the `package` keyword exists,
# the braceless form is invalid (to catch a "perl5ism"). This will error out:
# package Foo; # because Perl 6 will think the entire file is Perl 5
# Just use `module` or the brace version of `package`.
# 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; #=> [1]
# 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
# 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` (happens at `INIT` time -- see "Phasers" below)
# Along with `my`, there are several others declarators you can use.
# The first one you'll want for the previous part is `our`.
# (All packagish things (`class`, `role`, etc) are `our` by default)
# it's like `my`, but it also creates a package variable:
module Foo::Bar {
our $n = 1; # note: you can't put a type constraint on an `our` variable
our sub inc {
our sub available { # if you try to make scoped `sub`s `our` ... Better know what you're doing (Don't !).
say "Don't do that. Seriously. You'd get burned.";
}
my sub unavailable { # `my sub` is the default
say "Can't access me from outside, I'm my !";
}
}
say ++$n; # lexically-scoped variables are still available
}
say $Foo::Bar::n; #=> 1
Foo::Bar::inc; #=> 2
Foo::Bar::inc; #=> 3
## * `constant` (happens at `BEGIN` time)
# You can use the `constant` keyword to declare a compile-time variable/symbol:
constant Pi = 3.14;
constant $var = 1;
## * `state` (happens at run time, but only once)
# State variables are only executed one time
# (they exist in other langages such as C as `static`)
sub fixed-rand {
state $val = rand;
say $rand;
}
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 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
# Phasers in Perl 6 are blocks that happen at determined points of time in your program
# When the program is compiled, when a for loop runs, when you leave a block, when
# an exception gets thrown ... (`CATCH` 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, instead 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)" }
POST { say "Asserts a postcondition at every block exit, after LEAVE (especially useful for loops)" }
## * 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 trick or clever code ...:
say "This code took " ~ (time - CHECK time) ~ "s to run";
# ... or clever organization:
sub do-db-stuff {
ENTER $db.start-transaction; # create a new transaction everytime we enter the sub
KEEP $db.commit; # commit the transaction if all went well
UNDO $db.rollback; # or rollback if all hell broke loose
}
### More operators thingies !
## Everybody loves operators ! Let's get more of them
## The precedence list can be found here : http://perlcabal.org/syn/S03.html#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 !
# Okay, you've been reading all of that, so I guess I should try to show you something exciting.
# I'll tell you a little secret (actually not):
# In Perl 6, all operators are actually just funny-looking subroutines.
# You can declare an operator just like you declare a sub:
sub prefix:<win>($winner) { # refer to the operator categories
# (yes, it's the "words operator" `<>`)
say "$winner Won !";
}
win "The King"; #=> The King Won !
# (prefix is before)
# you can still call the sub with its "full name"
say prefix:<!>(True); #=> False
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 $r) { # infix in the middle
for ^$n {
$r(); # You need the explicit parentheses to call the function in `$r`,
# else you'd be referring at the variable itself, kind of like with `&r`.
}
}
3 times -> { say "hello" }; #=> hello
#=> hello
#=> hello
# You're very 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 is around. Again, not 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 (named argument)
%h{$key}:delete;
# equivalent to:
postcircumfix:<{ }>(%h, $key, :delete);
# It's *all* using the same building blocks! Syntactic categories (prefix infix ...),
# named arguments (adverbs), ..., used to build the language are available to you.
# (you are, obviously, recommended against making an operator out of *everything* --
# with great power comes great responsibility)
## Meta operators !
# Oh boy, get ready. Get ready, because we're dwelving deep into the rabbit's hole,
# and you probably won't want to go back to other languages after reading that.
# (I'm guessing you don't want to already at that point).
# Meta-operators, as their name suggests, are *composed* operators.
# Basically, they're operators that apply another operator.
## * Reduce meta-operator
# It's a prefix meta-operator that takes a binary functions and one or many lists.
# If it doesn't get passed any argument, it either return a "default value" for this operator
# (a value that'd be non-meaningful if contained in a list) or `Any` if there's none.
# Otherwise, it pops an element from the list(s) one at a time, and applies the binary function
# to the last result (or the list's first element) and the popped element.
# To sum a list, you could use the reduce meta-operator with `+`, i.e.:
say [+] 1, 2, 3; #=> 6
# equivalent to `(1+2)+3`
say [*] 1..5; #=> 120
# equivalent to `((((1*2)*3)*4)*5)`.
# You can reduce with any operator, not just with mathematical ones.
# For example, you could reduce with `//` to get the first defined element of a list:
say [//] Nil, Any, False, 1, 5; #=> False
# (Falsey, but still defined)
# Default value examples:
say [*] (); #=> 1
say [+] (); #=> 0
# In both cases, they're results that, if they were contained in the lists,
# wouldn't have any impact on the final value (since N*1=N and N+0=N).
say [//]; #=> (Any)
# There's no "default value" for `//`
# You can also call it with a function you made up, using double brackets:
sub add($a, $b) { $a + $b }
say [[&add]] 1, 2, 3; #=> 6
## * Zip meta-operator
# This one is an infix meta-operator than 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 runs the an array with all these new elements.
(1, 2) Z (3, 4); # ((1, 3), (2, 4)), since by default, the function makes an array
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:
## * Sequence operator
# 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 for when to stop, or even Whatever for a lazy infinite list.
my @list = 1, 2, 3 ... 10; # basic deducing
#my @list = 1, 3, 6 ... 10; # this throws you into an infinite loop, because Perl 6 can't figure out the end
my @list = 1, 2, 3 ...^ 10; # as with ranges, you can exclude the last element (when the predicate matches)
my @list = 1, 3, 9 ... * > 30; # you can use a predicate (with the Whatever Star, here)
my @list = 1, 3, 9 ... { $_ > 30 }; # (equivalent to the above)
my @fib = 1, 1, *+* ... *; # lazy infinite list of prime numbers, computed using a closure !
my @fib = 1, 1, -> $a, $b { $a + $b } ... *; # (equivalent to the above)
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 be instant
## * Sort comparison
# They return one value of the `Order` enum : `Less`, `Same` and `More` (which numerify to -1, 0 or +1).
1 <=> 4; # sort comparison for numerics
'a' leg 'b'; # sort comparison for string
$obj eqv $obj2; # sort comparison using eqv semantics
## * Generic ordering
3 before 4; # True
'b' after 'a'; # True
## * Short-circuit default operator
# Like `or` and `||`, but instead returns the first *defined* value :
say Any // Nil // 0 // 5; #=> 5
## * Short-circuit exclusive or (XOR)
# Returns `True` if one (and only one) of its arguments is true
say True ^^ False; #=> True
## * Flip Flop
# The flip flop operators (`ff` and `fff`, equivalent to Perl 5/Ruby's `..` and `...`).
# are operators that take two predicates to test:
# They are `False` until their left side returns `True`, then are `True` until their right side returns `True`.
# Like for ranges, you can exclude the iteration when it became `True`/`False` by using `^` on either side.
# Let's start with an example :
for <well met young hero we shall meet later> {
# by default, `ff`/`fff` smart-match (`~~`) against `$_`:
if 'met' ^ff 'meet' { # won't enter the if for "met" (explained in details below).
.say
}
if rand == 0 ff rand == 1 { # compare variables other than `$_`
say "This ... probably will never run ...";
}
}
# This will print "young hero we shall meet" (excluding "met"):
# the flip-flop will start returning `True` when it first encounters "met"
# (but will still return `False` for "met" itself, due to the leading `^` on `ff`),
# until it sees "meet", which is when it'll start returning `False`.
# The difference between `ff` (awk-style) and `fff` (sed-style) is that
# `ff` will test its right side just as its left side changes to `True`,
# and can get back to `False` right away (*except* it'll be `True` for the iteration that matched)
# while `fff` will wait for the next iteration to try its right side, once its left side changed:
.say if 'B' ff 'B' for <A B C B A>; #=> B B
# because the right-hand-side was tested directly (and returned `True`).
# "B"s are still printed since it matched that time
# (it just went back to `False` right away)
.say if 'B' fff 'B' for <A B C B A>; #=> B C B
# because the right-hand-side wasn't tested until `$_` became "C"
# (and thus did not match directly).
# A flip-flop can change state as many times as needed:
for <test start print this stop you stopped printing start printing again stop not anymore> {
.say if $_ eq 'start' ^ff^ $_ eq 'stop'; # exclude both "start" and "stop",
#=> "print this printing 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"
.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 execute 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
}