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diff --git a/perl6.html.markdown b/perl6.html.markdown new file mode 100644 index 00000000..599f207c --- /dev/null +++ b/perl6.html.markdown @@ -0,0 +1,915 @@ +--- +name: perl6 +category: language +language: perl6 +filename: learnperl6.pl +contributors: + - ["Nami-Doc", "http://github.com/Nami-Doc"] +--- + +Perl 6 is a highly capable, feature-rich programming language made for the upcoming hundred years. + +Perl 6 runs on [the Parrot VM](http://parrot.org/), the JVM and [the MoarVM](http://moarvm.com). + +Meta-note : the triple pound signs are here to denote headlines, double paragraphs, single notes. +`#=>` represents the output of a command. + +```perl +# 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` + +# 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. They 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`), "flattened" to remove duplicated keys. + +my %hash = 1 => 2, + 3 => 4; +my %hash = autoquoted => "key", # keys are 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, $name !"; +} + +# since you can omit parenthesis to call a function with no arguments, you need to use `&` also to capture `say-hello` +my &s = &say-hello; +my &other-s = sub { say "anonymous function !" } + +# A sub can have a "slurpy" parameter, or "doesn't-matter-how-many" +sub as-many($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 +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 $def; +} +named-def; #=> 5 +named-def(:10def); #=> 10 +named-def(def => 15); #=> 15 + +### Containers +# In Perl 6, values are actually stored in "containers". +# the assignment operator asks the container on the left to store the value on its right +# When passed around, containers are marked as immutable. Which means that, in a function, +# you'll get an error if you try to mutate one of your argument. +# If you really need to, you can ask for a mutable container using `is rw` : +sub mutate($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 +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` +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 !! $value-if-false; + +# - `given`-`when` looks like other languages `switch`, but it's much more powerful thanks to smart matching. +# given just puts its argument into `$_` (like a block), and `when` uses it using the "smart matching" operator. +given "foo bar" { + when /foo/ { # you'll read about the smart-matching operator below -- just know `when` uses it + say "Yay !"; + } + when $_.chars > 50 { # smart matching anything with True gives 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 !"; +} + +# default variable is $_ (like a block) +for @array { + say "I've got $_"; +} + +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; # True. Calls `.Bool` on `3` +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 || $b; + +### More on subs ! + +## There's more to come, but we're going to end this paragraph with a really powerful feature: +## Unpacking ! It's the ability to "extract" arrays and keys. It'll work in `my`s and parameters. +my ($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 $fst; +} +fst(1); #=> 1 +fst(1, 2); # errors with "Too many positional parameters passed" + +# You can also destructure hashes (and classes, which you'll learn about later !) +sub key-of(% (:value($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 `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); # returns the correct value, even in a `if` +} + +# 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 ("P6Model") 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) + # + # 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 is after) + + +sub infix:<times>(Int $n, Block $r) { # infix in the middle + for ^$n { + $r(); # needs the parentheses because it's a scalar + } +} +3 times -> { say "hello" }; #=> hello + #=> hello + #=> hello + +# For circumfix and post-circumfix ones +sub circumfix:<[ ]>(Int $n) { + $n ** $n +} +say [5]; #=> 3125 + # circumfix is around + +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 sure you don't want to already at that point). + +# - Reduce meta-operator + +## End of 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 `$_` + say "This ... probably will never run ..."; + } +} +# This will print "young hero we shall meet" (excluding "met"): +# the flip-flop will start returning `True` when it first encounters "met" +# (but will still return `False` for "met" itself, due to the leading `^` on `ff`), +# until it sees "meet", which is when it'll start returning `False`. + +# The difference between `ff` (flip-flop) and `fff` (flip-flop) is that +# `ff` will test its right side just as its left side changes to `True`, +# and can get back to `False` right away (*except* it'll be `True` for the iteration that matched) +# while `fff` will wait for the next iteration to try its right side, once its left side changed: +.say if 'B' ff 'B' for <A B C B A>; #=> B B + # because the right-hand-side was tested directly (and returned `True`). + # "B"s are still printed since it matched that time + # (it just went back to `False` right away) +.say if 'B' fff 'B' for <A B C B A>; #=> B C B + # because the right-hand-side wasn't tested until `$_` 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) { + .say if $_ > 50 ff *; # Once the flip-flop reached 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 +} +``` |