learnxinyminutes-docs/perl6.html.markdown

category	language	filename	contributors
language	perl6	learnperl6.pl	vendethiel http://github.com/vendethiel

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, which runs on the JVM and the MoarVM.

Meta-note : the triple pound signs are here to denote headlines, double paragraphs, and 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`
my $variable;
# Perl 6 has 4 kinds of variables:

## * Scalars. They represent a single value. They start with a `$`

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 $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

## * Lists. They represent multiple values. Their name start with `@`.

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.
my @array = 1, 2, 3;

say @array[2]; # Array indices start at 0 -- This is the third element

say "Interpolate an array using [] : @array[]";
#=> Interpolate an array using [] : 1 2 3

@array[0] = -1; # Assign a new value to an array index
@array[0, 1] = 5, 6; # Assign multiple values

my @keys = 0, 2;
@array[@keys] = @letters; # Assign using an array
say @array; #=> a 6 b

## * Hashes, or key-value Pairs.
# Hashes are actually arrays of Pairs
# (you can construct a Pair object using the syntax `Key => Value`),
#  except they get "flattened" (hash context), removing duplicated keys.
my %hash = 1 => 2,
           3 => 4;
my %hash = foo => "bar", # keys 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 <>
                   # (`{key1}` doesn't work, as Perl6 doesn't have barewords)

## * Subs (subroutines, or functions in most other languages).
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 !";
}

## It can also have optional arguments:
sub with-optional($arg?) { # the "?" marks the argument optional
  say "I might return `(Any)` (Perl's 'null'-like value) 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 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 this fails:
with-named(1, 'named' => 6);

with-named(2, :named(5)); #=> 7

# To make a named argument mandatory, you can use `?`'s inverse, `!`
sub with-mandatory-named(:$str!)  {
  say "$str !";
}
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

## You can also provide your named arguments with defaults:
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.
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) { # `*@` (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 (the *) 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 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
# 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 using `is rw`:
sub mutate($n is rw) {
  $n++;
  say "\$n is now $n !";
}

# If what you want a copy instead, use `is copy`.

# 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

# - `if`
# Before talking about `if`, we need to know which values are "Truthy"
#  (represent True), and which are "Falsey" (or "Falsy") -- represent False.
# Only these values are Falsey: 0, (), {}, "", Nil, A type (like `Str` or `Int`),
#  and of course False itself.
# Every 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 brackets around the "body" block:
# if (true) say; # This doesn't work !

# You can also use their postfix versions, with the keyword after:
say "Quite truthy" if True;

# - 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 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` 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 `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.
                       # 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

# - `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` - Passes through an array
for @array -> $variable {
  say "I've got $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 `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; # the above and this are equivalent.
}

for @array {
  # You can...
  next if $_ == 3; # Skip to the next iteration (`continue` in C-like languages).
  redo if $_ == 4; # Re-do the iteration, keeping the same topic variable (`$_`).
  last if $_ == 5; # Or break out of a loop (like `break` in C-like languages).
}

# The "pointy block" syntax isn't specific to for.
# It's just a way to express a block in Perl6.
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 list of combinations, 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; # "has type" (check superclasses and roles)
1 ~~ True; # smart-matching against a boolean always returns that boolean
           # (and will warn).

# 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/infinite arrays,
#  using the Whatever Star:
my @array = 1..*; # 1 to Infinite ! `1..Inf` is the same.
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.
# 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 assigning to an array
my @numbers = ^20;

# Here numbers increase by "6"; more on `...` 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 `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 compound assignment operators:
$a *= 2; # multiply and assignment
$b %%= 5; # divisible by and assignment
@array .= sort; # calls the `sort` method and assigns the result back

### 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 (AKA "destructuring").
# It'll work in `my`s and in parameter lists.
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 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); # Throws an 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 `%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 => 'foo', qua => 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 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 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 $_ == $elem }, @array);
}
sub truthy-array(@array) {
  # this will produce an array of `True` and `False`:
  # (you can also say `anon sub` for "anonymous subroutine")
  map(sub ($i) { if $i { 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); # Same as `-> $a, $b { $a + $b + 3 }`
                                     # also `sub ($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 + $^b + 3 }, @array); # equivalent to following:
map(sub ($a, $b) { $a + $b + 3 }, @array); # (here with `sub`)

# Note : those are sorted lexicographically.
# `{ $^b / $^a }` is like `-> $a, $b { $b / $a }`

## About types...
# Perl6 is gradually typed. This means you can specify the type
#  of your variables/arguments/return types, or you can omit them
#  and they'll default to "Any".
# You obviously get access to a few base types, like Int and Str.
# The constructs for declaring types are "class", "role",
#  which you'll see later.

# For now, let us examine "subset":
# a "subset" 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" keyword:
subset VeryBigInteger of Int where * > 500;

## 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) { # a multi is a `sub` by 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 (remember subsets?):
multi is-big(Int $n where * > 50) { "Yes !" } # using a closure
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" } # "else"

# You can even dispatch based on a positional's argument presence !
multi with-or-without-you(:$with!) { # You need 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`, is simply a dispatch to a function with this signature:
# sub trait_mod:<is>(Routine $r, :$rw!) {}
#
# (commented because running this would be a terrible idea !)


### Scoping
# In Perl 6, contrarily to many scripting languages (like Python, Ruby, PHP),
#  you are to declare your variables before using them. You know `my`.
# (there are 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 $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 depth.
sub foo {
  my $*foo = 1;
  bar(); # call `bar` in-place
}
sub bar {
  say $*foo; # `$*foo` will be looked in the call stack, and find `foo`'s,
             #  even though the blocks aren't nested (they're call-nested).
             #=> 1
}

### Object Model

# You declare a class with the keyword `class`, fields with `has`,
# methods with `method`. Every attribute that is private is named `$!attr`.
# Immutable public attributes are named `$.attr`
#   (you can make them mutable with `is rw`)

# 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.
               # From inside the class, use `$!field` to modify it.
  has $.other-field is rw; # You can mark a public attribute `rw`.
  has Int $!private-field = 10;

  method get-value {
    $.field + $!private-field;
  }

  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` is `rw`.
  }

  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);
$a.get-value; #=> 15
#$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)

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`
}

# 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 properties too, so you can pass `val => 5`.
my B $b .= new(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 "import" a mixin (a "role") 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 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
# Exceptions are built on top of classes, 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`
    # Basically, if a `when` matches (or there's a `default`) marks the exception as
    #  "handled" so that it doesn't get re-thrown from the `CATCH`.
    # You still can re-throw the exception (see below) by hand.
  }
}

# You can throw an exception using `die`:
die X::AdHoc.new(payload => 'Error !');

# You can access the last exception with `$!` (use `$_` in a `CATCH` block)

# There are also some subtelties to exceptions. Some Perl 6 subs return a `Failure`,
#  which is a kind of "unthrown exception". They're not thrown until you tried to look
#  at their content, 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
# 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're not supposed to use the package keyword, usually:
#  you use `class Package::Name::Here;` to declare a class,
#  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 a "stub",
                      #  that can be redeclared as something else later.
  # ... declarations here ...
}
unit module Parse::Text; # file-scoped form
grammar Parse::Text::Grammar { # A grammar is a package, which you could `use`
}

# 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)
# It's like `my`, but it also creates a package variable.
# (All packagish things (`class`, `role`, etc) are `our` by default)
module Foo::Bar {
  our $n = 1; # note: you can't put a type constraint on an `our` variable
  our sub inc {
    our sub available { # If you try to make inner `sub`s `our`...
                        # Better know what you're doing (Don't !).
      say "Don't do that. Seriously. You'd get burned.";
    }
    my sub unavailable { # `my sub` is the default
      say "Can't access me from outside, I'm my !";
    }
    say ++$n; # increment the package variable and output its value
  }
}
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;

# 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` (happens at run time, but only once)
# State variables are only initialized 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 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
# 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, 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)" }
# exemple:
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)" }
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
# 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 somewhere else)
# Note: all of these (except start) don't need explicit brackets `{` and `}`.

# - `do` (that you already saw) - runs a block or a statement as a term
# You can't normally use a statement as a value (or "term"):
#
#    my $value = if True { 1 } # `if` is a statement - parse error
#
# This works:
my $a = do if True { 5 } # with `do`, `if` is now a term.

# - `once` - Makes sure a piece of code only runs once
for ^5 { once say 1 }; #=> 1
                       # Only prints ... once.
# Like `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
# Gather allows you to `take` several values in an array,
#  much like `do`, but allows you to take any expression.
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` - Evaluate 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

# - `lazy` - Defer actual evaluation until value is fetched (forces lazy context)
# Not yet implemented !!

# - `sink` - An `eager` that discards the results (forces sink context)
constant nilthingie = sink for ^3 { .say } #=> 0 1 2
say nilthingie.perl; #=> Nil

# - `quietly` - Supresses warnings
# Not yet implemented !

# - `contend` - Attempts side effects under STM
# Not yet implemented !

### 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 (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:
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, 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, 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 that)
# 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 delving 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 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`
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
            # 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

## * 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 returns an array with all of 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
#  that says when to stop (or Whatever for a lazy infinite list).
my @list = 1, 2, 3 ... 10; # basic deducing
#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 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 fibonacci series,
                           #  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 be instant.

### Regular Expressions
# 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.
#  (we won't be discussing this in this tutorial, however)
#
# 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 /; # 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`, but 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 as `$/` (implicitly lexically-scoped).
# You can also use the capture variables (`$0`, `$1`, ... starting at 0, not 1 !).
#
# You can also note that `~~` does not perform start/end checking
#  (meaning the regexp can be matched with just one char of the string),
#  we're going to explain later how you can do it.

# In Perl 6, you can have any alphanumeric as a literal,
# everything else has to be escaped, using a backslash or quotes.
say so 'a|b' ~~ / a '|' b /; # `True`. Wouln't mean the same 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) modifier.
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.

# It is, however, important as for how modifiers (that you're gonna see just below)
#  are applied ...

## Quantifying - `?`, `+`, `*` and `**`.
# - `?` - 0 or 1
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 just 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' ~~ / ab?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`

# - `+` - 1 or more
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

# - `*` - 0 or more
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)
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 wrote quoting:)
'he he' ~~ / <[ h e ' ' ]> /; # Warns "Repeated characters found in characters class"

# You can also negate them ... (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
# Group: you can group parts of your regexp with `[]`.
# These groups are *not* captured (like PCRE's `(?:)`).
so 'abc' ~~ / a [ b ] c /; # `True`. The grouping does pretty much nothing
so 'foo012012bar' ~~ / foo [ '01' <[0..9]> ] + bar /;
# The previous line returns `True`.
# We match the "012" 1 or more time (the `+` was applied to the group).

# But this does not go far enough, because we can't actually get back what
#  we matched.
# Capture: We can actually *capture* the results of the regexp, 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 available as `$/`:
say $/; # Will print some weird stuff (we'll explain) (or "Nil" if nothing matched).

# As we also said before, it has array indexing:
say $/[0]; #=> ｢ABC｣ ｢ABC｣
           # These weird 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 capture (indexed using `$0`, `$/[0]` or a named one) will be an array
#  IFF it can have more than one element
#  (so, with `*`, `+` and `**` (whatever the operands), but not with `?`).
# Let's use examples to see that:
so 'fooABCbar' ~~ / foo ( A B C )? bar /; # `True`
say $/[0]; #=> ｢ABC｣
say $0.WHAT; #=> (Match)
             # It can't be more than one, so it's only a single match object.
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,
             #  may it 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. Those contain
                     # a bunch of infos: where the match started/ended,
                     #    the "ast" (see actions later), etc.
                     # You'll see named capture below with grammars.

## Alternatives - the `or` of regexps
# WARNING: They are DIFFERENT from PCRE regexps.
so 'abc' ~~ / a [ b | y ] c /; # `True`. Either "b" or "y".
so 'ayc' ~~ / a [ b | y ] c /; # `True`. Obviously enough ...

# The difference between this `|` and the one you're used to is LTM.
# LTM means "Longest Token Matching". This means that the engine will always
#  try to match as much as possible in the strng
'foo' ~~ / fo | foo /; # `foo`, 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.
# Declarative prefixes include alternations (`|`), conjuctions (`&`),
#  sub-rule calls (not yet introduced), literals, characters classes and quantifiers.
# The latter 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.
# Exemples:
# 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 `||`
'foo' ~~ / fo || foo /; # `fo` now.




### Extra: the MAIN subroutine
# 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`
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 haz multi-dispatch:
# (using a "Bool" for the named argument so that we can do `--replace`
#  instead of `--replace=1`)
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:
#      t.pl [--replace] add <key> <value>
#      t.pl remove <key>
#      t.pl [--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
###

# It's considered 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 bloat it up

## Operators


## * 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; #=> 0

## * 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 P5's `..`/`...`).
#  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 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
}


# - `===` is value identity and uses `.WHICH` on the objects to compare them
# - `=:=` is container identity and uses `VAR()` on the objects to compare them

If you want to go further, you can:

• Read the Perl 6 Advent Calendar. This is probably the greatest source of Perl 6 information, snippets and such.
• Come along on #perl6 at irc.freenode.net. The folks here are always helpful.
• Check the source of Perl 6's functions and classes. 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. They explain P6 from an implementor point-of-view, but it's still very interesting.