learnxinyminutes-docs/perl6.html.markdown at 93fc04dbb35921cf08d7b6aa16bfdfa2758b87ac

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Merge pull request #3045 from bmusin/patch-2

[perl6/en]: fix spelling

2018-01-18 22:11:41 +05:45

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Variables

## In Perl 6, you declare a lexical variable using `my`
my $variable;
## Perl 6 has 3 basic types of variables: scalars, arrays, and hashes.

Scalars

# Scalars 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 values.
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 and Lists

## Arrays represent multiple values. Their name start with `@`.
## Lists are similar but are an immutable type.

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 all elements of an array using [] : @array[]";
#=> Interpolate all elements of 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; # Assignment using an array containing index values
say @array; #=> a 6 b

Hashes, or key-value Pairs.

## Hashes are pairs of keys and values.
## You can construct a Pair object using the syntax `Key => Value`.
## Hash tables are very fast for lookup, and are stored unordered.
## Keep in mind that keys get "flattened" in hash context, and any duplicated
## keys are deduplicated.
my %hash = 1 => 2,
           3 => 4;
my %hash = foo => "bar", # keys get auto-quoted
            "some other" => "value", # trailing commas are okay
            ;

## Even though hashes are internally stored differently than arrays,
## Perl 6 allows you to easily create a hash from an even numbered array:
my %hash = <key1 value1 key2 value2>;

my %hash = key1 => 'value1', key2 => 'value2'; # same result as above

## 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 as most other languages call them, are
## created with the `sub` keyword.
sub say-hello { say "Hello, world" }

## You can provide (typed) arguments.
## If specified, the type will be checked at compile-time if possible,
## otherwise at runtime.
sub say-hello-to(Str $name) {
    say "Hello, $name !";
}

## A sub returns the last value of the block.
sub return-value {
    5;
}
say return-value; # prints 5
sub return-empty {
}
say return-empty; # prints Nil

## Some control flow structures produce a value, like if:
sub return-if {
	if True {
		"Truthy";
	}
}
say return-if; # prints Truthy

## Some don't, like for:
sub return-for {
    for 1, 2, 3 { }
}
say return-for; # prints Nil

## A sub can 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 "take everything else"
## Note: you can have parameters *before* a slurpy one (like here),
## 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 !";
}

my $m = 42;
mutate $m; # $n is now 43 !

## This works because we are passing the container $m to mutate.  If we try
## to just pass a number instead of passing a variable it won't work because
## there is no container being passed and integers are immutable by themselves:

mutate 42; # Parameter '$n' expected a writable container, but got Int value

## 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)
##   returns $value-if-true if the condition is true and $value-if-false
##   if it is false.
##   my $result = $value condition ?? $value-if-true !! $value-if-false;

my $age = 30;
say $age > 18 ?? "You are an adult" !! "You are under 18";

given/when, or switch

## - `given`-`when` looks like other languages' `switch`, but is much more
## powerful thanks to smart matching and 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 $_.chars > 50 { # smart matching anything with True is True,
                       # i.e. (`$a ~~ True`) 
                       # so you can also put "normal" conditionals.
                       # This `when` is equivalent to this `if`:
                       #  if $_ ~~ ($_.chars > 50) {...}
                       # Which means:
                       #  if $_.chars > 50 {...}
    say "Quite a long string !";
  }
  default { # same as `when *` (using the Whatever Star)
    say "Something else"
  }
}

Looping constructs

## - `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);

## - Smart Match Operator: `~~`
## Aliases the left hand side to $_ and then evaluates the right hand side.
## Here are some common comparison semantics:

## String or Numeric Equality

'Foo' ~~ 'Foo'; # True if strings are equal.
12.5 ~~ 12.50; # True if numbers are equal.

## Regex - For matching a regular expression against the left side.
## Returns a (Match) object, which evaluates as True if regexp matches.

my $obj = 'abc' ~~ /a/;
say $obj; # ｢a｣
say $obj.WHAT; # (Match)

## Hashes
'key' ~~ %hash; # True if key exists in hash

## Type - Checks if left side "has type" (can check superclasses and roles)

1 ~~ Int; # True

## Smart-matching against a boolean always returns that boolean (and will warn).

1 ~~ True; # True
False ~~ True; # True

## General syntax is $arg ~~ &bool-returning-function;
## For a complete list of combinations, use this table:
## http://perlcabal.org/syn/S03.html#Smart_matching

## You also, of course, have `<`, `<=`, `>`, `>=`.
## Their string equivalent are also available : `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.
## Trying 

## 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
#    Returns the first argument that evaluates to False, or the last argument.

my ( $a, $b, $c ) = 1, 0, 2;
$a && $b && $c; # Returns 0, the first False value

## || Returns the first argument that evaluates to True
$b || $a; # 1

## And because you're going to want them,
## you also have compound assignment operators:
$a *= 2; # multiply and assignment. Equivalent to $a = $a * 2;
$b %%= 5; # divisible by and assignment
@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 ($f, $g) = 1, 2;
say $f; #=> 1
my ($, $, $h) = 1, 2, 3; # keep the non-interesting anonymous
say $h; #=> 3

my ($head, *@tail) = 1, 2, 3; # Yes, it's the same as with "slurpy subs"
my (*@small) = 1;

sub unpack_array(@array [$fst, $snd]) {
  say "My first is $fst, my second is $snd ! All in all, I'm @array[].";
  # (^ remember the `[]` to interpolate the array)
}
unpack_array(@tail); #=> My first is 2, my second is 3 ! All in all, I'm 2 3


## If you're not using the array itself, you can also keep it anonymous,
## much like a scalar:
sub first-of-array(@ [$fst]) { $fst }
first-of-array(@small); #=> 1
first-of-array(@tail); # 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)

lambdas

## 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
              # Immediately 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, unlike many scripting languages, (such as Python, Ruby, PHP),
## you must declare your variables before using them. The `my` declarator
## you have learned uses "lexical scoping". There are a few other declarators,
## (`our`, `state`, ..., ) which we'll see later.
## This is called "lexical scoping", where in inner blocks,
## you can access variables from outer blocks.
my $file_scoped = 'Foo';
sub outer {
  my $outer_scoped = 'Bar';
  sub inner {
    say "$file_scoped $outer_scoped";
  }
  &inner; # return the function
}
outer()(); #=> 'Foo Bar'

## As you can see, `$file_scoped` and `$outer_scoped` were captured.
## But if we were to try and use `$bar` outside of `foo`,
## the variable would be undefined (and you'd get a compile time error).

Twigils

## There are many special `twigils` (composed sigil's) in Perl 6.
## Twigils define the variables' scope.
## The * and ? twigils work on standard variables:
## * Dynamic variable
## ? Compile-time variable
## The ! and the . twigils are used with Perl 6's objects:
## ! Attribute (class member)
## . Method (not really a variable)

## `*` Twigil: Dynamic Scope
## These variables use the`*` twigil to mark dynamically-scoped variables.
## Dynamically-scoped variables are looked up through the caller, not through
## the outer scope

my $*dyn_scoped_1 = 1;
my $*dyn_scoped_2 = 10;

sub say_dyn {
  say "$*dyn_scoped_1 $*dyn_scoped_2";
}

sub call_say_dyn {
  my $*dyn_scoped_1 = 25; # Defines $*dyn_scoped_1 only for this sub.
  $*dyn_scoped_2 = 100; # Will change the value of the file scoped variable.
  say_dyn(); #=> 25 100 $*dyn_scoped 1 and 2 will be looked for in the call.
             # It uses the value of $*dyn_scoped_1 from inside this sub's 
             # lexical scope even though the blocks aren't nested (they're 
             # call-nested).
}
say_dyn(); #=> 1 10
call_say_dyn(); #=> 25 100
                # Uses $*dyn_scoped_1 as defined in call_say_dyn even though
                # we are calling it from outside.
say_dyn(); #=> 1 100 We changed the value of $*dyn_scoped_2 in call_say_dyn
           #         so now its value has changed.

Object Model

## To call a method on an object, add a dot followed by the method name:
## => $object.method
## Classes are declared with the `class` keyword. Attributes are declared
## with the `has` keyword, and methods declared with `method`.
## Every attribute that is private uses the ! twigil for example: `$!attr`.
## Immutable public attributes use the `.` twigil.
## (you can make them mutable with `is rw`)
## The easiest way to remember the `$.` twigil is comparing it to how methods
## are called.

## Perl 6's object model ("SixModel") is very flexible,
## and allows you to dynamically add methods, change semantics, etc ...
## (these will not all be covered here, and you should refer to:
## https://docs.perl6.org/language/objects.html.

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

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

  method set-value($param) { # Methods can take parameters
    $!attrib = $param;   # This works, because `$!` is always mutable.
	# $.attrib = $param; # Wrong: You can't use the `$.` immutable version.

    $.other-attrib = 5; # This works, because `$.other-attrib` is `rw`.
  }

  method !private-method {
    say "This method is private to the class !";
  }
};

## Create a new instance of Attrib-Class with $.attrib set to 5 :
## Note: you can't set private-attribute from here (more later on).
my $class-obj = Attrib-Class.new(attrib => 5);
say $class-obj.get-value; #=> 15
# $class-obj.attrib = 5; # This fails, because the `has $.attrib` is immutable
$class-obj.other-attrib = 10; # This, however, works, because the public
                              # attribute is mutable (`rw`).

Object Inheritance

## Perl 6 also has inheritance (along with multiple inheritance)
## While `method`'s are inherited, `submethod`'s are not.
## Submethods are useful for object construction and destruction tasks,
## such as BUILD, or methods that must be overridden by subtypes.
## We will learn about BUILD later on.

class Parent {
  has $.age;
  has $.name;
  # This submethod won't be inherited by Child.
  submethod favorite-color {
    say "My favorite color is Blue";
  }
  # This method is inherited
  method talk { say "Hi, my name is $!name" }
}
# Inheritance uses the `is` keyword
class Child is Parent {
  method talk { say "Goo goo ga ga" }
  # This shadows Parent's `talk` method, This child hasn't learned to speak yet!
}
my Parent $Richard .= new(age => 40, name => 'Richard');
$Richard.favorite-color; #=> "My favorite color is Blue"
$Richard.talk; #=> "Hi, my name is Richard"
## $Richard is able to access the submethod, he knows how to say his name.

my Child $Madison .= new(age => 1, name => 'Madison');
$Madison.talk; # prints "Goo goo ga ga" due to the overridden method.
# $Madison.favorite-color # does not work since it is not inherited

## When you use `my T $var`, `$var` starts off with `T` itself in it,
## so you can call `new` on it.
## (`.=` is just the dot-call and the assignment operator:
##  `$a .= b` is the same as `$a = $a.b`)
## Also note that `BUILD` (the method called inside `new`)
## will set parent properties too, so you can pass `val => 5`.

Roles, or Mixins

## 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 attributes are put together, which means a class can access
  ## the private attributes/methods of its roles (but not the inverse !):
  method access {
    say $!counter++;
  }

  ## However, this:
  ## method print {}
  ## is ONLY valid when `print` isn't a `multi` with the same dispatch.
  ## (this means a parent class can shadow a child class's `multi print() {}`,
  ##  but it's an error if a role does)

  ## NOTE: You can use a role as a class (with `is ROLE`). In this case, 
  ##       methods will be shadowed, since the compiler will consider `ROLE` to 
  ##       be a class.
}

Exceptions

## Exceptions are built on top of classes, in the package `X` (like `X::IO`).
## In Perl6 exceptions are automatically 'thrown'
open 'foo'; #> Failed to open file foo: no such file or directory
## It will also print out what line the error was thrown at and other error info

## You can throw an exception using `die`:
die 'Error!'; #=> Error!

## Or more explicitly:
die X::AdHoc.new(payload => 'Error!');

## In Perl 6, `orelse` is similar to the `or` operator, except it only matches
## undefined variables instead of anything evaluating as false.
## Undefined values include: `Nil`, `Mu` and `Failure` as well as `Int`, `Str`
## and other types that have not been initialized to any value yet.
## You can check if something is defined or not using the defined method:
my $uninitialized;
say $uninitiazilzed.defined; #> False
## When using `orelse` it will disarm the exception and alias $_ to that failure
## This will avoid it being automatically handled and printing lots of scary
## error messages to the screen.
## We can use the exception method on $_ to access the exception
open 'foo' orelse say "Something happened {.exception}";

## This also works:
open 'foo' orelse say "Something happened $_"; #> Something happened
              #>  Failed to open file foo: no such file or directory
## Both of those above work but in case we get an object from the left side that
## is not a failure we will probably get a warning.  We see below how we can use
## `try` and `CATCH` to be more specific with the exceptions we catch.

Using `try` and `CATCH`

## By using `try` and `CATCH` you can contain and handle exceptions without
## disrupting the rest of the program. `try` will set the last exception to
## the special variable `$!` Note: This has no relation to $!variables.
try open 'foo';
say "Well, I tried! $!" if defined $!; #> Well, I tried! Failed to open file
                                       #foo: no such file or directory
## Now, what if we want more control over handling the exception?
## Unlike many other languages, in Perl 6, you put the `CATCH` block *within*
## the block to `try`. Similar to how $_ was set when we 'disarmed' the
## exception with orelse, we also use $_ in the CATCH block.
## Note: ($! is only set *after* the `try` block)
## By default, a `try` has a `CATCH` block that catches
## any exception (`CATCH { default {} }`).

try { my $a = (0 %% 0);  CATCH { say "Something happened: $_" } }
 #=> Something happened: Attempt to divide by zero using infix:<%%>

## You can redefine it using `when`s (and `default`)
## to handle the exceptions you want:
try {
  open 'foo';
  CATCH {     # In the `CATCH` block, the exception is set to $_
    when X::AdHoc { say "Error: $_" }
     #=>Error: Failed to open file /dir/foo: no such file or directory

    ## 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.
  }
}

## There are also some subtleties 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 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]

## You should not declare packages using the `package` keyword (unlike Perl 5).
## Instead, use `class Package::Name::Here;` to declare a class, or if you only
## want to export variables/subs, you can use `module`.

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 will learn more about grammars in the regex section

## As said before, any part of the six model is also a package.
## Since `JSON::Tiny` uses its own `JSON::Tiny::Actions` class, you can use it:
my $actions = JSON::Tiny::Actions.new;

## We'll see how to export variables and subs in the next part:

Declarators

## 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` declarations happen 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 Var::Increment {
  our $our-var = 1; # Note: you can't put a type constraint like Int on an
  my $my-var = 22;  # `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'll get burned.";
    }

    my sub unavailable { # `my sub` is the default
      say "Can't access me from outside, I'm 'my'!";
    }
    say ++$our-var; # Increment the package variable and output its value
  }

}
say $Var::Increment::our-var; #=> 1 This works
say $Var::Increment::my-var; #=> (Any) This will not work.

Var::Increment::Inc; #=> 2
Var::Increment::Inc; #=> 3 # Notice how the value of $our-var was
                         # retained.
Var::Increment::unavailable; #> Could not find symbol '&unavailable'

## * `constant` (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 languages such as C as `static`)
sub fixed-rand {
  state $val = rand;
  say $val;
}
fixed-rand for ^10; # will print the same number 10 times

## Note, however, that they exist separately in different enclosing contexts.
## If you declare a function with a `state` within a loop, it'll re-create the
## variable for each iteration of the loop. See:
for ^5 -> $a {
  sub foo {
    state $val = rand; # This will be a different value for every value of `$a`
  }
  for ^5 -> $b {
    say foo; # This will print the same value 5 times, but only 5.
             # Next iteration will re-run `rand`.
  }
}

Phasers

## Phasers in Perl 6 are blocks that happen at determined points of time in your
## program.  They are called phasers because they mark a change in the phase
## of a program.  For example, when the program is compiled, a for loop runs,
## you leave a block, or an exception gets thrown. 
## (`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)";
    say "If this block doesn't return a truthy value,
        an exception of type X::Phaser::PrePost is thrown.";
}

## example:
for 0..2 {
    PRE { $_ > 1 } # This is going to blow up with "Precondition failed"
}

POST {
    say "Asserts a postcondition at every block exit,
        after LEAVE (especially useful for loops)";
    say "If this block doesn't return a truthy value,
        an exception of type X::Phaser::PrePost is thrown, like PRE.";
}
for 0..2 {
    POST { $_ < 2 } # This is going to blow up with "Postcondition failed"
}

## * Block/exceptions phasers
sub {
    KEEP { say "Runs when you exit a block successfully
                (without throwing an exception)" }
    UNDO { say "Runs when you exit a block unsuccessfully 
                (by throwing an exception)" }
}

## * Loop phasers
for ^5 {
  FIRST { say "[*] The first time the loop is run, before ENTER" }
  NEXT { say "At loop continuation time, before LEAVE" }
  LAST { say "At loop termination time, after LEAVE" }
}

## * Role/class phasers
COMPOSE { "When a role is composed into a class. /!\ NOT YET IMPLEMENTED" }

## They allow for cute tricks or clever code ...:
say "This code took " ~ (time - CHECK time) ~ "s to compile";

## ... or clever organization:
sub do-db-stuff {
  $db.start-transaction; # start a new transaction
  KEEP $db.commit; # commit the transaction if all went well
  UNDO $db.rollback; # or rollback if all hell broke loose
}

Statement prefixes

## Those act a bit like phasers: they affect the behavior of the following code.
## Though, they run in-line with the executable code, so they're in lowercase.
## (`try` and `start` are theoretically in that list, but explained elsewhere)
## Note: all of these (except start) don't need explicit 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

Iterables

## Iterables are objects that can be iterated similar to the `for` construct
## `flat`, flattens iterables:
say (1, 10, (20, 10) ); #> (1 10 (20 10)) Notice how grouping is maintained
say (1, 10, (20, 10) ).flat; #> (1 10 20 10) Now the iterable is flat

## - `lazy` - Defer actual evaluation until value is fetched 
## (forces lazy context)
my @lazy-array = (1..100).lazy;
say @lazy-array.is-lazy; #> True # Check for laziness with the `is-lazy` method.
say @lazy-array; #> [...] List has not been iterated on!
my @lazy-array { .print }; # This works and will only do as much work as
                           # is needed.
[//]: # ( TODO explain that gather/take and map are all lazy)
## - `sink` - An `eager` that discards the results (forces sink context)
constant nilthingie = sink for ^3 { .say } #=> 0 1 2
say nilthingie.perl; #=> Nil

## - `quietly` blocks will suppress warnings:
quietly { warn 'This is a warning!' }; #=> No output

## - `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:
## https://docs.perl6.org/language/operators#Operator_Precedence
## But first, we need a little explanation about associativity:

## * Binary operators:
$a ! $b ! $c; # with a left-associative `!`, this is `($a ! $b) ! $c`
$a ! $b ! $c; # with a right-associative `!`, this is `$a ! ($b ! $c)`
$a ! $b ! $c; # with a non-associative `!`, this is illegal
$a ! $b ! $c; # with a chain-associative `!`, this is `($a ! $b) and ($b ! $c)`
$a ! $b ! $c; # with a list-associative `!`, this is `infix:<>`

## * Unary operators:
!$a! # with left-associative `!`, this is `(!$a)!`
!$a! # with right-associative `!`, this is `!($a!)`
!$a! # with non-associative `!`, this is illegal

Create your own operators !

## 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 that also can be used as a "normal"
## operator.  It takes an optional binary function (by default, it just creates
## a pair), and will pop one value off of each array and call its binary 
## function on these until it runs out of elements. It returns an array with 
## all of these new elements.
(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 /; #=> True #  More readable with some spaces!

## In all our examples, we're going to use the smart-matching operator against
##  a regexp. We're converting the result using `so`, 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 which start at 0:
##    `$0`, `$1', `$2`...
##
## You can also note that `~~` does not perform start/end checking
##  (meaning the regexp can be matched with just one 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`. Wouldn'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) adverb.
say so 'a b c' ~~ / a  b  c /; #> `False`. Space is not significant here
say so 'a b c' ~~ /:s a b c /; #> `True`. We added the modifier `:s` here.
## If we use only one space between strings in a regex, Perl 6 will warn us:
say so 'a b c' ~~ / a b c /; #> 'False' #> Space is not significant here; 
## please use quotes or :s (:sigspace) modifier (or, to suppress this warning,
## omit the space, or otherwise change the spacing)
## To fix this and make the spaces less ambiguous,  either use at least two
## spaces between strings or use the `:s` adverb.

## As we saw before, we can embed the `:s` inside the slash delimiters, but we 
## can also put it outside of them if we specify `m` for 'match':
say so 'a b c' ~~ m:s/a  b  c/; #> `True`
## By using `m` to specify 'match', we can also use delimiters other 
## than slashes:
say so 'abc' ~~ m{a  b  c}; #> `True`

## Use the :i adverb to specify case insensitivity:
say so 'ABC' ~~ m:i{a  b  c}; #> `True`

## 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' ~~ / a  b?  c /; # `False`. Whitespace is not significant
so 'def' ~~ / 'abc'? /; # `True`. The whole "abc" group is optional.

## Here (and below) the quantifier applies only to the `b`

## - `+` - 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:

## Note: We quoted A B C to demonstrate that the whitespace between them isn't
##       significant. If we want the whitespace to *be* significant there, we 
##       can use the :sigspace modifier.
so 'fooABCbar' ~~ / foo ( "A" "B" "C" )? bar /; # `True`
say $/[0]; #=> ｢ABC｣
say $0.WHAT; #=> (Match)
             # There can't be more than one, so it's only a single match object.
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 (`|`), conjunctions (`&`),
## 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.
## Examples:
## DECLARATIVE | PROCEDURAL
/ 'foo' \d+     [ <subrule1> || <subrule2> ] /;
## DECLARATIVE (nested groups are not a problem)
/ \s* [ \w & b ] [ c | d ] /;
## However, closures and recursion (of named regexps) are procedural.
## ... There are also more complicated rules, like specificity
##  (literals win over character classes)

## Note: the first-matching `or` still exists, but is now spelled `||`
'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 have 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 Docs. This is a great resource on Perl6. If you are looking for something, use the search bar. This will give you a dropdown menu of all the pages referencing your search term (Much better than using Google to find Perl 6 documents!)
Read the Perl 6 Advent Calendar. This is a great source of Perl 6 snippets and explanations. If the docs don't describe something well enough, you may find more detailed information here. This information may be a bit older but there are many great examples and explanations. Posts stopped at the end of 2015 when the language was declared stable and Perl 6.c was released.
Come along on #perl6 at irc.freenode.net. The folks here are always helpful.
Check the source of Perl 6's functions and classes. 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.

66 KiB Raw Blame History Unescape Escape

Variables

Scalars

Arrays and Lists

Hashes, or key-value Pairs.

Subs

Containers

Control Flow Structures

Conditionals

given/when, or switch

Looping constructs

Operators

More on subs !

Unpacking !

lambdas

About types...

Multiple Dispatch

Scoping

Twigils

Object Model

Object Inheritance

Roles, or Mixins

Exceptions

Using try and CATCH

Packages

Declarators

Phasers

Statement prefixes

Iterables

More operators thingies !

Create your own operators !

Meta operators !

Regular Expressions

Grouping and capturing

Extra: the MAIN subroutine

APPENDIX A:

List of things

66 KiB

Raw Blame History

Using `try` and `CATCH`