Perl 6
Perl 6 Variable Tablet: Revision 63

Overview - Chapter: 0:Intr, 1:Hist, 2:Design, 3:Var, 4:Op, 5:IO, 6:{}, 7:Sub, 8:OOP, 9:Rx - Appendices: A:Index, B:Tables, C:Best of, D:Delta, E:Links




Allison Randal: The most basic building blocks of a programming language are its nouns, the chunks of data that get sucked in, pushed around, altered in various ways, and spat out to some new location.

Primary Types

Perl 6 (as Perl 5) knows 3 basic types of variables: Scalars (single values), Arrays (ordered and indexed lists of several values) and Hashes (2 column table, with ID and associated value pairs). They can be easily distinguished, because in front of their name is a special character called sigil (latin for sign). Its the $ (similar to S) for Scalars, @ (like an a) for Arrays and a % (kv pair icon) for a Hash. They are now invariant, which means for instance, an array vaiable starts always with an @, even if you just want a slice of the content. The sigils mark also distinct namespaces, meaning: in one lexical scope you can have 3 different variables named $stuff, @stuff and %stuff. These sigils can also be used as an operator to enforce a context in which the following data will be seen.

The fourth namespace where you can store and retrieve something under specified names is the one of subroutines and alike, even if you don't might think of them as variables. It's sigil & has to be used only rarely.

Special namespaces of Perl 5 (often marked with special syntax) like tokens (__PACKAGE__), formats, file or dir handle and builtins are now regular (mostly scalar) variables or routines.

Because variables are (as anything in Perl 6) objects, they have methods. In fact, any operator, including these square or curly brackets you get specific array and hash values with, are just methods of a variable object with a fancy name.

The primary sigil can be followed by a secondary sigil, called twigil, which mostly indicate special scope of that variable.

Scalar

This type is known as a storage room for one value, but it's more like a reference that can point to anything: to values of any data type, to code, to objects or to a compound of values like a pair, junction, array or hash. The scalar context is now called item context hence the scalar instruction from Perl 5 was renamed to item.

$CHAPTER = 3;
$pi = 3.14159_26535_89793; # the underscores just ease reading
$float = 6.02e23;
$complex = 1+2i;
$text = 'Welcome all!';    # single quoted string
$text = "What is $pi?";    # double quoted string, does eval $pi to its content
$text = q:to'EOT';         # heredoc string, more about quoting in the 5th tablet
   handy for multiline text
   like HTML templates or email
EOT
$handle = open $file_name; # file handle
$object = Class::Name.new();
$arrayref = [0,1,1,2,3,5,8,13,21];
$hashref = {'audreyt'=>'pugs', 'pm'=>'pct', 'damian'=>'larrys evil henchman'};
$coderef = sub { do_something_completely_diffenent(@_) };

Unlike Perl 5, references are automatically dereferenced to a fitting context. So you could use these $arrayref and $hashref in same way as an array or hash, making $ the variable highlighter, pretty much like in PHP.

Array

is an ordered and indexed list of scalar variables. Their context is forced with a @ operator or list command. If not specified otherwise they can be changed, prolonged and shorten anytime and used as a list, stack, queue and much more. As in Haskell, lists are processed lazily, which means: the compiler looks only at the part he currently needs. This way Perl 6 can handle infinite lists or do computation on lists that are still building up. The lazy command enforces and the eager command prevents that behaviour on any expression.

@primes = (2,3,5,7,11,13,17,19,23); # an array gets filled
@primes =  2,3,5,7,11,13,17,19,23 ; # same thing, since () just do group
@primes = <2 3 5 7 11 13 17 19 23>; # dito, <> is the new qw()
$arrayref = 2,3,5,7,11,13,17,19,23; # in scalar context you get automatically a reference
$arrayref = 13,;                    # comma is the new array generator
@primes = 2;                        # array with one element
@primes = [2,3,5,7,11,13,17,19,23]; # array with one element (arrayref)
@dev    = {'dan' => 'parrot'};      # array with one element (hashref)
@data   = [1..5],[6..10],[11..15];  # Array of Arrays (AoA)

Array Slices

@primes                       # all values
"@primes[]"                   # insert all values in a string, use [] as distinction from mail adresses
$prime = @primes[0];          # get the first prime
$prime = @primes[*-1];        # get the last one
@some = @primes[2..5];        # get several
$cell = @data[1][2];          # get 8, third value of second value (list)
$cell = @data[1;2];           # same thing, shorten syntax
@numbers = @data[1];          # get a copy of the second subarray (6..10)
@copy = @data;                # copy the whole AoA, no more reference passing, use binding instead

Array Methods

Some of the more important things you can do with lists. All the methods can also used like ops in "elems @rray;"

+ @rray;             # numeric context does the same
@rray.elems;         # number of elements (like in Perl 5 scalar @a)
@rray.end;           # number of the last element, equal to @rray.elems-1
@rray.exists(@ind);  # Bool::True if all indecies of @ind have a value (can be 0 or '')
@rray.delete(@ind);  # delete all cell with indecies of @ind
~ @rray;             # in string context you get all content, stringified and joined
@rray.cat;           # does the same
@rray.join('');      # also same result, you can put another string as parameter that gets between all values
? @rray;             # Bool::True if array has any value in it, even if its a 0
@rray.unshift;       # prepend one value to the array
@rray.shift;         # remove the first value and return it
@rray.push;          # add one value on the end
@rray.pop;           # remove one value from the end and return it
@rray.splice($pos,$n)# remove on $pos $n values and replace them with values that follow that two parameter
@rray.pick([$n]);    # return $n (default is 1) randomly selected values, without duplication
@rray.roll([$n]);    # return $n (default is 1) randomly selected values, duplication possible (like roll dice)
@rray.reverse;       # all elements in reversed order
@rray.rotate($n);    # returns a list where $n times first item is taken to last position if $n is positive, if negative the other way around
@rray.sort($coderef);# returns a sorted list by a userdefined criteria, default is alphanumerical sorting
@rray.min;           # numerical smallest value of that array
@rray.max;           # numerical largest value of that array
$a,$b= @rray.minmax; # both at once, like in .sort . min or .max a sorting algorith can be provided 
@rray.map($coderef); # high oder map function, runs $coderef with every value as $_ and returns the list or results
@rray.classify($cr); # kind of map, but creates a hash, where keys are the results of $cr and values are from @rray
@rray.grep({$_>1});  # high order grep, returns only these elements that pass a condition ($cr returns something positive)
@rray.first($coder); # kind of grep, return just the first matching value
@rray.zip;           # join arrays by picking first element left successively from here and then there

There is even a whole class of metaoperators that work upon lists.

Hash

is in Perl 6 an unordered list of Pairs. A Pair is a single key => value association and appears in many places of the language syntax.

%dev =  'pugs'=>'audreyt', 'pct'=>'pm', "STD"=>'larry';
%dev = :rakudo('jnthn'), :testsuite('moritz');            # adverb (pair) syntax works as well
%dev = ('audreyt', 'pugs', 'pm', 'pct', 'larry', "STD");  # lists get autoconverted in hash context
%compiler = Parrot => {Rakudo => 'jnthn'}, SMOP => {Mildew => 'ruoso'};       # hash of hashes (HoH)

Hash Slices

my $table = %dev;          # nicely formatted 2 column table using \t and \n
say %dev;                  # only $key and $value are separated by \t

 ~ %dev                    # string context, same thing
 ? %dev                    # bool context, true if hash has any pairs
 + %dev                    # numeric context, returns number of pairs(keys)

$value = %dev{'key'};      # just give me the value related to that key, like in P5
$value = %dev<pm>;         # <> autoquotes like qw() in P5
$value = %dev<<$name>>;    # same thing, just with eval
@values = %dev{'key1', 'key2'};
@values = %dev<key1 key2>;
@values = %dev<<key1 key2 $key3>>;

%dev\ .{'patrick'};        # "long dot style", because its its an object in truth 
%dev   {'audrey'};         # error, only syntax before will work
%dev\  {'allison'};        # works,
%dev   .<dukeleto>;        # error

%compiler<Parrot><Rakudo>; # value in a HoH, returns 'jnthn'
%compiler<SMOP>;           # returns the Pair: Mildew => 'ruoso'

Hash Methods

%dev.keys                  # returns the list with all keys
%dev.values                # list of all values
%dev.kv                    # flat list with key1, value1, key 2 ...
%dev.pairs                 # list of pair objects, list context default (same as in @a = %dev) 
%dev.invert                # reverse all key => value relations    

Callable

Internally subroutines, methods and alike are variables with the sigil "&" and stored in a fourth namespace. They are no more builtins with an own namespace, that can't be overwritten or augmented with your programming.

Object Types

Perl 6 has much more variable types then the primary 4. They are defined by classes or partial classes (roles).

Pair

are very new and their syntax is used nearly everywhere the language where you have associations between a name and a value.

$pair = 'jakub' => 'helena';  # "=>" is the pair constructor
$pair = :jakub('helena');     # same in adverbial notation
$pair.key                     # returns 'jakub'
$pair.value                   # returns 'helena'
$pair.isa(Pair)               # Bool::True

Typing

Properties and Traits

Scoping

Twigils

Assignment and Binding

Assignment

Binding

Special Variables

are listed heir table. To understand their secondary sigil look here


Overview - Chapter: 0:Intr, 1:Hist, 2:Design, 3:Var, 4:Op, 5:IO, 6:{}, 7:Sub, 8:OOP, 9:Rx - Appendices: A:Index, B:Tables, C:Best of, D:Delta, E:Links


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