=pod

=head1 NAME

Magazine_Article_05 - Yet Another Perl 6 Operator

=head1 AUTHOR

Adriano Ferreira <a.r.ferreira@gmail.com>

=head1 VERSION

Its published between September 18, 2007 and January 7, 2008 as a series
of microarticles on perl.com and not changed since. Find the original under:

http://www.oreillynet.com/onlamp/blog/2007/09/yet_another_perl_6_operator_th_1.html

=head1 ARTICLE

=head2 Introduction

You surely heard about the upcoming Perl 6 (L<http://dev.perl.org/perl6/>)
language. This language will be endowed with a set of features so rich
that every Greek and Trojan is eager to see a full working implementation.
Among these features, there are operators, many of them. Perl 6 was even
said () to be an operator-oriented language, with a yet larger diversity
than Perl 5 already has.

=head2 The zip operator

Perl 6 has an operator C<Z>, named B<zip>, to interleave elements of two
or more arrays.

    my @a = 1,2 Z -1,-2; # (1,-1),(2,-2)

The zip is one of the list generating operators that gives the language some
flavor of functional programming. This gets further as the usual semantics
for lists is to get lazy generation, which means easy/efficient handling of
large lists (and also the extreme case of infinite lists).

=head2 String concatenation

Today’s operator is a very simple one, the string concatenation operator.

    my $a = 'ab' ~ 'c'; # 'abc'

    my $b = 'def';
    my $c = $a ~ $b;    # 'abcdef'

=head2 Repeat Operators

Perl 6 has two repeat operators: one for replicating a string/buffer
and the other for replicating lists.

String repeat C<'x'> takes a string as the left argument and the number
of times to replicate as the right argument.

    $string x $count

    my $s = 'a' x 3; # 'aaa'
    my $empty = 'foo' x 0; # ''
    my $n = 2; my $dots = '.' x ($n - 3); # '' because ($n-3)<1

=head2 Coercion operators

In Perl 5, we expect values to DWIM (”do what I mean”) in various contexts.
For example, if we use a string containg “42? as a number we expect it
automagically act as a number. Perl 6 keeps this tradition of DWIMmery and
introduces several new explicit coercion operations.

    ? to get booleans
    + to get numbers
    ~ to get strings

=head2 Comparisons - Part I

As expected, Perl 6 supports the usual comparison operators. This includes
the numeric comparison operators:

    == != < <= > >=

(where C<'!='> is a short for C<'!=='>, the negated version of C<'=='>). These
operators convert their terms into numbers before comparison.

The string comparisons operators are here as well.

=head2 Comparisons - Part II

In the the last article, we’ve seen some of the usual relational operators
in Perl 6 and their enhanced syntax through chaining (which allows
expressions like C<'a < b < c'>).

Another kind of comparison operators are those that, instead of true/false
returns, identify the relative order between its operands: C<before>, C<equal>, or
C<after>.

=head2 Boolean Operators

In the article on coercion operators, we got to know the prefix operator C<'?'>
which converts values into C<Bool::True> or C<Bool::False>. Like it happens with C<'~'>
for strings, C<'?'> is recurrent for boolean operators.

In Perl 6, the usual infix boolean operators are:

    ?& - and
    ?| - or
    ?^ - xor

=head2 The Default Operator

Among the new Perl 6 operators, there is the handy operator C<'//'>, known as
defined-or or the default operator. This novelty was anticipated by the
introduction of this syntactic bit in Perl 5 (see the upcoming 5.10 release) —
so you won’t need to wait for Perl 6 to start using it.

    # dor.pl
    use 5.010;
    print "arg: '", shift // "?", "'\n";

    $ perl dor.pl one
    arg: 'one'
    $ perl dor.pl ""
    arg: ''
    $ perl dor.pl
    arg: '?'

=head2 Range Operators

In Perl 6, you may construct ranges with expressions like

    $min  ..  $max
    $min ^..  $max
    $min  ..^ $max
    $min ^..^ $max

and even

    ^$limit
    
=head2 Conditional Operator

The syntax of an if-then-else expression in Perl 6 is composed by the 
conditional operator.

    say "My answer is: ", $maybe ?? 'yes' !! 'no';

The expression above is equivalent to that, which uses the if-then-else
statement within a do.

    say "My answer is: ", do {
        if $maybe {
            'yes';
        }
        else {
            'no';
        }
    };

=head2 The Cross Operator

Perl 6 provides an operator C<'X'>, the cross operator, which combines its list
operands into a sort of cartesian product of these arguments.

    1,2 X 3,4        # (1,3), (1,4), (2,3), (2,4)

    1,2 X 3,4 X 5,6  # (1,3,5), (1,3,6), (1,4,5), ..., (2,4,6)

=head2 Iterate Operator

If you are wondering how processing the lines of a file will look in Perl 6,
the answer is something like this:

    my $h = open '<', $filename;

    for =$h {
       ...
    }

=head2 Reduce operators

And that’s time to take a look at another of the Perl 6 meta-operators: the 
reduction operator.

By surrounding with square brackets an (associative) infix operator, a new 
list operator is created.

    [*] 1..10      # that's 1*2*...*10 = 10!
    [~] <m oo s e> # 'moose' - [~] is basically Perl 5 join
    [,] 'a'..'e'   # <a b c d e> - [,] is a list builder

=head2 Mutating Operators

We already have seen two Perl 6 meta-operators in articles of this series:
namely, the negate and the reduction operators. These are two of the five
standard meta-operators of the language. What makes meta-operators interesting
is how Perl automatically generates new operators from others (user-defined or
builtins) with some straightforward semantics derived from the transformation
of the base operators.

This time, we approach mutating operators, which are a shortcut for typical
assignments where the assignment target and the first operand are the same
variable.

    my $s = 'foo';
    $s x= 3;          # $a = 'foofoofoo'

    my $x;
    $x //= 'default'; # $x = 'default'

=head2 The Pair Constructor

Binary C<'=>'> is no longer just a “fancy comma”. In Perl 6, it now constructs 
a Pair object that can, among other things, be used to pass named arguments 
to functions.

    my $pair = (one => 1);
    $pair.isa(Pair)        # Bool::True
    $pair.key              # 'one'
    $pair.value            # 1

=head2 Reduce Operators - Part II

In a previous article , we introduced the reduction operators (like C<'[*]'> and
C<'[~]'>) which produced list operators from infix operators (like C<'*'> and C<'~'>).

There is a variant of the reduction operator that operates over its list
argument producing all intermediate results along with the final result of the
ordinary reduction.

    [\+] 1..5   # (1, 3, 6, 10, 15)

which is equivalent to

    ([+] 1),
    ([+] 1, 2),
    ([+] 1, 2, 3),
    ([+] 1, 2, 3, 4),
    ([+] 1, 2, 3, 4, 5)

=head2 Filetests?

This article is not about some set of Perl 6 operators, but rather about what
happened to Perl 5 filetests operators. Short answer: They are not operators
anymore.

Where programmers were used to write

    # good ol' Perl 5
    if ( -e $filename ) { print "exists\n" }

they will now use pair methods that may be expressed as methods or smart 
patterns.

    if $filename.:e { say "exists" }
    # or
    if $filename ~~ :e { say "exists" }

=head2 Junction Operators

Perl 6 introduces a new scalar data-type: the “junction”. A junction is a
single scalar value that can act like two or more values at once.

    example                 a value which acts like

    any(1,2,3)              1 or 2 or 3
    all(@vals)              all members of @vals at the same time
    one(<moe curly larry>)  one of the three stooges
    none(@bad_guys)         none of the listed bad guys

The operators '|', '&' and '^' are now junction constructors, providing a
syntactical complement to the functional variants any, all, one and none.

    $a  | $b                 any($a, $b)
    $x  & $y                 all($x, $y)
    $me ^ $you               one($me, $you)

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