Synopsis 3

Mar 18, 2004 by Luke Palmer

Operator Renaming

Several operators have been given new names to increase clarity and better Huffman-code the language:

• -> becomes ., like the rest of the world uses.
• The string concatenation . becomes ~. Think of it as “stitching” the two ends of its arguments together.
• Unary ~ now imposes a string context on its argument, and + imposes a numeric context (as opposed to being a no-op in Perl 5). Along the same lines, ? imposes a Boolean context.
• Bitwise operators get a data type prefix: +, ~, or ?. For example, | becomes either +| or ~| or ?|, depending on whether the operands are to be treated as numbers, strings, or Boolean values. Left shift << becomes +<, and correspondingly with right shift. Unary ~ becomes either +^ or ~^ or ?^, since a bitwise NOT is like an exclusive-or against solid ones. Note that ?^ is functionally identical to !. ?| differs from || in that ?| always returns a standard Boolean value (either 1 or 0), whereas || returns the actual value of the first of its arguments that is true.
• x splits into two operators: x (which concatenates repetitions of a string to produce a single string), and xx (which creates a list of repetitions of a list or scalar).
• Trinary ? : becomes ?? ::.
• qw{ ... } gets a synonym: « ... ». For those still living without the blessings of Unicode, that can also be written: << ... >>.
• The scalar comma , now constructs a list reference of its operands. You have to use a [-1] subscript to get the last one.

New Operators

• Binary // is just like ||, except that it tests its left side for definedness instead of truth. There is a low-precedence form, too: err.
• Binary => is no longer just a “fancy comma.” it now constructs a Pair object that can, among other things, be used to pass named arguments to functions.
• ^^ is the high-precedence version of xor.
• Unary . calls its single argument (which must be a method, or an de-referencer for a hash or array) on $_.
• ... is a unary postfix operator that constructs a semi-infinite (and lazily evaluated) list, starting at the value of its single argument.
• However, ... as a term is the “yada, yada, yada” operator, which is used as the body in function prototypes. It dies if it is ever executed.
• $(...) imposes a scalar context on whatever it encloses. Similarly, @(...) and %(...) impose a list and hash context, respectively. These can be interpolated into strings.

Hyperoperators

The Unicode characters » (\x[BB]) and « (\x[AB]) and their ASCII digraphs >> and << are used to denote “hyperoperations” – “list” or “vector” or “SIMD” operations that are applied pairwise between corresponding elements of two lists (or arrays) and which return a list (or array) of the results. For example:

     (1,1,2,3,5) »+« (1,2,3,5,8);  # 
(2,3,5,8,13)

If one argument is insufficiently dimensioned, Perl “upgrades” it:

     (3,8,2,9,3,8) >>-<< 1;          # 
(2,7,1,8,2,7)

This can even be done with method calls:

     ("f","oo","bar")».«length; 
   # (1,2,3)

When using a unary operator, only put it on the operand’s side:

     @negatives = -« @positives;

      @positions»++;            # Increment all positions

      @objects».run();

Junctive Operators

|, &, and ^ are no longer bitwise operators (see Operator Renaming) but now serve a much higher cause: they are now the junction constructors.

A junction is a single value that is equivalent to multiple values. They thread through operations, returning another junction representing the result:

     1|2|3 + 4;                              # 5|6|7
     1|2 + 3&4;                              # (4|5) & (5|6)

Note how when two junctions are applied through an operator, the result is a junction representing the operator applied to each combination of values.

Junctions come with the functional variants any, all, one, and none.

This opens doors for constructions like:

     unless $roll == any(1..6) { print "Invalid roll" }

     if $roll == 1|2|3 { print "Low roll" }

Chained Comparisons

Perl 6 supports the natural extension to the comparison operators, allowing multiple operands.

     if 3 < $roll <= 6              { print "High roll" }
     
     if 1 <= $roll1 == $roll2 <= 6  { print "Doubles!" }

Binding

A new form of assignment is present in Perl 6, called “binding,” used in place of typeglob assignment. It is performed with the := operator. Instead of replacing the value in a container like normal assignment, it replaces the container itself. For instance:

    my $x = 'Just Another';
    my $y := $x;
    $y = 'Perl Hacker';

After this, both $x and $y contain the string “Perl Hacker,” since they are really just two different names for the same variable.

There is another variant, spelled ::=, that does the same thing at compile time.

There is also an identity test, =:=, which tests whether two names are bound to the same underlying variable. $x =:= $y would return true in the above example.

List Flattening

Since typeglobs are being removed, unary * may now serve as a list-flattening operator. It is used to “flatten” an array into a list, usually to allow the array’s contents to be used as the arguments of a subroutine call. Note that those arguments still must comply with the subroutine’s signature, but the presence of * defers that test until runtime.

    my @args = (\@foo, @bar);
    push *@args;

Is equivalent to:

    push @foo, @bar;

Piping Operators

The new operators ==> and <== are akin to UNIX pipes, but work with functions that accept and return lists. For example,

     @result = map { floor($^x / 2) }
                 grep { /^ \d+ $/ }
                   @data;

Can also now be written:

     @data ==> grep { /^ \d+ $/ }
           ==> map { floor($^x / 2) }
           ==> @result;

Or:

     @result <== map { floor($^x / 2) }
             <== grep { /^ \d+ $/ }
             <== @data;

Either form more clearly indicates the flow of data. See Synopsis 6 for more of the (less-than-obvious) details on these two operators.

Invocant Marker

An appended : marks the invocant when using the indirect-object syntax for Perl 6 method calls. The following two statements are equivalent:

    $hacker.feed('Pizza and cola');
    feed $hacker: 'Pizza and cola';

zip

In order to support parallel iteration over multiple arrays, Perl 6 has a zip function that interleaves the elements of two or more arrays.

    for zip(@names, @codes) -> $name, $zip {
        print "Name: $name;   Zip code: $zip\n";
    }

zip has an infix synonym, the Unicode operator ¦.

Minimal Whitespace DWIMmery

Whitespace is no longer allowed before the opening bracket of an array or hash accessor. That is:

    %monsters{'cookie'} = Monster.new;  # Valid Perl 6
    %people  {'john'}   = Person.new;   # Not valid Perl 6

One of the several useful side-effects of this restriction is that parentheses are no longer required around the condition of control constructs:

    if $value eq $target {
        print "Bullseye!";
    }
    while 0 < $i { $i++ }

It is, however, still possible to align accessors by explicitly using the . operator:

     %monsters.{'cookie'} = Monster.new;
     %people  .{'john'}   = Person .new;
     %cats    .{'fluffy'} = Cat    .new;