Contexts

NAME

"Perl 5 to 6" Lesson 06 - Contexts

SYNOPSIS

    my @a = <a b c>
    my $x = @a;
    say $x[2];          # c
    say (~2).WHAT       # Str()
    say +@a;            # 3
    if @a < 10 { say "short array"; }

DESCRIPTION

When you write something like this

    $x = @a

in Perl 5, $x contains less information than @a - it contains only the number of items in @a. To preserve all information, you have to explicitly take a reference: $x = \@a.

In Perl 6 it's the other way round: by default you don't lose anything, the scalar just stores the array. This was made possible by introducing a generic item context (called scalar in Perl 5) and more specialized numeric, integer and string contexts. Void and List context remain unchanged.

You can force contexts with special syntax.

    syntax       context

    ~stuff       String
    ?stuff       Bool
    +stuff       Numeric
    -stuff       Numeric (also negates)
    $( stuff )   Generic item context
    @( stuff )   List Context
    @@( stuff )  Slice context

Slice Context

There's also a new context called slice context. It is a list context in which sublists don't interpolate.

     @( <a b> Z <c d> )     # <a c b d>
    @@( <a b> Z <c d> )     # (['a', 'c'], ['b', 'd'])

This was introduced after the observation that many built in list functions had two versions - one that returned a flat list, and one that returned a list of lists.

You can force slice context with @@( stuff ).

MOTIVATION

More specific contexts are a way to delay design choices. For example it seems premature to decide what a list should return in scalar context - a reference to the list would preserve all information, but isn't very useful in numeric comparisons. So every possible choice disappoints somebody.

With more specific context you don't need to make this choice - it returns some sensible default, and all operators that don't like this choice can simply propagate a more specific context.

For some things like the match object the different contexts really enhance their usefulness and beauty.

SEE ALSO

http://perlcabal.org/syn/S02.html#Context