
=encoding utf8

=head1 TITLE

DRAFT: Synopsis 32: Setting Library - Basics

=head1 AUTHORS

    Rod Adams <rod@rodadams.net>
    Larry Wall <larry@wall.org>
    Aaron Sherman <ajs@ajs.com>
    Mark Stosberg <mark@summersault.com>
    Carl Mäsak <cmasak@gmail.com>
    Moritz Lenz <moritz@faui2k3.org>
    Tim Nelson <wayland@wayland.id.au>

=head1 VERSION

    Created: 19 Mar 2009 extracted from S29-functions.pod

    Last Modified: 30 Apr 2009
    Version: 2

The document is a draft.

If you read the HTML version, it is generated from the pod in the pugs
repository under /docs/Perl6/Spec/S32-setting-library/Basics.pod so edit it there in
the SVN repository if you would like to make changes.

=head1 Roles

=head2 Object

The following are defined in the C<Object> role:

 role Object {
     our Bool multi method defined ($self:) is export {...}
     our Bool multi method defined ($self: ::role ) is export {...}

     our multi method undefine( $self: ) is export {...}

     method not() {...}
     method true() {...}
 }

=item defined

  our Bool multi method defined ( $self: ) is export
  our Bool multi method defined ( $self: ::role ) is export

C<defined> returns true if the parameter has a value and that value is
not the undefined value (per C<undef>), otherwise false is returned.

Same as Perl 5, only takes extra optional argument to ask if value is defined
with respect to a particular role:

  defined($x, SomeRole);

A value may be defined according to one role and undefined according to another.
Without the extra argument, defaults to the definition of defined supplied by
the type of the object.

=item undefine

  our multi undefine( Any $thing )
  our multi method undefine( Any $self )

Takes any variable as a parameter and attempts to "remove" its
definition. For simple scalar variables this means assigning
the undefined value to the variable. For objects, this is equivalent
to invoking their undefine method. For arrays, hashes and other
complex data, this might require emptying the structures associated
with the object.

In all cases, calling C<undefine> on a variable
should place the object in the same state as if it was just
declared.

=item not

     method not() {...}

=item true

     method true() {...}

XXX Copied from S02 -- should it be deleted from there?

The definition of C<.Bool> for the most ancestral type (that is, the
C<Object> type) is equivalent to C<.defined>.  Since type objects are
considered undefined, all type objects (including C<Object> itself)
are false unless the type overrides the definition of C<.Bool>
to include undefined values.  Instantiated objects default to true
unless the class overrides the definition.  Note that if you could
instantiate an C<Object> it would be considered defined, and thus true.
(It is not clear that this is allowed, however.)

=head2 Any

The following are defined in the C<Any> role:

 role Any does Object does Pattern {
     our Bool multi sub eqv (Ordering @by, $a, $b) {...}
     our Bool multi sub eqv (Ordering $by = &infix:<eqv>, $a, $b) {...}

     our multi method clone (::T $self --> T) {...}
     our multi method clone (::T $self, *%attributes --> T) {...}

     our Order multi sub cmp (Ordering @by, $a, $b) {...}
     our Order multi sub cmp (Ordering $by = &infix:<cmp>, $a, $b) {...}

     our Callable multi method can ($self:, Str $method) {...}
     our Bool     multi method does ($self:, $type) {...}
     our Bool     multi method isa  ($self:, $type) {...}
     our Str      multi method perl ( Object $o: ) is export {...}
     our          multi method warn ( Object $o: ) is export {...}
 }

=over

=item eqv

 our Bool multi sub eqv (Ordering @by, $a, $b)
 our Bool multi sub eqv (Ordering $by = &infix:<eqv>, $a, $b)

Returns a Bool indicating if the parameters are equivalent,
using criteria C<$by> or C<@by> for comparisons. C<@by> differs
from C<$by> in that each criterion is applied, in order,
until a non-zero (equivalent) result is achieved.

=item can

 our Callable multi method can ($self:, Str $method)

If there is a multi method of name C<$method> that can be called on
C<$self>, then a closure is return which has C<$self> bound to the position
of the invocant.

Otherwise an undefined value is returned.

=item clone

 our multi method clone (::T $self --> T)
 our multi method clone (::T $self, *%attributes --> T)

The first variant returns  an independent copy of C<$o> that is equivalent
to C<$o>.

The second variant does the same, but any named arguments override an
attribute during the cloning process.

=item cmp

 our Order multi sub cmp (Ordering @by, $a, $b)
 our Order multi sub cmp (Ordering $by = &infix:<cmp>, $a, $b)

Returns C<Order::Increase>, or C<Order::Same>, or C<Order::Decrease>
(which numify to -1, 0, +1 respectively) indicating if paramater C<$a> should
be ordered before/tied with/after parameter C<$b>, using criteria
C<$by> or C<@by> for comparisons. C<@by> differs from C<$by>
in that each criterion is applied, in order, until a non-zero
(tie) result is achieved.  If the values are not comparable,
returns a proto C<Order> object that is undefined.

=item does

 our Bool multi method does ($self:, $type)

Returns C<True> if and only if C<$self> conforms to type C<$type>.

=item isa

 our Bool multi method isa ($self:, $type)

Returns C<True> if a the invocant an instance of class C<$type>, or
of a subset type or a derived class (through inheritance) of C<$type>.

=item perl

 our Str multi method perl ( Object $o: ) is export

Returns a perlish representation of the object, so that calling C<eval>
on the returned string reproduces the object as accurately as possible.

=item warn

 our multi method warn ( Object $o: ) is export

Prints a warning to C<$*ERR>, which is usually finds C<$PROCESS::ERR>. See
C<Synopsis 16: IPC / IO / Signals> for details.

=back

=head2 Pattern

 role Pattern {
     method ACCEPTS($self:, $other) {...}
     method REJECTS($self:, $other) {...}
 }

=item ACCEPTS

Used in smartmatching; see S03.

=item REJECTS

Used in smartmatching; see S03.

=head2 Scalar

B<API document>: L<Scalar>

C<Scalar> provides the basic tools for operating on undifferentiated
scalar variables. All of the following are exported by default.

=over

=item undef

  constant Scalar Scalar::undef

Returns the undefined scalar object. C<undef> has no value at
all, but for historical compatibility, it will numify to C<0>
and stringify to the empty string, potentially generating a
warning in doing so. There are two ways to determine if a
value equal to undef: the C<defined> function (or method) can
be called or the C<//> (or C<orelse>) operator can be used.

C<undef> is also considered to be false in a boolean context.
Such a conversion does not generate a warning.

Perl 5's unary C<undef> function is renamed C<undefine> to avoid
confusion with the value C<undef> (which is always 0-ary now).

=item VAR

This is not really a method, but some kind of macro.  See L<S12> for details.

=back

=head1 Additions

Please post errors and feedback to perl6-language.  If you are making
a general laundry list, please separate messages by topic.
