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RFC 115 (v3) Overloadable parentheses for objects

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September 30, 2000 23:22
RFC 115 (v3) Overloadable parentheses for objects
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=head1 TITLE

Overloadable parentheses for objects

=head1 VERSION

  Maintainer: pdl-porters team <>
  Date: 16 Aug 2000
  Last Modified: 30 Sep 2000
  Mailing List:
  Number: 115
  Version: 3
  Status: Frozen


There was hardly any response to this RFC. That can be
interpreted in a number of ways C<;)>. RFC 231 suggests
an alternative way to get what we tried to get from
this RFC: Syntactical ease of making slices of multidim
array objects. RFC 231 suggests

   $a->[1..3; 4] * $b->[];

>From our point of view this is not too bad but we would like
to get rid of the requirement for C<-E<gt>>. An extended C<tie>
mechanism with possibility of overloaded operations on those
arrays might do it. But then again, if we get all those
polymorphic methods we could also allow the one suggested by


This RFC proposes syntactic support for a polymorphic method
that can be defined for blessed references. It would
allow the parentheses C<()> to be used for a variety of convenient
syntaxes for objects. This is principally motivated by a need
of PDL for a simple syntax of PDL object slicing.


=head2 Motivation

Currently, PDL objects have to use quite an unwieldy syntax
for the all important slicing and indexing. In perl5 the syntax is

  $n1 = $n-1;                       # since we need to stringify
  $y = $x->slice("0:$n1:4");
  $y = $x->slice("0:${\($n-1)}:4"); # even more horrible
This should be contrasted with the less cluttered syntax offered
by numerical Python and commercial systems such as Matlab and IDL:

  y = x[0:n-1:4]; 
In perl we desire to say:

 $y = $x(0:$n-1,4); # or, depending on the choice of separator
 $y = $x(0:$n-1;4); # see also RFC 169

Note that we need to keep l-value subs in perl6 to avoid related
types of syntactical clumsiness if C<$x()> can invoke a subroutine
(see below).

  $x(0:$n-1:4) *= 2;

should be allowed, as well as the long form

  $x->slice(0:$n-1:4) *= 2;

L<RFC 81> and related RFCs propose introducing ranges as part of the
syntax cleanup, this RFC proposes C<()> overloading for

=head2 Overloading C<()>'s

If classes allowed the definition of a method that is invoked with a
syntax akin to one used with sub refs (but without the need for the
C<&> dereferencing) we could have the cake and eat it.  The parentheses
method notion seems general enough to be useful for other classes.

=head2 Examples:

A possible scenario could be as follows. The class PDL
defines a default method that is invoked when a syntax


is used, where C<$E<lt>variable_nameE<gt>> is supposed to be an
instance of the class in question (here PDL).

The PDL package would contain the definition of the
method C<PARENTHESES> (a polymorphic method along the lines of RFC 159):

  package PDL;


	my $this = shift;

This would allow for the creation of a variety of powerful syntaxes
for different kinds of objects. For example in PDL we might
wish to use C<()> to slice by index value and in a derived class
PDL::World C<()> would slice by physical real-value coordinate.

One could think of this as saying that C<()> is an
operators which can be overloaded.

Note that we still would like to have such a feature even if
Perl 6 provided its own multi-dim array type. It would
give us the freedom to provide an object oriented interface
to these arrays and/or derive classes from it I<and> have
convenient slicing syntax.


Changes to the parser to allow the new syntax.
We are not aware of any other conflicts with existing
or proposed language features.

=head1 SEE ALSO

RFC 159: True Polymorphic Objects

RFC 117: Perl syntax support for ranges

RFC 81: Lazily evaluated list generation functions

RFC 169: Proposed syntax for matrix element access and slicing

RFC 231: Data: Multi-dimensional arrays/hashes and slices

L<PDL> (

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