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RFC 98 (v1) context-based method overloading

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From:
Perl6 RFC Librarian
Date:
August 14, 2000 11:50
Subject:
RFC 98 (v1) context-based method overloading
Message ID:
20000814162005.31865.qmail@tmtowtdi.perl.org
This and other RFCs are available on the web at
  http://dev.perl.org/rfc/

=head1 TITLE

context-based method overloading

=head1 VERSION

  Maintainer: David Nicol <perl6rfc@davidnicol.com>
  Date: 14 Aug 2000
  Version: 1
  Mailing List: perl6-language-subs@perl.org
  Number: 98

=head1 ABSTRACT

The other way C++ allows you to overload a named function is
by return type.  This document is a companion piece to
a similarly named one about protoypes.  It replaces old Perl's
"wantscalar" and "wantarray" kluges, which can now be deprecated,
with a cleaner interface allowing decisions which are possible to
make at compile time to be made then.

=head1 DESCRIPTION

Defining multiple subroutines with both the same name and the same calling
interface that explicitly return items of differing types is now
allowed. subject to the following:

=over

=item compile-time

the return-type expectation of all method calls meaning to take advantage
of this feature must be clear from explicit type tags available at compile time.

=item what constitutes a context?

We are familiar with "scalar context" and "array context" from perl5 and
previous.  Perl6 gives us user-defined contexts as well as user defined
types or all sorts.  Any unique string can be a context, for the purposes
of context-based method overloading, and Perl will attempt to guess a
reasonable base class for the given "context."  Use of undeclared context
specifiers will of course generate warnings.

=item interaction with prototype-based and traditional overloading

A traditional perlsub may be considered the simultaneous
definition of two routines,
C<$ sub(PROTO){body}> and
C<@ sub(PROTO){body}>.

It is now possible to differentiate between these two explicitly.

=item guaranteed sane values

The return context is considered before considering the types of the
arguments, and in the case of no exact match, the fallback is to a
default method defined to work in the given context rather than to
the default method for the name, should one exist.

=item specification incomplete

Further points to discuss and agree on include the relation of this
mechanism to inheritance, and multiple inheritance, and the meta-alteration
of all aspects of the mechanism.

=back

=head1 IMPLEMENTATION

At compile time, the keys in the big hash (be it global or per-package
or per-class) that holds the mapping from the names of the classes
to their coderefs are further extended to include, 
as well as the prototype,
the declared type of the return values, when specified,
as part of the
name of each method. 

=head1 REFERENCES

RFC 21: Replace C<wantarray> with a generic C<want> function

RFC 57: Subroutine prototypes and parameters

RFC 61: Interfaces for linking C objects into perlsubs

RFC 75: first class interface definitions


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