# Copyright (C) 2007-2010, Parrot Foundation.
# $Id: pdd27_multiple_dispatch.pod 45652 2010-04-14 02:57:43Z coke $

=head1 PDD 27: Multiple Dispatch

=head2 Abstract

This PDD defines Parrot's multiple dispatch system.

=head2 Version

$Revision: 45652 $

=head2 Definitions

Multiple dispatch allows a single function name to be associated with several
alternate implementations. It selects the alternate to execute for a
particular call at runtime, based on the types of the arguments passed into
the call. The selection process compares the arguments of the call and the
types declared in the signatures of the alternates, as well as their inherited
or composed parents, to find the best match.

A similar selection of alternates based on types at compile time is generally
called overloading.

=head2 Description

=over 4

=item - Parrot supports multiple dispatch in opcodes and user-defined
routines.

=item - A single dispatch system supports MMD in both contexts.

=item - For user-defined subroutines and methods, the dispatch system is
pluggable, allowing users to swap in their own type-matching algorithms.

=item - For opcodes, the dispatch system is merely extensible, allowing users
to define alternates with their own types.

=item - Dispatch considers both low-level (register) types and high-level
(PMC) types, as well as inherited and composed types.

=item - Dispatch does not consider argument values. HLLs with dispatch systems
that do consider argument values will implement their own pluggable matching.

=back

=head2 Implementation

The aim of Parrot's multiple dispatch system is not to be an exact match to
any one HLL, but to provide the features and tools needed to support multiple
dispatch in a number of different HLLs.

Parrot has a single multiple dispatch system, used at the HLL level and
internally. {{NOTE: I appreciate the history that led us to have two largely
independent MMD systems, but it will cause problems down the road, if we don't
fix it now.}}

The heart of the system is the MultiSub PMC. All multiple dispatch routines
are MultiSub PMCs, subclasses of MultiSub, or polymorphic equivalents of
MultiSub. Calls to multiple dispatch routines use the Parrot calling
conventions.

=head3 MultiSub PMC API

A MultiSub stores a list of subroutines. When a MultiSub is invoked, it calls
an MMD sorting routine to select the best candidate for the current arguments,
and invokes that candidate.

A MultiSub can be used anywhere a Sub can be used. It can be stored in a
namespace or as a method or vtable override in a class.

An alternate multiple dispatch matching algorithm may be plugged in by
subclassing MultiSub and overriding C<invoke>.

=head4 Vtable Functions

MultiSub defines the following vtable functions in addition to the ones
inherited from ResizablePMCArray.

=over 4

=item push_pmc

Add a Sub or NCI sub to the list of candidates. Throw an exception if the
value passed in is anything other than a Sub or NCI sub.

=item set_pmc_keyed_int [deprecated]

Add a Sub or NCI sub to the list of candidates at a particular position in the
list. Throw an exception if the value passed in is anything other than a Sub
or NCI sub.

{{ NOTE: deprecated because MultiSub is no longer an array. }}

=item get_pmc

Return an array of matching candidates, sorted from best matching to worst
matching, for the current call arguments.

=item get_pmc_keyed

Return an array of matching candidates, sorted from best matching to worst
matching, for a passed in signature. The signature passed in is an array of
PMCs, and the types are extracted from the types of the array elements.

=item get_pmc_keyed_str

Return an array of matching candidates, sorted from best matching to worst
matching, for a passed in signature. The signature passed in is a simple type
string for the signature ('P' for PMC, 'I' for integer, 'N' for number, and
'S' for string).

=item get_iter

Return an iterator object of matching candidates, sorted from best matching to
worst matching, for the current call arguments.

=item invoke

Return a function pointer to the best matching candidate for the current call
arguments, and set up the interpreter preparing for invocation. This vtable
function calls C<Parrot_mmd_sort_manhattan> from the public MMD API, but
may be changed to call C<Parrot_mmd_invoke>.

=item set_integer_keyed_int, set_number_keyed_int, set_string_keyed_int
[deprecated]

Throw an exception, as an integer, float, or string value is not a Sub or NCI
sub. (Masks the vtable functions inherited from ResizablePMCArray.)

{{NOTE: no longer needed, since not inheriting from ResizablePMCArray.}}

=back

=head4 Methods

=over 4

=item get_iter

Return an array of matching candidates, sorted from best matching to worst
matching, for a passed in signature. If the signature passed in is an array of
PMCs, the types are extracted from the types of the array elements. If the
signature passed in is a simple type string for the signature ('P' for PMC,
'I' for integer, 'N' for number, and 'S' for string), the types are matched as
simple types.

=back

=head4 PIR subroutine attributes

The following attributes are used when declaring a MultiSub in PIR.

=over 4

=item :multi

=begin PIR

  .sub mymultisub :multi(Integer, Integer)
    # ...
  .end

=end PIR

The C<:multi> attribute marks a subroutine or method as participating in
multiple dispatch.

=item :invocant

=begin PIR_INVALID

  .sub mymultisub
  .param pmc first :invocant
  .end

=end PIR_INVALID

Not all elements of a multi-sub's signature are relevant for the purposes of
multiple dispatch. The subroutine parameters that participate in dispatch are
marked with the attribute C<:invocant>, and must be contiguous.
Subs that are not marked with C<:multi> may not mark parameters with
C<:invocant>.

{{NOTE: I'm open to other names for the attribute. An English form of the
Latin I<invocare>, "to call upon", is pretty appropriate to the act of using
an argument as a selector for multiple dispatch.}}

=back

=head3 C Multiple Dispatch API

These functions are the public interface to common multiple dispatch
operations, and may be called from opcodes, vtable functions, or other C
functions.

=over 4

=item Parrot_mmd_sort_manhattan

Performs a multiple dispatch lookup on an array of subroutines. The call
signature to match is pulled from the current interpreter, following the
Parrot calling conventions. Returns a sorted array of candidates that match
the call signature, with the best matching candidate as the 0th element.

=item Parrot_mmd_invoke

Make a multiple dispatch call. Similar in syntax to C<Parrot_PCCINVOKE>, but
doesn't separate out the invocant before the signature since the call can have
multiple invocants. A significant part of the code from C<Parrot_PCCINVOKE>
can be factored out to helper routines used by both C<Parrot_PCCINVOKE> and
C<Parrot_mmd_invoke>.

  void
  Parrot_mmd_invoke(PARROT_INTERP, NOTNULL(STRING *sub_name),
        ARGIN(const char *signature), ...)>

=item mmd_dispatch* [deprecated]

Dispatch a routine with a fixed low-level (register) type signature, with
multiple dispatch on the high-level (PMC) types. The low-level call and return
types are specified in the name of the function: 'i' for integer, 'n' for
float, 's' for string, 'p' for PMC, and 'v' for void. So, a dispatch of one
PMC and one integer argument that returns void is a call to
C<mmd_dispatch_v_pi()>.

=back

=head3 Opcode Multiple Dispatch

Parrot's basic opcodes are not multiple dispatch. They appear to be so, since
a single opcode name may have multiple signatures. But, behind the scenes,
this is essentially compile-time overloading. (Each different signature is
actually a different opcode number, hidden behind the abstraction of a single
name.)

Multiple dispatch is only used by a limited set of opcodes. These opcodes
directly correspond to standard vtable functions. Multiple dispatch opcodes
may take PMC, integer, float, or string arguments and return a PMC, integer,
float, or string result.

Multiple dispatch opcodes are standard opcodes that internally call the
C<Parrot_mmd_invoke> routine from the public MMD API.

{{NOTE: It is no longer necessary to name multisubs used for opcode dispatch
using the "__" names.}}

=head3 Vtable Multiple Dispatch

Some PMCs call multiple dispatch routines from their vtable functions.
ResizablePMCArray, for example, calls the multiple dispatch equality operation
on each element of two arrays from the 'is_equal' vtable function.

Multiple dispatch calls from within vtable functions call the
C<Parrot_mmd_invoke> routine from the public MMD API.

=head2 References

F<docs/mmd.pod>, F<src/multidispatch.c>, F<src/pmc/multisub.pmc>

=cut

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