# Copyright (C) 2009-2010, Parrot Foundation.
# $Id: pdd31_hll.pod 46867 2010-05-21 19:13:51Z pmichaud $

=head1 [DRAFT] PDD 31: HLL Compilers and Libraries

=head2 Abstract

This PDD describes the standard compiler API and support for
cross-library communication between high-level languages (HLLs).

=head2 Version

$Revision: 46867 $

=head2 Description

Parrot's support for HLL interoperability is primarily focused on
enabling programs written in one language to be able to use libraries
and code written in a different language.  At the same time, language
implementors should not be overly restricted by a global specification.

This PDD describes an API for HLL compiler objects to use to
promote library sharing among languages.  It's intended to make it
easy for a program to request loading of a local or foreign module,
determine the capabilities provided by the module, and potentially
import and integrate them into its own namespaces.  In general,
the API treats library-level interoperability as a negotiation
among HLL compiler objects, with each HLL compiler maintaining
primary control over the operations performed in its HLL space.

In particular, this HLL API does not attempt to prescribe how
languages should organize their internal capabilities, PMCs,
namespaces, methods, data structures, and the like.

=head2 Implementation

=head3 Compiler API

This section describes the abstract API for HLL compiler objects.

=head4 Locating a compiler object

Generally HLL compilers are loaded via the C<load_language>
opcode, and register themselves using the C<compreg> opcode.
By convention, each HLL compiler should at minimum register
itself using the name of its HLL namespace (see PDD 26),
although a compiler can choose to register itself under other
names as well.

=head4 Methods

=over 4

=item C<compile>

    $P0 = compiler.'compile'(source [, options :named :slurpy])

Return the result of compiling C<source> according to C<options>.
Common options include:

=over 4

=item target

Stop the compilation process when the stage given by target
has been reached.  Common values for target include "parse",
"past", "pir", and "pbc".

=item outer_ctx

Use the supplied context as the outer (lexical) context for
the compilation.  Some languages require this option to be
able to look up lexical symbols in outer scopes when performing
a dynamic compilation at runtime.

=back

=item C<eval>

    $P0 = compiler.'eval'(source [, args :slurpy] [, options :named :slurpy])

Compile and evaluate (execute) the code given by C<source>
with C<args> and according to C<options>.  The available options 
are generally the same as for the C<compile> method above; in particular,
the C<outer_ctx> option can be used to specify the outer lexical
context for the evaluated source.

=item C<parse_name>

    $P0 = compiler.'parse_name'(name)

Parse the string C<name> using the rules specific to C<compiler>,
and return an array of individual name elements.

For example, a Java compiler would turn 'C<a.b.c>' to C<['a','b','c']>,
while a Perl compiler would turn 'C<a::b::c>' into the same result.
Perl's sigil rules would likely turn 'C<$a::b::c>' into C<['a','b','$c']>.

=item C<load_module>

    module = compiler.'load_module'(name)

Locate and load the module given by C<name> using the rules for
libraries specific to C<compiler>, and return a C<module> handle for
the module just loaded.  The C<name> argument is typically an array
or a string to be processed as in C<parse_name> above.  In general
the module handle returned should be considered opaque by the
caller, but specific HLL compilers are allowed to specify the
nature of the handle returned (e.g., a namespace for the loaded
module, or a specific "handle" object).

=item C<get_module>

    module = compiler.'get_module'(name)

Similar to C<load_module> above, this method returns a handle
to an already-loaded module given by C<name>.

=item C<get_exports>

    $P0 = compiler.'get_exports'(module [,name,name,...] [, 'tagset'=>tagset])

Requests the exported objects given by C<name> and/or C<tagset> for
C<module> within the given C<compiler>.  The C<module> argument
should be a module handle as obtained by C<load_module> or
C<get_module> above.

A C<tagset> argument provides an identifier that a compiler and/or
module can use to supply their own lists of items to be exported.
By convention, a C<tagset> of "DEFAULT" refers to the default set
of exported items for the module, while "ALL" returns all available
exports.  Compilers and modules are free to define their own custom
tagsets beyond these.

Any C<name> arguments supplied generally limit the export list to
the tagset items corresponding to the supplied names (as determined by
the compiler invocant).  If names are provided without an explicit
tagset, then "ALL" is assumed.  If neither names nor a tagset are
provided, then symbols from "DEFAULT" are returned.

The returned export list is a hash of hashes; each entry in the
top level hash has a key identifying the type of exported
object (one of C<'namespace'>, C<'sub'>, or C<'var'>) and a
value hash containing the corresponding exported symbol names
and objects.  This hash-of-hashes approach is intended to generally
correspond to the "Typed Interface" section of PDD 21 ("Namespaces"),
and allows the module's source HLL to indicate the type of exported
object to the caller.  The hash-of-hash approach also accommodates
languages where a single name might be used to refer to several
objects that differ in type.  (This PDD explicitly rejects the
notion that a HLL should be directly exporting or injecting symbols
into a foreign HLL's namespaces.)

=back

=head3 HLL::Compiler class

HLL::Compiler is a common base class for compiler objects
based on the Parrot Compiler Toolkit (PCT) and NQP (Not Quite Perl)
libraries.  It provides a default implementation of the abstract
Compiler API above, plus some additional methods for simple symbol
table export and import.  The default methods are intended to support
importing and exporting symbols using standard Parrot namespace
objects (PDD 21).  However, it's normal (and expected) that
languages will subclass HLL::Compiler to provide language-specific
semantics where needed.

=head4 Methods

=over 4

=item C<language>

    $S0 = compiler.'language'([name])

If C<name> is provided, sets the language name of the invocant
and registers the invocant as the compiler for C<name> via
the C<compreg> opcode.

Returns the language name of the compiler.

=item C<parse_name>

    $P0 = compiler.'parse_name'(name)

Splits a name based on double-colons, such that "C<A::B::C>"
becomes C<['A','B','C']>.

=item C<get_module>

    module = compiler.'get_module'(name)

Returns a handle to the HLL namespace associated with C<name>
(which is processed via the invocant's C<parse_name> method
if needed).

=item C<load_module>

    module = compiler.'load_module'(name)

Loads a module C<name> via the C<load_bytecode> opcode
using both ".pbc" and ".pir" extensions.  Parrot's standard
library paths for C<load_bytecode> are searched.

Returns the HLL namespace associated with C<name> (which may
be PMCNULL if loading failed or if the requested module did
not create an associated namespace).

=item C<get_exports>

    $P0 = compiler.'get_exports'(module [,name,name,...] [, 'tagset'=>tagset])

Implements a simple exporting interface that meets the "Compiler API"
above.  The C<module> argument is expected to be something that
supports a hash interface, such as NameSpace or LexPad.  (Note
that this is what gets returned by the default C<get_module>
and C<load_module> methods above.)  The C<module["EXPORT"]> entry
should return another hash-like object keyed by tagset names; each
of those tagset names then identify the exportable symbols
associated with that tagset.

With this default arrangement, it's entirely possible for a
module to indicate its tagsets by using symbol entries in
namespaces.  For example, a module with namespace C<['XYZ']>
can define its default exports by binding symbols in the
C<['XYZ';'EXPORT';'DEFAULT']> namespace.  (Modules aren't
required to use exactly this mechanism; it's just one possibility
of many.)

If the "ALL" tagset is requested and there is no "ALL" entry
in the C<module['EXPORT']> hash, then C<module> itself is used
as the source of exportable symbols for this method.  This
enables C<get_exports> to be used to obtain symbols from
modules that do not follow the "EXPORT" convention above
(e.g., core Parrot modules).

As described in the Compiler API section above, the return value
from C<get_exports> is a hash-of-hashes with exported namespaces
in the C<namespace> hash, exported subroutines in the C<sub> hash,
and all other exports in the C<var> hash.

=item C<import>

    compiler.'import'(target, export_hash)

Import the entries from C<export_hash> (typically obtained
via C<get_exports> above) into C<target> according to the rules
for C<compiler>.  Any entries in C<export_hash['namespace']>
are imported first, followed by entries in C<export_hash['sub']>,
followed by entries in C<export_hash['var']>.

Note that this method is not part of the abstract Compiler API --
a HLL compiler is able to implement importing in any way it
deems appropriate.  The C<HLL::Compiler> class provides this
method as a useful default for many HLL compilers.

For each exported item of C<export_hash>, import takes place by
checking the invocant for an C<import_[type]> method and using
that if it exists (where C<[type]> is one of "namespace", "sub",
or "var").  These methods are used to implemented "typed imports",
and allows the compiler object to perform any name mangling or
other operations needed to properly import an object.

If the compiler invocant doesn't define an C<import_[type]> method,
C<import> attempts to use any C<add_[type]> method that exists on
C<target> (e.g., for the case where C<target> is a namespace PMC
supporting the typed interface defined by PDD 21).

If neither of these methods are available, then C<import>
simply binds the symbol using C<target>'s hash interface.

=back

=head3 Examples

=head4 Importing a module Acme::Boom from language xyz into language abc

=begin PIR_FRAGMENT

    # Load the HLL library and get its compiler
    .local pmc xyzcompiler, module, exports
    load_language 'xyz'
    xyzcompiler = compreg 'xyz'

    # load xyz's module "Acme::Boom"
    module = xyzcompiler.'load_module'("Acme::Boom")

    # get the default exports for the module
    # (note that 'tagset'=>'DEFAULT' is optional here
    exports = xyzcompiler.'get_exports'(module, 'tagset'=>'DEFAULT')

    # import into current namespace
    .local pmc abccompiler
    abccompiler = compreg 'abc'
    $P0 = get_namespace
    abccompiler.'import'($P0, exports)

=end PIR_FRAGMENT

=head2 References

F<pdd21_namespaces.pod>

=cut

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