=pod

=head1 HLLs and Interoperation

Z<CHP-10>

=head2 Parrot HLL Environment

In the earliest days Parrot was designed to be the single-purpose backend
for the Perl 6 language. It quickly blossomed beyond that, and now has a
much grander purpose: to host all dynamic languages, and to host them
together on a single platform. If we look back through the history of
dynamic programming languages, they've had a more difficult time
interoperating with each other then compiled languages have because
compiled languages operate at the same machine-code level and typically
can make use of the same application binary interface (ABI). With the
right compiler settings, programs written in Visual Basic can interoperate
with programs written in C N<On some systems anyway>, which can call
functions written in C++, in Ada, Fortran, Pascal and so on. To try to mix
two common dynamic languages, like Perl and Python, or Ruby and PHP, you
would need to write some kind of custom "glue" function to try to include
an interpreter object from one language as a library for another language,
and then write code to try and get the parser for one to interact nicely
with the parser for the other. It's a nightmare, frankly, and you don't
see it happen too often.

In Parrot, the situation is different because high level languages (HLL)
are almost all written with the PCT tools, and are compiled to the same
PIR and PBC code. Once compiled into PBC, a library written in any HLL
language can be loaded and called by any other HLL N<Well, any HLL which
supports loading libraries>. A language can have a syntax to include
code snippets from other languages inline in the same file. We can write
a binding for a popular library such as opengl or xlib once, and include
that library into any language that needs it. Compare this to the current
situation where a library like Gtk2 needs to have bindings for every
language that wants to use it. In short, Parrot should make interoperation
easier for everybody.

This chapter is going to talk about HLLs, the way they operate, and the
way they interoperate on Parrot.

=head2 HLLs on Parrot

=head2 Working with HLLs

=head3 Fakecutables

It's possible to turn compilers created with PCT into stand-alone
executables that run without the Parrot executable. To do this, the
compiler bytecode is linked together with a small driver program in
C and the Parrot library, C<libparrot> X<libparrot>. These programs
have been given a special name by the Parrot development community:
I<fakecutables> X<fakecutables>. They're called fake because the PBC
is not converted to native machine code like in a normal binary
executable file, but instead is left in original PBC format.

=head3 Compiler Objects

The C<compreg> opcode has two forms that are used with HLL compilers. The
first form stores an object as a compiler object to be retrieved later, and
the second form retrieves a stored compiler object for a given language.
The exact type of compiler object stored with C<compreg> can vary for each
different language implementation, although most of the languages using PCT
will have a common form. If a compiler object is in register C<$P0>, it can
be stored using the following C<compreg> syntax:

  compreg 'MyCompiler', $P0

There are two built-in compiler objects: One for PIR and one for PASM. These
two don't need to be stored first, they can simply be retrieved and used.
The PIR and PASM compiler objects are Sub PMCs that take a single string
argument and return an array PMC containing a list of all the compiled
subroutines from the string. Other compiler objects might be different
entirely, and may need to be used in different ways. A common convention is
for a compiler to be an object with a C<compile> method. This is done with
PCT-based compilers and for languages who use a stateful compiler.

Compiler objects allow programs in Parrot to compile arbitrary code strings
at runtime and execute them. This ability, to dynamically compile
code that is represented in a string variable at runtime, is of fundamental
importance to many modern dynamic languages.  Here's an example using
the PIR compiler:

  $P0 = compreg 'PIR'      # Get the compiler object
  $P1 = $P0(code)          # Compile the string variable "code"

The returned value from invoking the compiler object is an array of PMCs
that contains the various executable subroutines from the compiled source.
Here's a more verbose example of this:

  $P0 = compreg 'PIR'
  $S0 = << "END_OF_CODE"

    .sub 'hello'
       say 'hello world!'
    .end
  
    .sub 'goodbye'
       say 'goodbye world!'
    .end

  END_OF_CODE

  $P1 = $P0($S0)
  $P2 = $P1[0]      # The sub "hello"
  $P3 = $P1[0]      # The sub "goodbye"

  $P2()             # "hello world!"
  $P3()             # "goodbye world!"

Here's an example of a Read-Eval-Print-Loop (REPL) in PIR:

  $P0 = getstdin
  $P1 = compreg 'PIR'
  
  loop_top:
    $S0 = readline $P0
    $S0 = ".sub '' :anon\n" . $S0
    $S0 = $S0 . "\n.end\n"
    $P2 = $P1($S0)
    $P2()
  
    goto loop_top

The exact list of HLL packages installed on your system may vary. Some
language compiler packages will exist as part of the Parrot source code
repository, but many will be developed and maintained separately. In any
case, these compilers will typically need to be loaded into your program
first, before a compiler object for them can be retrieved and used.

=head2 HLL Namespaces

Let's take a closer look at namespaces then we have in previous chapters.
Namespaces, as we mentioned before can be nested to an arbitrary depth
starting with the root namespace. In practice, the root namespace is
not used often, and is typically left for use by the Parrot internals.
Directly beneth the root namespace are the X<HLL Namespaces> HLL
Namespaces, named after the HLLs that the application software is written
in. HLL namespaces are all lower-case, such as "perl6", or "cardinal",
or "pynie". By sticking to this convention, multiple HLL compilers can
operate on Parrot simultaneously while staying completely oblivious to
each other.

=head2 HLL Mapping

=head2 Interoperability Guidelines

=head3 Libraries and APIs

As a thought experiment, imagine a library written in Common Lisp that
uses Common Lisp data types. We like this library, so we want to include
it in our Ruby project and call the functions from Ruby. Immediately
we might think about writing a wrapper to convert parameters from Ruby
types into Common Lisp types, and then to convert the Common Lisp return
values back into Ruby types. This seems sane, and it would probably even
work well. Now, expand this to all the languages on Parrot. We would need
wrappers or converters to allow every pair of languages to communicate,
which requires C<N^2> libraries to make it work! As the number of languages
hosted on the platform increases, this clearly becomes an untennable
solution.

So, what do we do? How do we make very different languages like Common
Lisp, Ruby, Scheme, PHP, Perl and Python to interoperate with each other
at the data level? There are two ways:

=over 4

=item* VTable methods

VTable methods are the standard interface for PMC data types, and all PMCs
have them. If the PMCs were written properly to satisfy this interface
all the necessary information from those PMCs. Operate on the PMCs at the
VTable level, and we can safely ignore the implementation details of them.

=item* Class Methods

If a library returns data in a particular format, the library reuser should
know, understand, and make use of that format. Classes written in other
languages will have a whole set of documented methods to be interfaced with
and the reuser of those classes should use those methods. This only works,
of course, in HLLs that allow object orientation and classes and methods,
so for languages that don't have this the vtable interface should be used
instead.

=back

=head3 Mixing and Matching Datatypes

=head2 Linking and Embedding

Not strictly a topic about HLLs and their interoperation, but it's important
for us to also mention another interesting aspect of Parrot: Linking and
embedding. We've touched on one related topic above, that of creating
the compiler fakecutables. The fakecutables contain a link to C<libparrot>,
which contains all the necessary guts of Parrot. When the fakecutable is
executed, a small driver program loads the PBC data into libparrot through
its API functions. The Parrot executable is just one small example of how
Parrot's functionality can be implemented, and we will talk about a few other
ways here too.

=head3 Embedding Parrot

C<libparrot> is a library that can be statically or dynamically linked
to any other executable program that wants to use it. This linking process
is known as I<embedding parrot>, and is a great way to interoperate

=head3 Creating and Interoperating Interpreters

Parrot's executable, which is the interface which most users are going
to be familiar with, uses a single interpreter structure to perform a
single execution task. However, this isn't the only supported structural
model that Parrot supports. In fact, the interpreter structure is not a
singleton, and multiple interpreters can be created by a single program.
This allows separate tasks to be run in separate environments, which can
be very helpful if we are writing programs and libraries in multiple
languages. Interpreters can communicate and share data between each other,
and can run independently from others in the same process.

=head3 Small, Toy, and Domain-Specific Languages

How many programs are out there with some sort of scripting capability?
You can probably name a few off the top of your head with at least some
amount of scripting or text-based commands. In developing programs like
this, typically it's necessary to write a custom parser for the input
commands, and a custom execution engine to make the instructions do what
they are intended to do. Instead of doing all this, why not embed an
instance of Parrot in the program, and let Parrot handle the parsing
and executing details?

Small scripting components which are not useful in a general sense like
most programming languages, and are typically limited to use in very
specific environments (such as within a single program) are called
I<Domain-Specific Languages> (DSL). DSLs are a very popular topic because
a DSL allows developers to create a custom language that makes dealing
with a given problem space or data set very easy. Parrot and its suite
of compiler tools in turn make creating the DSLs very easy. It's all
about ease of use.

=head3 Parrot API

=cut

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