====== Typhon

Typhon is a virtual machine for Monte. It loads and executes Kernel-Monte from Monte AST files.

How To Monte

You will need Nix, either by dint of being on NixOS or by installing single-user Nix.

Once you have Nix, you can build a basic toolchain by building the monte target::

$ nix-build -A monte

Note: Translation is not cheap. It will require approximately 0.5GiB memory and 5min CPU time on a 64-bit x86 system to translate a non-JIT Typhon executable, or 1GiB memory and 15min CPU time with the JIT enabled. It will take another few minutes to build all of the Monte support libraries.

For only a bit more extra CPU time, one can build a toolchain which includes Capn Proto support::

$ nix-build -A fullMonte

The pure-Monte portions of the build are incremental, so after building monte, the build of fullMonte should be relatively quick.

For development, it can be useful to build only the VM::

$ nix-build -A typhonVm

And to run the VM untranslated, use a Nix shell::

$ nix-shell default.nix -A typhonVm
[nix-shell:~/typhon]$ pypy main.py ...

Contributing

Contributions are welcome. Please ensure that you're okay with the license!

Diffing Typhon Binaries

By default, git won't show diffs of binary files. I don't especially blame it. However, with a bit of a filter, we can give git what it needs::

$ nix-env -iA monte -f default.nix
$ git config --replace-all diff.typhon.textconv 'monte dump-mast'

This configuration option, along with the .gitattributes in the repository, will let git display textual diffs of the binary ASTs.

RPython Quirks

Here's what you need to know about RPython and things imported from rpython.rlib.

Immutability

Like Monte, RPython values immutable structures. Whenever a class is
immutable, adding the ``_immutable_ = True`` annotation will cause RPython to
enforce an immutability variant: The fields of an instance of that class can
only be assigned to once.

Don't use ``_immutable_`` unless the class is *totally* immutable. It's
possible to make only some fields immutable; just list all the immutable
fields in a tuple with ``_immutable_fields_ = "this", "that"``. To make a
field an immutable list with immutable elements, use a ``[*]``, as in
``_immutable_fields_ = "this", "that", "those[*]"``.

The JIT (``rpython.rlib.jit``)

r.r.jit is mostly about hints to inform the JIT about the behavior of code paths. Some hints are safe and some are not safe.

The JIT colors all values as "red" or "green"; a red value is non-constant and a green value is constant. promote accepts a red value and turns it into a green value. The JIT will reflect this with a guard on the value of the given object. When a value is expected to have a relatively small number of possibilities, a promote can be very effective at improving the performance of the code. promote is safe; it will never cause the JIT to generate wrong code, although it can cause the JIT to perform too much compilation.

jit_debug() can print one string and up to four integers to the JIT log. The computation which prepares the debug message is part of the JIT trace, so it is ideal to have the inputs be green values.

elidable functions must be referentially transparent. In return, the JIT accepts the promise of referential transparency and will try to reorder or remove the call to the elidable function when it can. The function need not actually be pure; it is sufficient for it to appear pure in all cases. If the function is not pure, then elidable is unsafe, since the JIT will not second-guess a promise of elidability.

.. note:: Do not mark elidable if you want the JIT to inline the function. The JIT will not enter an elidable function.

elidable_promote changes a function so that it is elidable and all of its arguments will be promote'd before entering the function body. It is unsafe, like elidable.

dont_look_inside forces the JIT to not inline calls to a function. It is sometimes necessary to protect events like GIL handling or I/O. It can also be a big improvement for calls to functions which don't inline well due to recursive or other strange behavior. It should be safe.

unroll_safe forces the JIT to consider inlining calls to functions which were not inlinable due to containing loops. This is important because the JIT will otherwise refuse to look inside those functions. Usage of unroll_safe is an informal promise to the JIT that the loops in the function are tightly bounded in the number of iterations which will be performed. While not unsafe, unroll_safe can cause exponential amounts of overcompilation and overtracing, so it should be used sparingly.

How are these used within the codebase? Values that are expected to be green but aren't green-inferred by the JIT are promote'd. Functions that do I/O have dont_look_inside. Functions which are pure and called often are elidable. Lots of factoring has been done to make small chunks of code elidable.

If a function has a loop that is conditionally called, it is useful to factor the loop to a separate function and then consider whether to mark the new function with unroll_safe. Even if the function isn't actually safe to unroll, merely the factorization of code is sufficient to allow the JIT to look into the original function. This happens with every object which is defined in RPython; the dispatch function, callAtom() or similar, is factored to not have loops within it. Since atoms are (usually) green values during execution, this means that callAtom() gets specialized for that atom, and the actual work can usually be inlined.

Unicode (rpython.rlib.unicodedata)

We use RPython's Unicode database. The magic incantation::

    from rpython.rlib.unicodedata import unicodedb_6_2_0 as unicodedb

``unicodedb`` will have plenty of useful functions, like ``islower()`` and
``isalpha()``. These functions are *not* available as methods on ``unicode``
objects.

.. _reference Monte: https://github.com/monte-language/monte

Documentation
-------------

If you create a new object by subclassing ``Object`` or calling ``@runnable``,
please give it a docstring. The docstrings will be reflected into Monte, so
please follow these guidelines:

* The first line should describe the object.
* Subsequent lines should describe specifics of the object's nature which
  might be helpful to somebody calling ``help()`` on the object.
* Docstrings should refer to their object as "this object".
* In-jokes are sometimes allowed. Ask on IRC.
* Dry language is always allowed.
* Unicode is encouraged; do not be afraid to use symbols which are generally
  available in Unicode fonts. Ask on IRC if unsure.

An example:

    ▲> help(Any)
    Result: Object type: AnyGuard
    A guard which admits the universal set.
    This object specializes to a guard which admits the union of its
    subguards: Any[X, Y, Z] =~ X ∪ Y ∪ Z

Also document objects and methods written in Monte. For methods:

* "this method" is the correct self-reference.
* To refer to names defined in the method specification, surround the name in
  backticks.
* To refer to methods, use atom syntax and backticks: A method with name
  "meth" and arity 2 would appear as \`meth/2\`.

Autohelp would like to remind you that subclasses of ``Object`` should
decorate themselves with ``@autohelp`` in order to maintain compliance and
safety.

To override pretty-printing for an object, add a ``toString()`` method which
should return a Unicode string.