=====================
Overview over RPython
=====================

RPython
=======

 - implementation language of the PyPy Python interpreter
 - restrictions necessary to allow efficient translation
 - quite close to Python: e.g. fully garbage collected
 - fully testable on top of CPython
 - feels a bit like Java without the type annotations
 - "being RPython" is a whole-program property



Guide to writing RPython programs
=================================

 - *Important*: First make it work on top of CPython, then worry about RPython


Type restrictions
=================

You cannot arbitrarily mix types. The following is not allowed::

    # NOT RPYTHON
    if condition:
        x = 42
    else:
        x = "fortytwo"

This, however is fine::
    
    x = 42
    x = "fortytwo"

(not that it's useful)


Control Flow
============

 - all control flow statements allowed
 - no generators (yield) and generator expressions
 - global variables are constant
 - closures are not supported

Classes
=======

 - single inheritance::

    class A(object):
        def __init__(self, a, b):
            self.a = a
            self.b = b
        def method(self):
            return self.a + self.b

    class B(A):
        def __init__(self, a, b, c):
            A.__init__(self, a, b)
            self.c = c
        def method(self):
            return A.method(self) + self.c



Data Types
==========

 - immutable:

   - ``int``, ``float``, ``bool``
   - tuples: (1, 2, 3)
   - ``string``

 - mutable:
  
   - ``list``
   - ``dict``


Numerical Types
===============

 - Python ``int`` are C ``long``
 - have kind of the obvious C/Java-like behaviour
 - difference between Python and RPython: RPython ``int`` wrap around silently


Strings
=======

Fully supported in RPython::

      >>> a = "hello"
      >>> b = " world"
      >>> print a + b * 2
      hello world world

Most string methods work fine


Lists
=====

Restriction: Lists must be homogeneous::

    >>> l = []
    >>> l.append(1)
    >>> l.extend([5, 7, 9])
    >>> l[0] = "this is wrong" # NOT RPYTHON

Otherwise, most list methods supported (apart from sorting)

Dicts
=====

Restriction: Dictionary keys and values must each be homogeneous::

    >>> d = {"hello": 1, "world": 2}
    >>> d["moon"] = 3
    >>> del d[5] # NOT RPYTHON

The backends typically create specialized versions suited for the keys and
values.


Init-time metaprogramming
=========================

 - analysis starts from live objects
 - before analysis starts, you can do all sorts of non-RPython thing
 - can use init-time metaprogramming::

    def adder(x): # NOT RPYTHON
        def add(y):
            return x + y
        return add

    add4 = adder(4)
    # now add4 is RPython


Things you also can do (but usually don't)
==========================================

 - manipulate raw memory addresses (to write GCs)
 - explicit specialization of functions
 - explicit loop unrolling