.. _module:

######
Module
######


What is a Theano Module
=======================

Theano 'Module' is a structure which implements what could be called a
"theano class". A ``Module`` can contain ``Members``, which act like
instance variables ("state"). It can also contain an arbitrary number
of ``Methods``, which are functions that share the same ``Members`` in
addition to their own inputs. Last but not least, ``Modules`` can be
nested (explanations and examples follow). ``Module`` is meant to:

 #. ease the sharing of variables between several Theano functions,
 #. streamline automatic naming, and
 #. allow a hierarchy of "modules" whose states can interact.


import
======

All examples suppose that you have done those import:

.. code-block:: python

    #!/usr/bin/env python
    import theano
    import numpy as N
    from theano import tensor as T
    from theano.tensor import nnet as NN
    from theano.compile import module as M

Module
======

A ``Module`` can contain ``Members``, ``Methods`` and inner ``Modules``. Each type has a special meaning.

.. code-block:: python

    module = M.Module()

``Member``
------------

Usage:

.. code-block:: python

    #module.state = variable
    module.state = T.scalar()

A ``Member`` represents a state variable (i.e., whose value remains after a ``Method`` is called). It will be named automatically after that field and it will be an implicit input of all ``Methods`` of the ``Module``. Its storage (i.e. where the value is stored) will be shared by all ``Methods`` of the ``Module``.

A ``Variable`` which is the variable of a previous computation (by opposition to being ``updated``) is not a ``Member``. Internally this is called an External. You should not need to care about this.

For sharing state between modules, see ``Inner Module`` section.

``Method``
------------

Usage:

.. code-block:: python

    module.method = M.Method(inputs, outputs, **updates)

Each key in the updates dictionary must be the name of an existing ``Member`` of the ``Module`` and the value associated to that key is the update expression for the state. When called on a ``ModuleInstance`` produced by the ``Module``, the method will calculate the outputs from the inputs and will update all the states as specified by the update expressions. See the basic example below.

Inner Module
------------

To share a ``Member`` between modules, the modules must be linked through the inner module mechanism. 

Usage:

.. code-block:: python

    module2.submodule = module

``ModuleInstance``
====================

A ``Module`` can produce a ``ModuleInstance`` with its ``make`` method. Think of this as a class and an object in C++/Java. If an attribute was a ``Member``, it will become a read/write access to actual data for the state. If it was a ``M.Method``, a function will be compiled with the proper signature and semantics.


Module Interface
================

.. code-block:: python

    def make(self, mode = {'FAST_COMPILE', 'FAST_RUN', ... }, **init)

'''make''' compiles all ``Methods`` and allocates storage for all ``Members`` into a ``ModuleInstance`` object, which is returned. The ``init`` dictionary can be used to provide initial values for the members.

.. code-block:: python

    def resolve(self, symbol, filter = None)

Resolves a symbol in this module. The symbol can be a string or a ``Variable``. If the string contains dots (eg ``"x.y"``), the module will resolve the symbol hierarchically in its inner modules. The filter argument is None or a class and it can be used to restrict the search to ``Member`` or ``Method`` instances for example.


.. code-block:: python

    def _instance_initialize(self, inst, **init)

The inst argument is a ``ModuleInstance``. For each key, value pair in init: ``setattr(inst, key, value)`` is called. This can be easily overriden by ``Module`` subclasses to initialize an instance in different ways. If you don't know what to put their, don't put it and it will execute a default version. If you want to call the parent version call: ``M.default_initialize(inst,**init)``


Basic example
=============

The problem here is to create two functions, ``inc`` and ``dec`` and a shared state ``c`` such that ``inc(n)`` increases ``c`` by ``n`` and ``dec(n)`` decreases ``c`` by ``n``. We also want a third function, ``plus10``, which return 10 + the current state without changing the current state. Using the function interface, the feature can be implemented as follows:

.. code-block:: python

    n, c = T.scalars('nc')
    inc = theano.function([n, ((c, c + n), 0)], [])
    dec = theano.function([n, ((c, c - n), inc.container[c])], []) # we need to pass inc's container in order to share
    plus10 = theano.function([(c, inc.container[c])], c + 10)
    assert inc[c] == 0
    inc(2)
    assert inc[c] == 2 and dec[c] == inc[c]
    dec(3)
    assert inc[c] == -1 and dec[c] == inc[c]
    assert plus10() == 9

Now, using ``Module``:

.. code-block:: python

    m = M.Module()
    n = T.scalar('n')
    m.c = T.scalar() # state variables
    m.inc = M.Method(n, [], updates = {m.c: m.c + n}) # m.c <= m.c + n
    m.dec = M.Method(n, [], updates = {m.c: m.c - n}) # k.c <= k.c - n
    #m.dec = M.Method(n, [], updates = {c: m.c - n})#global c don't exist
    #m.plus10 does not update the state
    m.plus10 = M.Method([], m.c + 10) # m.c is always accessible since it is a member of this mlass
    
    inst = m.make(c = 0) # here, we make an "instance" of the module with c initialized to 0
    assert inst.c == 0
    inst.inc(2)
    assert inst.c == 2
    inst.dec(3)
    assert inst.c == -1
    assert inst.plus10() == 9
    
Benefits of ``Module`` over ``function`` in this example:
 * There is no need to manipulate the containers directly
 * The fact inc and dec share a state is more obvious syntactically.
 * ``Method`` does not require the states to be anywhere in the input list.
 * The interface of the instance produced by ``m.make()`` is simple and coherent, extremely similar to that of a normal python object. It is directly usable by any user.


Nesting example
===============

The problem now is to create two pairs of ``inc dec`` functions and a function ``sum`` that adds the shared states of the first and second pair.

Using function:

.. code-block:: python

    def make_incdec_function():
           n, c = T.scalars('nc')
           inc = theano.function([n, ((c, c + n), 0)], [])
           dec = theano.function([n, ((c, c - n), inc.container[c])], [])#inc and dec share the same state.
           return inc,dec
    
    
    inc1, dec1 = make_incdec_function()
    inc2, dec2 = make_incdec_function()
    a, b = T.scalars('ab')
    sum = theano.function([(a, inc1.container['c']), (b, inc2.container['c'])], a + b)
    inc1(2)
    dec1(4)
    inc2(6)
    assert inc1['c'] == -2 and inc2['c'] == 6
    assert sum() == 4 # -2 + 6

Using Module:

.. code-block:: python

    def make_incdec_module():
        m = M.Module()
        n = T.scalar('n')
        m.c = T.scalar() # state variables
        m.inc = M.Method(n, [], updates = {m.c: m.c + n}) # m.c <= m.c + n
        m.dec = M.Method(n, [], updates = {m.c: m.c - n}) # m.c <= m.c - n
        return m
    
    m = M.Module()
    m.incdec1 = make_incdec_module()
    m.incdec2 = make_incdec_module()
    m.sum = M.Method([], m.incdec1.c + m.incdec2.c)
    inst = m.make(incdec1 = dict(c=0), incdec2 = dict(c=0))
    inst.incdec1.inc(2)
    inst.incdec1.dec(4)
    inst.incdec2.inc(6)
    assert inst.incdec1.c == -2 and inst.incdec2.c == 6
    assert inst.sum() == 4 # -2 + 6

Here, we make a new ``Module`` and we give it two inner ``Modules`` like
the one defined in the basic example. Each inner module has methods inc
and dec as well as a state c and their state is directly accessible from
the outer module, which means that it can define methods using them. The
instance(inst) we make from the ``Module``(m) reflects the hierarchy
that we created. Unlike the method using function, there is no need to
manipulate any containers directly.


Advanced example
================

Complex models can be implemented by subclassing ``Module`` (though that is not mandatory). Here is a complete, extensible (and working) regression model implemented using this system:

.. literalinclude:: ../code/regression.py

Here is how we use the model:

.. code-block:: python

    data_x = N.random.randn(4, 10)
    data_y = [ [int(x)] for x in N.random.randn(4) > 0]
    
    
    model = SoftmaxXERegression(regularize = False).make(input_size = 10,
                       target_size = 1,
                       stepsize = 0.1)
    
    for i in xrange(1000):
       xe = model.update(data_x, data_y)
       if i % 100 == 0:
           print i, xe
           pass
       #for inputs, targets in my_training_set():
           #print "cost:", model.update(inputs, targets)
    
    print "final weights:", model.w
    print "final biases:", model.b


Extending ``Methods``
=======================

``Methods`` can be extended to update more parameters. For example, if we wanted to add a variable holding the sum of all costs encountered so far to ``SoftmaxXERegression``, we could proceed like this:

.. code-block:: python

    model_module = SoftmaxXERegression(regularize = False)
    model_module.sum = T.scalar() # we add a module member to hold the sum
    model_module.update.updates.update(sum = model_module.sum + model_module.cost) # now update will also update sum!
    
    model = model_module.make(input_size = 4,
                             target_size = 2,
                             stepsize = 0.1,
                             sum = 0) # we mustn't forget to initialize the sum
    
    test = model.update([[0,0,1,0]], [[0,1]]) + model.update([[0,1,0,0]], [[1,0]])
    assert model.sum == test

The inputs and outputs list of a ``Method`` can be doctored as well, but it is trickier, arguably less useful and not fully supported at the moment.


