Discussion of Function Specification for Dataset Types
======================================================

Some talking points from the September 2 meeting:

 * Datasets as views/tasks (Pascal Vincent's idea): our dataset specification
 needs to be flexible enough to accommodate different (sub)tasks and views of
 the same underlying data.
 * Datasets as probability distributions from which one can sample.
    * That's not something I would consider to be a dataset-related problem to
        tackle now: a probability distribution in Pylearn would probably be a
        different kind of beast, and it should be easy enough to have a
        DatasetToDistribution class for instance, that would take care of viewing a
        dataset as a probability distribution. -- OD
 * Our specification should allow transparent handling of infinite datasets (or
 simply datasets which cannot fit in memory)
 * GPU/buffering issues.

Commiteee: DE, OB, OD, AB, PV
Leader: DE

Some ideas from existing ML libraries:

- PyML: notion of dataset containers: VectorDataSet, SparseDataSet, KernelData,
  PairDataSet, Aggregate. Ultimately, the learner decides	
- mlpy: very primitive notions of data
- (still going through the other ones)

A few things that our dataset containers should support at a minimum:

    - streams, possibly infinite
    - task/views of the data for different problems
    - indexing & slicing 
    - pairs or triples or etc of examples
    - a 'distance/gram matrix' container (imagine that the data is given to you
      as a distance matrix)
    - multi-dimensional time-series (again, maybe with pairs/triples, maybe
      given to you as a distance matrix over time)

Another question to consider is the following: how tight should it integrate
with Theano? Do we want to be able to store data as shared variables or just
have an option for that? Theano + GPU constrains things that we can do (in terms
of sizes, buffering, etc): these are things we need to think about, but it's not
clear whether we should aim for building them into the interface.

Task views of the data for different problems: How can we achieve this? Should
we simply have a set of standard dataset descriptors ('classification',
'regression', 'multi-label', 'density_estimation') and have a set_view method
that changes the current dataset view type?

There is then the question of how to approach the design of a Dataset class from
an OOP perspective. So far, my (Dumi's) idea is to have an almost 'abstract class' 
Dataset that doesn't implement any methods except a few setters/getters. The reason
to have the methods listed that way is to have a common 'specification', but classes
that inherit from Dataset need not implement every single method (only the ones
that are relevant) and can obviously implement other methods as appropriate. The
reason to have a common specification (as abstract as it might be) is to, well,
have a common specification that would make our code clearer and cleaner.

An example of what I (Dumi) am thinking in terms of concrete API:

class Dataset:
    def __init__(self):
        self.type = None
        self.in_memory = None
        self.inputs = None # list of filepaths, or objects in memory, or...
        self.outputs = None

    def get_example(self,example_index):
        raise NotImplementedError()

    def get_next_example(self):
        raise NotImplementedError()

    def get_batch(self,batch_index):
        raise NotImplementedError()

    def get_next_batch(self):
        raise NotImplementedError()

    def get_slice(self,slice_object):
        raise NotImplementedError()

    def set_view(self,view_type):
        self.view_type = view_type
        self.n_classes = None

    def set_n_classes(self,n_classes):
        self.n_classes = n_classes

    def set_batch_size(self,batch_size):
        self.batch_size = batch_size

You will note that there is no notion of train/valid/test in this class: I think we should
just have a train dataset, a valid one and a test one instead or (if it's in one
big file or infinite stream) just handle the split ourselves (via slicing, for
instance). I (Dumi) am of the opinion that it keeps things cleaner, but the
specification does not preclude more fine-grained 'splitting' of the data.

A concrete implementation would look like this (we would have one class per
dataset that we use, and the class declaration contains essentially everything
there is to know about the dataset):

class MNIST(Dataset):
    def  __init__(self,inputs=['train_x.npy'],outputs=['train_y.npy']):
        self.type='standard_xy'
        self.in_memory = True
        self.inputs = inputs # load them or create 
        self.outputs = outputs
        self.set_view('classification') 
        self.set_n_classes(10)
        self.set_batch_size(20)
        self.n_batches = self._compute_n_batches()

    def get_batch(self,batch_index):
        x,y = self._fetch_batch(batch_index)
        if self.view_type == 'classification':
            return x,numpy.int32(y)
        elif self.view_type == 'density_estimation':
            return x
        else:
            raise NotImplementedError()

    def shared_data(self):
        shared_x = theano.shared(numpy.asarray(self.inputs, dtype=theano.config.floatX))
        shared_y = theano.shared(numpy.asarray(self.outputs, dtype=theano.config.floatX))
        return shared_x, T.cast(shared_y, 'int32')

    def _compute_n_batches(self):
        pass

    def _fetch_batch(self,batch_index):
        pass

But nothing stops you from defining get_train_batch, get_valid_batch and stuff
like that! 

So we'd use it as:

train_mnist = MNIST(inputs = ['train_x.npy'], outputs = ['train_y.npy'])
valid_mnist = MNIST(inputs = ['valid_x.npy'], outputs = ['valid_y.npy'])

x,y = train_mnist.get_batch(0)
train_mnist.set_view('density_estimation')
x = train_mnist.get_batch(0)

or

mnist_data = MNIST(inputs = ['x.npy'], outputs = ['y.npy'])
batches_train = range(int(mnist_data.n_batches*0.8))
batches_valid = range(int(mnist_data.n_batches*0.8),mnist_data.n_batches)

xt,yt = mnist_data.get_batch(batches_train[0])
xv,yv = mnist_data.get_batch(batches_valid[0])