FunctionGraph

TODO How do we initialize a FunctionGraph?

TODO What does a FunctionGraph do that a variable given by a TensorVariable does not?

TODO What are Features?

TODO What functionalities of FunctionGraphs are needed where?

Need to draw a map of where the different FunctionGraph functionalities could be used.

Misc

From the documentation:

A FunctionGraph represents a subgraph bound by a set of input variables and a set of output variables, ie a subgraph that specifies an Aesara function. The inputs list should contain all the inputs on which the outputs depend. Variable``\s{=latex} of type Constant are not counted as inputs.

The FunctionGraph supports the replace operation which allows to replace a variable in the subgraph by another, e.g. replace (x + x).out by (2 \* x).out. This is the basis for optimization in Aesara.

This class is also responsible for verifying that a graph is valid (ie, all the dtypes and broadcast patterns are compatible with the way the Variable``\s{=latex} are used) and for tracking the Variable``\s{=latex} with a :attr:FunctionGraph.clients dict that specifies which Apply nodes use the Variable. The :attr:FunctionGraph.clients field, combined with the :attr:Variable.owner and each :attr:Apply.inputs, allows the graph to be traversed in both directions.

It can also be extended with new features using :meth:FunctionGraph.attach~feature~. See Feature for event types and documentation. Extra features allow the FunctionGraph to verify new properties of a graph as it is optimized.

What I am wondering is: when is a FunctionGraph needed? When does it become cumbersome?

import aesara
from aesara.graph.fg import FunctionGraph
import aesara.tensor as at
 
x = at.vector('x')
z = at.log(at.exp(x + x)) + x + x
 
fg = FunctionGraph(outputs=[z])
 
aesara.dprint(fg)

print(fg.clients)

TODO What is the clients dictionary used for?

It looks like this is what allows to traverse the graph?

TODO Keep track of where a FunctionGraph is necessary, where it is produced, where it is cloned, etc.