This rule raises an issue when a `tensorflow.function` is recursive.

== Why is this an issue?

When defining a `tensorflow.function` it is generally a bad practice to make this function recursive. TensorFlow does not 
support recursive `tensorflow.function` and will in the majority of cases throw an exception. However it is possible as 
well that the execution of such function succeeds, but with multiple tracings which has strong performance implications.
When executing `tensorflow.function`, the code is split into two distinct stages. 
The first stage call `tracing` creates a new `tensorflow.Graph`, runs the Python code normally, 
but defers the execution of TensorFlow operations (i.e. adding two Tensors). These operations are added to the graph without being ran.
The second stage which is much faster than the first, runs everything that was deferred previously.
Depending on the input of the `tensorflow.function` the first stage may not be needed, see: https://www.tensorflow.org/guide/function#rules_of_tracing[Rules of tracing].
Skipping this first stage is what provides the user with TensorFlow's high performance.

Having a recursive `tensorflow.function` prevents the user from benefiting of TensorFlow's capabilities.

== How to fix it

To fix this issue, refactor the `tensorflow.function` so that is it not recursive.

=== Code examples

==== Noncompliant code example

[source,python,diff-id=1,diff-type=noncompliant]
----
import tensorflow as tf

@tf.function
def factorial(n):
     if n == 1:
        return 1
    else:
        return (n * factorial(n-1)) # Noncompliant: the function is recursive
----

==== Compliant solution

[source,python,diff-id=1,diff-type=compliant]
----
import tensorflow as tf

@tf.function
def factorial(n):
     return tf.exp(tf.lgamma(n + 1)) # Compliant
----


== Resources
=== Documentation

* TensorFlow Documentation - https://www.tensorflow.org/guide/function#recursive_tffunctions_are_not_supported[Recursive tf.functions are not supported]
* TensorFlow Documentation - https://www.tensorflow.org/guide/function#rules_of_tracing[Rules of tracing]