Documenting Known Qualitative Limitations

[willtebbutt]

This issue is for keeping track of and documenting known large limitations of the form "You should not expect Taped to do things of type X".

Known Limitations:

• If a (mutable) data structure x contains a circular reference, it will not be possible to construct a zero_tangent / random_tangent MutableTangent to it -- an infinite recursion will occur. It should be possible, however, to differentiate through its construction. If you find this to be a problem in practice, please open an issue.
• zero_tangent and random_tangent do not work for pointers, because we don't know how large a chunk of memory a given pointer points to, so cannot allocate a corresponding chunk of tangent memory. Your best bet when testing rrule!!s for things involving pointers is currently to do integration testing. See the tests for blas functionality for examples.
• If you pass active data through a global variable, AD will fail. Furthermore / worse still, the failures will probably be silent. Globals #41 (comment)

[yebai]

We can probably move these limitations to README or docs since they are design choices (of strictly typed Tangent), which means we will not fix them.

[willtebbutt]

Good idea. I'll close this when the docs are written and discuss them.

[willtebbutt]

These items have been documented in #175 -- will close when that is merged.