Using dependent types in the deeper functions and
requiring a Proxy to reach them meant we required
dictionary passing to get the Nats. This made the
pad and crop layers almost 1000 times slower than
they should have been.
I had a basic monadic interface on, thinking that it might be nice
to use for dropout and the like.
In retrospect I think that was too heavy. Being a purely functional
heterogeneous list, substituting layers is easy, so it one wants to
do that using MonadRandom it can still be done.
