Congratulations on the nice work! It is very elegant to use combinators to formulate and solve this problem. Though my worry with combinators is that they introduce awkwardness with setting up 'long range' DAG edges, you can have to do a lot of shuttling of things around manually through tuples and whatnot, I'm not sure how it is with your framework.

Am I right in thinking that there is no bucketing going on here? In other words, each batch is fixed length, and a short-lived DAG is planned and then simulated with tf.while to accommodate the set of shapes in that particular batch? Are there any problems when the input shapes are wildly different in expense? For example, imagine a size-agnostic convnet. Maybe some of the images in the training set are small, others are large, how would that look in your framework, if it can be done? Is junk padding part of picture, to help match almost-equal tensors to allow them to be batched?