TensorFlow Fold is a library for creating TensorFlow models that consume structured data, such as nested lists, dictionaries, and protocol buffers. Examples of such models are tree-recursive neural networks such as models of the Stanford sentiment treebank, tree LSTMs, hierarchical LSTMs, and graph-convolutional neural networks.
TensorFlow by itself was not designed to work with tree or graph structured data. It does not natively support any data types other than tensors, nor does it support the complex control flow, such as recursive functions, that are typically used to run models like tree-RNNs. When the input consists of trees (e.g. parse trees from a natural language model), each tree may have a different size and shape. A standard TensorFlow model consists of a fixed graph of operations, which cannot accommodate variable-shaped data. Fold overcomes this limitation by using the dynamic batching algorithm.
Fold consists of a high-level API called Blocks, and a low-level API called Loom. Blocks are pure Python, whereas Loom is a mixture of Python and C++. Internally, Blocks uses Loom as its execution engine. Loom is an abstraction layer on top of TensorFlow that makes it possible to easily express computations over structures of varying sizes and shapes without the need to modify the underlying computation graph at run-time.