Interesting, thanks! On https://www.tensorflow.org/tutorials/seq2seq I found a link to https://arxiv.org/abs/1406.1078, which says

> $One RNN encodes a sequence of symbols into a fixed-length vector representation, and the other decodes the representation into another sequence of symbols.$

To me it sounds like they use an RNN to learn a hash function.

Thanks for the NTM link, I'll check it out.