Bags of words, edit distance (as see in bioinformatics, hamming distances, cunning kernels and vector spaces over documents. Vector spaces induced by document structures. Metrics based on generation by finite state machines, *-omics Maybe co-occurrence metrics would also be useful as musical metrics? Inference complexity.
Blei, David M., Andrew Y. Ng, and Michael I. Jordan. 2003. “Latent Dirichlet Allocation.” Journal of Machine Learning Research 3 (Jan): 993–1022. http://www.jmlr.org/papers/v3/blei03a.html.
Selinger, Peter. 2009. “A Survey of Graphical Languages for Monoidal Categories.” In A Survey of Graphical Languages for Monoidal Categories, 813:289–355. Lecture Notes in Physics. Springer Berlin / Heidelberg. http://arxiv.org/abs/0908.3347.