"Standard ML is a functional programming language, in the sense that the full power of mathematical functions is present." From a Pdf linked on the repo (I didn't know what Standard ML was, I was hoping for a mark up language)

For those interested in the Standard ML compilers, Mlton is another one worth checking out. Standard ML of New Jersey (smlnj) is another interesting one although it tends to be more of a research vehicle than polyml or mlton. If you are inclined toward verification, the cakeml project is quite cool as well. For those unfamiliar with standard ML, if you’ve heard of ocaml or F#, they’re relatives in the ML language family.