The codecommit article is a good intro to H-M as in Haskell, OCaml, F#, SML (AKA Damas-Milner by at least one dev I respect), here is codecomit guy re: why Scala can't use HM / DM, rather, Scala is flow-based, or "local type inference".

http://scala-programming-language.1934581.n4.nabble.com/scal...

http://stackoverflow.com/questions/3689407/disadvantages-of-...

http://stackoverflow.com/questions/7234095/why-is-scalas-typ...

this sentence in OP objectionable

Erlang and Clojure: two other functional, concurrency-focused languages which many developers find inscrutable.