Implementations of various type systems in OCaml

You have read Types and Programming Languages, right? :-)

Haskell and Scheme are the other two perennial choices, with Haskell getting the advantage in type-system work. But Pierce's shadow and Ocaml is getting hard to avoid.

I've been looking for a hands-on tutorial for H-M typology for a while now.

For an introduction, I would also recommend "Algorithm W Step by Step"

http://www.grabmueller.de/martin/www/pub/AlgorithmW.pdf

It has a small error, which I have corrected here: https://github.com/KMahoney/Algorithm-W-Step-By-Step

I've been working my way through this which in my opinion is excellent: http://okmij.org/ftp/Haskell/AlgorithmsH.html#teval