This is a nice introductory tutorial on how to implement Hindley-Milner type inference: https://github.com/jozefg/hm https://github.com/jozefg/hm This is a mor

This is a nice introductory tutorial on how to implement Hindley-Milner type inference: https://github.com/jozefg/hm https://github.com/jozefg/hm This is a more advanced tutorial that illustrates a nice but tricky optimization that OCaml's type checker internally uses: http://okmij.org/ftp/ML/generalization.html http://okmij.org/ftp/ML/generalization.html Finally, TAPL's type checkers are pretty good. They aren't designed for efficiency, though. They're designed to closely follow the book's contents: http://www.cis.upenn.edu/~bcpierce/tapl/checkers/ http://www.cis.upenn.edu/~bcpierce/tapl/checkers/