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Last week I went to a presentation by Jeff Foster about GLR parser for Ruby grammar with extensions for type annotations and is written in OCaml. Internally it uses a normalized and simplified ruby subset to perform the type analysis in order to remove ambiguities and simplify flow analysis.

Type inference is fed a prebuilt set of type annotations derived from the core library that look very similar to rdoc. The annotation system can handle variable arguments to methods, type intersections and type unions. Basically, this means it’s expressive enough to handle String#slice which can be called six different ways.

For details on the type inferencing itself, you should read