RoelTyper is an extremely fast optional pluggable type system for Smalltalk, based on heuristics. It type-checks instance variables of classes. Typing a complete VisualWorks Smalltalk image takes less than one minute and a half! For this speed you get the types for about 80% of instance variables in the system. The approach is quite similar to what the Refactoring Browser does, but somewhat more refined.
This talk delves into the implementation details for the VisualWorks and Squeak versions, and ends with a (fast) demo of the system.