Soft Typing with Conditional Types

We present a simple and powerful type inference method for dynamically typed languages where no type information is supplied by the user. Type inference is reduced to the problem of solvability of a system of type inclusion con-straints over a type language that includes function types, constructor types, union, intersection, and recursive types, and conditional types. Conditional types enable us to ana-lyze control ow using type inference, thus facilitating com-putation of accurate types. We demonstrate the power and practicality of the method with examples and performance results from an implementation.