Flexible types: robust type inference for first-class polymorphism

We present HML, a type inference system that supports full first-class polymorphism where few annotations are needed: only func-tion parameters with a polymorphic type need to be annotated. HML is a simplification of MLF where only flexibly quantified types are used. This makes the types easier to work with from a programmers perspective, and we found that it simplifies the im-plementation of the type inference algorithm. Still, HML retains much of the expressiveness of MLF, it is robust with respect to small program transformations, and has a simple specification of the type rules with an effective type inference algorithm that infers principal types. A reference implementation of the type system i