Lambda the Ultimate

Flexible types: Robust type inference for first-class polymorphism, by Dan Leijen:

We present HML, a type inference system that supports full firstclass polymorphism where few annotations are needed: only function 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 implementation of the type inference algorithm significantly. 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 is available at: http://research.microsoft.com/users/daan/pubs.html.

Seems there's been a flurry of activity on first-class polymorphism recently, with HML, FPH, and HM^F as simpler alternatives to full ML^F.