The pure Hindley-Milner type system combines function types, type inference and let polymorphism. It solves constraints by type unification and generalizes eligible type variables at
let bindings. Robin Milner's 1978 paper presents Algorithm W and proves its soundness for the paper's language; it also acknowledges Roger Hindley's earlier work on finding general types. [ref] Articles by others on the same topic
The Hindley–Milner type system is a well-known type system used in functional programming languages, particularly those that support first-class functions and polymorphism. It was developed by Roger Hindley and Robin Milner in the 1970s and is the foundation for type inference in languages such as ML (Meta Language), Haskell, and others.