Type Inference Verified: Algorithm script W sign in Isabelle/HOL

This paper presents the first machine-checked verification of Milner's type inference algorithm script D sign for computing the most general type of an untyped λ-term enriched with let-expressions. This term language is the core of most typed functional programming languages and is also known as Mini-ML. We show how to model all the concepts involved, in particular types and type schemes, substitutions, and the thorny issue of "new" variables. Only a few key proofs are discussed in detail. The theories and proofs are developed in Isabelle/HOL, the HOL instantiation of the generic theorem prover Isabelle.