MDL is a functional programming language created by the Programming Methodology Group at Stanford University. It was designed to be used in conjunction with the theorem prover Isabelle/HOL.
MDL is based on the ML programming language, with some additions to support higher-order logic. MDL programs are compiled to Isabelle/HOL theories, which can be used to prove theorems.
MDL has been used to formalize a variety of concepts in mathematics and computer science, including the Knuth-Bendix completion algorithm, the Four Color Theorem, and the Boyer-Moore string search algorithm.