Building on a previous lecture in the summer school, the introduction to interactive proof, this lecture demonstrates a specific application of interactive proof assistants: the semantics of programming languages. In particular, I show how to formalise a small imperative programming language in the theorem prover Isabelle/HOL, how to define its semantics in different variations, and how to prove properties about the language in the theorem prover.

The emphasis of the lecture is not on formalising a complex language deeply, but to teach formalisation techniques and proof strategies using simple examples. To this purpose, we cover big- and small step semantics, typing and type safety, as well as a small machine language with compiler and compiler correctness proof.