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Multi-methods are a straightforward extension of traditional (single) dynamic dispatch, which is the core of most object oriented languages. With multi-methods, a method call will select an appropriate implementation based on the values of multiple arguments, and not just the first/receiver. Language support for both single and multiple dispatch is typically designed to be used in conjunction with other object oriented features, in particular classes and inheritance. But are these extra features really necessary?Mµl is a dynamic language designed to be as simple as possible but still supporting flexible abstraction and polymorphism. Mµl provides only two forms of abstraction: (object) identities and (multi) methods. In Mµl method calls are dispatched based on the identity of arguments, as well as what other methods are defined on them. In order to keep Mµls design simple, when multiple method definitions are applicable, the most recently defined one is chosen, not the most specific (as is conventional with dynamic dispatch).In this paper we show how by defining methods at runtime, we obtain much of the power of classes and meta object protocols, in particular the ability to dynamically modify the state and behaviour of 'classes' of objects.CCS Concepts • Software and its engineering → Object oriented languages; Procedures, functions and subroutines; • Theory of computation → Object oriented constructs;

Static verification relying on an automated theorem prover can be very slow and brittle: since static verification is undecidable, correct code may not pass a particular static verifier. In this work we use metaprogramming to generate code that is correct by construction. A theorem prover is used only to verify initial "traits": units of code that can be used to compose bigger programs.In our work, meta-programming is done by trait composition, which starting from correct code, is guaranteed to produce correct code. We do this by extending conventional traits with pre-and postconditions for the methods; we also extend the traditional trait composition (+) operator to check the compatibility of contracts. In this way, there is no need to re-verify the produced code.We show how our approach can be applied to the standard "power" function example, where metaprogramming generates optimised, and correct, versions when the exponent is known in advance.

Formal methods for software correctness are critical to the future of software engineering -and so must be an essential part of software engineering education. Unfortunately, formal methods are often resisted by students due to perceived difficulty, mathematicity, and practical irrelevance. We redeveloped our software correctness course by taking a programming intensive approach, using the solver-aided language Dafny to provide instant formative feedback via automated assessment. Our redeveloped course increased student retention and resulted in the best evaluation for the course for at least ten years.