Local Reasoning for Global Properties

- Laurie Tratt is a professor of programming language design and implementation at King's College London, known for work on language engineering, parsing, and the Converge programming language. - The article is about a deep problem in software engineering: local reasoning (understanding a piece of code in isolation) vs. global properties (behaviors that emerge from the whole system). Modern programming increasingly makes it hard to reason locally because features like async/await, concurrency, and distributed systems introduce implicit dependencies. - Tratt argues that much of programming language design is an attempt to make local reasoning capture more global properties — but there are fundamental limits, and when those limits are hit, programmers must rely on documentation, testing, or formal methods instead. - The piece is aimed at practicing programmers and language designers, not complete beginners, though its core insight is widely accessible.