Local Reasoning for Global Properties

- Laurie Tratt is a professor of programming language engineering at King's College London who writes a well-regarded technical blog. - "Local reasoning" means understanding a piece of code in isolation without examining the entire program; "global properties" are system-wide guarantees (e.g. security, encapsulation, data integrity) that can be undermined by local changes. - The post uses the **Expression Problem** (a classic software design dilemma: extending a program with new data types vs. new operations without modifying existing code) as a running example. - Tratt argues that in practice, codebases are large and nobody holds the full program in their head, so languages/designs that let you reason locally are critical; he contrasts this with approaches that require global knowledge (e.g. many OOP or pattern-matching designs). - The post is aimed at experienced programmers familiar with type systems, design patterns, and the trade-offs between functional and object-oriented styles.