Nicolas Fränkel • 4/19/2026

Making illegal state unrepresentable

This article discusses the concept of making illegal states unrepresentable in software design, focusing on state machines and the Builder pattern. The author uses a pizza-building model to demonstrate how static typing systems (Java, Kotlin, Rust, Gleam) can prevent invalid transitions at compile-time, while dynamic typing (Python) relies on runtime validation. The post covers the trade-offs between type safety and boilerplate code, providing code examples for each language. It is a technical tutorial relevant to programming, software engineering, and type system design.

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#Python #finite state machines #static typing