Introduction to Hindley-Milner Type Inference: Build a Type Checker

Static type systems catch bugs before your code even runs, but requiring developers to annotate every single type can be tedious. Understanding Hindley-Milner type inference allows you to design languages and tools that automatically figure out types with mathematical certainty. This course guides you from the fundamental mathematics of types to implementing a fully functional type checker in code. You will transition from writing basic expressions to constructing an inference engine that resolves complex types without manual annotations, mastering the core mechanics that power languages like Haskell, OCaml, and modern functional compilers. What you'll learn: - Understand the core principles of type theory, terms, and typing rules. - Implement Algorithm W for Hindley-Milner type inference from scratch. - Manage substitution, unification, and let-polymorphism in type resolution. - Practice diagnosing and generating clear, helpful type error messages. - Apply modern type system concepts such as bidirectional type checking and row polymorphism. - Design a robust abstract syntax tree optimized specifically for type analysis. You will start by learning essential terminology and the mathematical notation of typing rules before moving step-by-step through unification and inference algorithms. Through clear written explanations and structured coding exercises, you will build and test your type checker incrementally. This course is designed for programmers, aspiring compiler engineers, and language design enthusiasts who want to understand how modern type systems work under the hood. No prior background in type theory or advanced mathematics is required. Begin reading today to unlock the power of automated type inference and level up your compiler engineering skills.