algow

A small, experimental type checker written in TypeScript. This is a learning project — I'm not an academic, just someone trying to understand how compilers and type systems work by building one from scratch.

What's implemented

• Hindley-Milner type inference (Algorithm W)
• Basic types: number, string, boolean
• Functions (lambda abstractions and application)
• Let bindings with polymorphism
• Binary operators (+, -, *, /, <, >, ==, etc.)
• Type classes for operator overloading (Eq, Ord, Add)
• Parameterized types (List a, Maybe a, Either a b)
• Algebraic data types (ADTs) with dataDecl
• Pattern matching with match expressions
• Exhaustiveness checking for pattern matches
• Tuples with proper arity: (number, string, boolean)
• Record types with field access: { x: number, y: string }
• Row polymorphism: fn r => r.x types as { x: t | ρ } -> t
• Pattern matching on records and tuples

What's next

• Lexer and parser
• Runtime/interpreter for code execution

Running

bun install
bun run src/index.ts

Resources that helped

• Write You a Haskell
• Algorithm W Step by Step
• Types and Programming Languages (Pierce)

Disclaimer

This is not production code. It's messy, incomplete, and probably wrong in places. But that's the point — learning by doing.