Type System Overview

FScript’s type system is strict, structural, predictable, and mostly inferred. It aims to feel familiar to TypeScript users while giving the compiler stronger guarantees.

Main goals

• reject ill-typed programs before execution
• avoid implicit any
• support structural typing for plain data
• keep local code lightweight through inference where that is safe
• make exported APIs explicit and readable

The core type categories

• primitive types
• record types
• array types
• function types
• generator sequence types
• union and intersection types
• literal types
• generic types
• Unknown
• Never

Comparison to TypeScript

The overall feel is intentionally familiar, but Draft 0.1 is stricter in several ways:

• no implicit any
• more emphasis on explicit annotations at module boundaries
• stronger preference for closed record shapes
• a design goal of avoiding internal runtime type mismatches in well-typed code

Best reading order

1. Inference
2. Primitive Types
3. Records
4. Functions
5. Unions
6. Tagged Unions