Pragmatic types

Introduction

What

Small practical guide on statically-typed languages for developers who switching from dynamically-typed languages to statically-typed versions. For example from JavaScript to TypeScript or Flow.

For who

This guide assumes at least some prior knowledge of dynamically-typed languages. The idea of this guide is to help make a mental shift from dynamic to static types.

Most of the time I will use JavaScript and TypeScript or Flow examples. Maybe this will be useful for other languages - I do not know yet, let's see.

If you have prior knowledge of statically typed languages, but Hindley-Milner doesn't ring a bell for you, this guide can be useful for you too.

Why

This is my diary of personal discoveries. I find myself explaining those things again and again to different people. I decided to write it down once and for all.

Table of contents

• Exhaustive check and why it is useful for event processing, enums, unions or disjoint unions
• Opaque types and how they could have saved Mars Climate Orbiter
• IO validation or how to handle JSON-based APIs in statically typed language
• What are types?
• Dynamically-, statically-, gradually-, weakly-, strongly- and un-typed languages
• Is JavaScript an untyped language?
• Type system vs null
• Making Impossible States Impossible
• Immutability and type system
• Classes, types, sets, structural and nominal subtyping, Liskov substitution principle
• Redux and impossible states
• Redux and impossible state transitions or FSM
• Redux and side effects
• Types and proofs
• Why JavaScript Promise is unsound?
• Terminology: soundness, completeness, totality, decidability
• Rosetta stone, Curry–Howard correspondence
• To be continued...

Similar works

• To type or not to type: quantifying detectable bugs in JavaScript Gao et al., ICSE 2017
• Type systems will make you a better JavaScript programmer by Jared Forsyth 2017
• Types, and Why You Should Care
• Lauren Tan Swipe Left, Uncaught TypeError Learning to Love Type Systems

Contribution

Ideas, questions, and fixes are welcome. Sharing on social networks is also a big help. Thank you.