Building eDSLs in functional TypeScript

Business logic could be expressed in a limited subset of host language, leading to correct by construction, robust, optimisable code. This process is known as building eDSL – embedded domain-specific languages – and interpreting them, and is a widely used practice in functional languages like Haskell, Scala, OCaml. Still, this topic is terra incognita for many JS/TS developers.

During this workshop I will give an overview of two ways of building eDSLs in functional TypeScript using fp-ts library:

1. Free Monads
2. Tagless Final

We will compare those approaches and discuss their trade-offs.

You will get hands-on experience in describing a business domain in a way that allows separating the construction of business logic from its interpretation and execution.

List of Requirements

• A notebook with code editor OR browser with CodeSandbox.
• Working Node.js 10+ environment.
• Downloaded workshop template.
• Understanding basic concepts of functional programming: immutability, totality, purity, function composition, least power principle, etc.
• Understanding what a monad and a functor are.
• Experience with fp-ts would be a great plus.

Preparations for the workshop

1. Clone the workshop repository to your local computer.
2. Install all the dependencies using npm ci.
3. Open the repository in editor of your choice and follow along with the explanations.
4. If you stuck, feel free to use one of recovery points (see below) to catch-up.

Recovery points are branches with implemented crucial for understaning the material checkpoints. Their names are:

• 01-free-api
• 02-free-example
• 03-free-interpreters
• 04-tagless-api
• 05-tagless-examples
• 06-tagless-interpreters