Property-based testing is a testing paradigm supporting regular example-based unit tests. In the Pragmatic Programmer book, the authors recommend to use property-based testing for verifying assumptions about their code. It forces you to think about the actual preconditions, postconditions and invariants of your code instead of implicitly coming up with such rules through hard-coded examples.
Optics are a functional programming toolbox for zooming into nested data structures. They are designed for composability, allowing you to create complex operations step-by-step by composing simple components. Optics also never modify their input data structures, ensuring your objects stay nice and immutable.
Static typing is awesome. It helps to detect bugs, acts as in-code documentation and makes development more enjoyable. Recently I've started to use Python's typing module to add static typing to all of my Python projects. Python's typing system may not be as as powerful as one might hope, but I think once you go typed, you don't go back.
fp-ts is a great TypeScript library for functional programming. I haven't found that many real world usage examples out there, so I thought it might be fun to take a repository and try adding fp-ts there. More specifically, I wanted to learn more about making HTTP requests with fp-ts.