Adapted from A Tale of 3 Nightclubs Act Zero: 10:15 Saturday Night In which we will see how to use the type system to handle failure with the Either type and the Validation type. Here we define some checks that all nightclubs make. The checks can be specialised by providing a function that creates…

Note. See here for a reference implementation

Immutability is one of the building blocks of functional programming. An immutable object is an object whose state cannot be modified after it is created. Immutability allows for referential transparency: there is no difference between a value, and a reference to that value. …

Optics are a very useful tool in functional programming. They can reduce the amount of code we have to write significantly, as well as making operations clearer and more readable. We’ll talk about two main types: Lens and Prism. Both propose a way for “getting” and “setting” values in a…

Adapted from Algebraic Structure and Protocols by @mbrandonw Mathematicians love to study objects abstractly. When the object is a set of elements that is equipped with some operation(s) (and some laws!) we call it an algebra. …

TL;DR You don’t really need a different language (Elm) or an adt library (folktale, daggy), often you can just use vanilla JavaScript and a good type checker. The problem This problem is presented in Slaying a UI Antipattern in Fantasyland by Stefan Oestreicher which in turn is based on How Elm Slays…

Written by @GiulioCanti and @alpacaaa “Type driven development” is a technique used to split a problem into a set of smaller problems, letting the type checker suggest the concrete implementation, or at least helping us getting there. Here’s a practical example The Problem Say for instance that we like to reimplement the…

Written by @controlplusb and @GiulioCanti Flow declaration files, like any other piece of software, must be thoroughly tested. The base technique for doing so, which is also used by the flow-typed repo, relies on the usage of the suppress_comment configuration option of the .flowconfig: [options] suppress_comment=\\(.\\|\n\\)*\\$ExpectError include_warnings=true The above configuration…