sum-types VS immutable-js

I entirely agree about using symbols. sum-types does that.

With fp-ts and sum-types you'd handle this functionally like so:

If you're looking to have discriminated unions like Rust, follow that section to roll your own or look for a library that's already implemented them. fp-ts has everything under the sun in a Haskell style, neverthrow has Rust-like Result and ResultAsync, and sum-types helps with the boilerplate of defining your own sum types.

I'd suggest sum-types. Instead of unsafe user-defined type guards you get safe pattern matching.

If you wanted to lean (more) into FP you could replace the Event type and buildPopup function with a sum type and pattern match respectively, as well as represent side effects like in popup with fp-ts' IO, make use of function composition, etc. The former in particular is great for readability:

You can test types, at least in terms of asserting happy path types and whether a piece of code, runtime or purely type-level, should trigger a tsc type error. Here's an example using eslint-plugin-expect-type.

Anywhere that overloads are needed will rely upon type assertions or the unsafety implicit in overloads, for example in fp-ts/function::pipe. It'll come up a lot with objects too when there isn't a preexisting primitive you can compose atop of, as in for example fp-ts-std/Record::pick. Something as generic as @unsplash/sum-types has assertions all over the place, though to be fair that's mostly again a case of struggles interacting with object types.

You might like @unsplash/sum-types also.

immutable for JavaScript.

Check out Object.freeze. There's also Immutable.js for working with immutable data.

Use immutability to manage state updates When updating state in your React application, it's important to ensure that you are not mutating the original state object. Instead, you should create a new copy of the state object with the updated values. Immutability makes it easier to manage state updates and ensures that the updates are performed in a predictable and safe manner. Libraries like Immutable.js provide a set of functions that simplify working with immutable data in React applications.

We're wanting to get rid of immutable so I'm trying to replicate what it is about the Record functionality and types that allow this dynamic property access to work. After pulling my hair out looking through the [email protected] type definitions and the actual code, to me it looks like the types are just kind of lying about what's going on ... and it's just working. Does anyone have any ideas how I can replicate this dynamic property access with strong typings?

The Immutable.js README has a much more complete description of immutability and why you might want to use the library. Also worth mentioning that Immer is an alternative which is a bit easier to get started with.

Examples: immutable-js Immer

I can't speak to C# and Java, but the suggestion in this post: ImmutableJS already uses Persistent data structures. (It's the second sentence of their introduction)

Avoiding mutation is just never modifiyng an object, ever. There are tons of implementations of this pattern, notably immutable.js (https://immutable-js.com/), Redux is also an example of this philosophy.

JavaScript is most programers' first introduction to map, filter, and reduce. Easy lambdas made those higher-order functions a staple of a lot of JS code.

Meanwhile, immutable.js[0] is at 10 million downloads per week and rising.

I would add that it's not just the ease of use of lambdas, but the fact that in JavaScript functions really are first class citizens. Most of the other widely used languages that people start on have lambda functions added in as a bit of a hack and only treat some functions as real values.

[0] https://www.npmjs.com/package/immutable