[phlip9]

Languages with first class effects, like koka[1], require you to declare up-front what side effects (sample randomness, do I/O, abort, etc...) your function might cause.

Then from another outer function, you can't call the effectful function unless the outer function also declares those same side-effects (or you provide handlers for them).

This way you can easily isolate non-determinism and I/O to the top-level part of your program, while keeping the interior business logic pure.

The "first-class effects" part is what lets you easily compose effects and inject custom handlers. Ideally in a way that's more ergonomic than juggling with Monad type system Jenga.

[1]: https://koka-lang.github.io/koka/doc/index.html