A little off-topic, but the author of this paper (Daan Leijen) is also the author of Koka[1], a programming language with algebraic effects.

IMO, Koka (or something similar) has more potential to become a mainstream language than "traditional" FP languages (Haskell, OCaml, Idris, etc.).

Effects seem to be easier to understand than monads (at least on a superficial level) and more modular (I don't have a lot of experience with Haskell, so take that with a grain of salt).

Its syntax is also very close to C-like languages.

Taken from the Koka book[2]:

  fun square1(x : int) : total int {
    return x*x
  }

  fun square2(x : int) : io int {
    println( "a not so secret side-effect" )
    return x*x
  }

`square1` is a pure mathematical function, so its effect is `total`. `square2` has a side-effect because of `println`, so its effect is `io`. This means that `square2` can raise exceptions, not terminate, be non-deterministic, read and write to the heap, and do any input/output operations.

Note that Koka can infer effects, so these annotations are optional.

[1]: https://www.microsoft.com/en-us/research/project/koka/?from=...

[2]: https://koka-lang.github.io/koka/doc/kokaspec.html#sec-effec...