package algaeff

Effects for any monad (subject to OCaml continuations being one-shot).

This is a general construction that uses effects to construct monadic expressions. Here is an alternative implementation of State using the standard state monad:

module StateMonad =
struct
  type state = int
  type 'a t = state -> 'a * state
  let ret x s = x, s
  let bind m f s = let x, s = m s in f x s
  let get s = s, s
  let set s _ = (), s
  let modify f s = (), f s
end

module StateUnmonad =
struct
  type state = int
  module U = Algaeff.Unmonad.Make (StateMonad)
  let get () = U.perform StateMonad.get
  let set s = U.perform @@ StateMonad.set s
  let modify f = U.perform @@ StateMonad.modify f
  let run ~init f = fst @@ U.run f init
end

Note that continuations in OCaml are one-shot, so the list monad will not work; it will quickly lead to the runtime error that the continuation is resumed twice. Also, monads do not mix well with exceptions, and thus the bind operation should not raise an exception unless it encounters a truly unrecoverable fatal error. Raising an exception within bind will skip the continuation, and thus potentially skipping exception handlers within the continuation. Those handlers might be crucial for properly releasing acquired resources.