How to update the state with async requests using the state monad?

Question

I'm doing an app using F# and I wonder if I could a) Keep the state under the State computation expression (using FSharpx's implementation) and b) be able to modify the state asynchronously (handling the user's input).

I believe I know how to handle each problem separately; either:

Keep the state in mutable variables and use async & MailboxProcessor to receive async requests to modify the state
Use the state computation expression but not being able to process the user input.

How could I use both the state and the async computation expressions in FSharp or Haskell?

How do you want the two to interact? For instance you could use an intermediary to store the asynchronous requests and then have a portion of the state computation expression pull from that the perform the actual update, but that would involve stalling the resultant updates until the state computation checks. I don't believe there is an alternative though without breaking a lot of guarantees about the state computation (since you no longer know when it can change, and can no longer simply stream it through your process as a single immutable value). — Guvante, Jan 28 '15 at 17:43
I'm finishing my first attempt. When I finish it, I'll post it as an edit to let you know my requirements. Then, any improvement would be welcome. — Lay González, Jan 28 '15 at 18:11

Gus · Accepted Answer · 2020-01-18T08:05:59.350

Basically there are 2 approaches to combine monads: use monad transformers or create a custom monad by hand which combines both monads.

Since there are no native type classes in F# you usually end up with the latter solution, but you can use F#+ to combine Async and State a-la Haskell and get the desired computation expression.

Here's an example translated from Haskell:

#r @"FSharpPlus.dll"

open FSharpPlus
open FSharpPlus.Data

let print x = async {printfn "%A" x}

#nowarn "0025" // Incomplete pattern match, list is assumed not to be empty.
let code  =
    let inline io (x: Async<_>)  : StateT<_,Async<_>> = liftAsync x
    let pop  = monad {
        let! (x::xs) = get
        do! put xs
        return x}
    monad {
        let! x = pop
        do! io <| print x
        let! y = pop
        do! io <| print y
        return () }

let main = StateT.run code [1..10] >>= fun _ -> result ()

// try Async.RunSynchronously main

See also this and this related questions.

How to update the state with async requests using the state monad?

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