リアクティブプログラミングの擬似コードでよく見るやつ

ExcelBuilder.fs

open System
open System.Reactive
open System.Reactive.Subjects
open System.Reactive.Linq
open Microsoft.FSharp.Control
open FSharp.Control.Reactive
open FSharp.Control.Reactive.Builders
open FSharp.Data

type BehaviorSubject<'a> with 
  member this.setValue x = this.OnNext x

type Cell<'a> = BehaviorSubject<'a>

type ExcelBuilder() =
  let mutable transit: IObservable<_> = null

  member __.Bind(m: IObservable<_>, f: _ -> IObservable<_>) =
              transit <- Observable.bind f m
              transit

  member __.Return(x) =
              let source = Observable.publishInitial x transit
              source :?> IObservable<_>

let excel = new ExcelBuilder()

let a1 = new Cell<int> 1
let b1 = new Cell<int> 2
let c1 = excel {
  let! x = a1
  let! y = b1
  return x + y
}

c1 |> Observable.subscribe (fun x -> printfn "%i" x) |> ignore

a1.setValue 3  // a1(3) + b1(2)

stdin.Read() |> ignore