Composing-Async-Example.fs

open System
open System.Net
open System.Threading

[<EntryPoint>]
let main _ =
  let url = "http://www.google.com"

  let (>>=) (asyncBlock: Async<'a>) (f: 'a -> Async<'b>)  = 
    async {
      let! v = asyncBlock
      return! f v
    }

  let (>=>) f g x =
    f x >>= g

  let getWebResponse () = 
    async {
      let req = WebRequest.Create(Uri(url))
      return! req.AsyncGetResponse ()
    }

  let readResponseAsString (response: WebResponse) = 
    async {
      use stream = response.GetResponseStream()
      use reader = new IO.StreamReader(stream)

      return reader.ReadToEnd()
    }

  let f =
    getWebResponse
      >=> readResponseAsString
      >=> fun s -> async { printfn "%s" s }
      >> Async.Start

  f ()
  getWebResponse () >>= (fun _ -> async { printfn "Ignoring the web response and just printing." }) |> Async.Start
  printfn "Where do I print?"

  Thread.Sleep 1000000 // Don't kill the main program thread while we are doing the async stuff
  0