How do I make this function correctly asyncronous? - f#

Question

I'm trying to get F# async working, and I just can't figure out what I'm doing wrong. Here's my sorta syncronous code that runs:
open System.Net
open System.Runtime.Serialization
open System.Threading.Tasks
[<DataContract>]
type Person = {
[<field: DataMember(Name = "name")>]
Name : string
[<field: DataMember(Name = "phone")>]
Phone : int
}
let url = "http://localhost:5000/app/plugins/anon/CCure"
let js = Json.DataContractJsonSerializer(typeof<Person>)
let main x =
let client = new WebClient()
let url = url + "/" + x
let reader = client.OpenRead(url)
let person = js.ReadObject(reader) :?> Person
printfn "Name: %s, Phone number: %d" person.Name person.Phone
printfn "starting x"
let x = Task.Factory.StartNew(fun () -> main "x")
printfn "starting y"
let y = Task.Factory.StartNew(fun () -> main "y")
Task.WaitAll(x, y)
I was thinking that to run it asyncronously this would work, but it doesn't:
open System.Net
open System.Runtime.Serialization
open System.Threading.Tasks
[<DataContract>]
type Person = {
[<field: DataMember(Name = "name")>]
Name : string
[<field: DataMember(Name = "phone")>]
Phone : int
}
let url = "http://localhost:5000/app/plugins/anon/CCure"
let js = Json.DataContractJsonSerializer(typeof<Person>)
let main x = async {
let client = new WebClient()
let url = url + "/" + x
let! reader = client.OpenReadAsync(url)
let person = js.ReadObject(reader) :?> Person
printfn "Name: %s, Phone number: %d" person.Name person.Phone }
printfn "starting x"
let x = Task.Factory.StartNew(fun () -> main "x")
printfn "starting y"
let y = Task.Factory.StartNew(fun () -> main "y")
Task.WaitAll(x, y)
$ fsharpc -r System.Runtime.Serialization foo.fs && ./foo.exe F#
Compiler for F# 3.1 (Open Source Edition) Freely distributed under the
Apache 2.0 Open Source License
/home/frew/code/foo.fs(19,18): error FS0001: This expression was
expected to have type
Async<'a> but here has type
unit
/home/frew/code/foo.fs(20,17): error FS0041: A unique overload for
method 'ReadObject' could not be determined based on type information
prior to this program point. A type annotation may be needed.
Candidates: XmlObjectSerializer.ReadObject(reader:
System.Xml.XmlDictionaryReader) : obj,
XmlObjectSerializer.ReadObject(reader: System.Xml.XmlReader) : obj,
XmlObjectSerializer.ReadObject(stream: System.IO.Stream) : obj
/home/frew/code/foo.fs(20,17): error FS0008: This runtime coercion or
type test from type
'a to
Person involves an indeterminate type based on information prior to this program point. Runtime type tests are not allowed on
some types. Further type annotations are needed.
What am I missing here?

Answer 1

OpenReadAsync is part of the .NET BCL and therefore wasn't designed with F# async in mind. You'll notice it returns unit, rather than Async<Stream>, so it won't work with let!.
The API is designed to be used with events (i.e. you have to wire up client.OpenReadCompleted).
You have a couple of options here.
There are some nice helper methods in FSharp.Core that can help
you to convert the API into a more F# friendly one (see
Async.AwaitEvent).
Use AsyncDownloadString, an extension method for WebClient that can be found in Microsoft.FSharp.Control.WebExtensions. This is easier so I've done it below although it does mean holding the whole stream in memory as a string so if you have a huge amount of Json this may not be the best idea.
It's also more idiomatic F# to use async instead of tasks for running things in parallel.
open System.Net
open System.Runtime.Serialization
open System.Threading.Tasks
open Microsoft.FSharp.Control.WebExtensions
open System.Runtime.Serialization.Json
[<DataContract>]
type Person = {
[<field: DataMember(Name = "name")>]
Name : string
[<field: DataMember(Name = "phone")>]
Phone : int
}
let url = "http://localhost:5000/app/plugins/anon/CCure"
let js = Json.DataContractJsonSerializer(typeof<Person>)
let main x = async {
printfn "Starting %s" x
let client = new WebClient()
let url = url + "/" + x
let! json = client.AsyncDownloadString(System.Uri(url))
let bytes = System.Text.Encoding.UTF8.GetBytes(json)
let st = new System.IO.MemoryStream(bytes)
let person = js.ReadObject(st) :?> Person
printfn "Name: %s, Phone number: %d" person.Name person.Phone }
let x = main "x"
let y = main "y"
[x;y] |> Async.Parallel |> Async.RunSynchronously |> ignore<unit[]>