Translate Polly HTTP request to F#

Question

I'm making an HTTP request with Polly. I would like to retry once after waiting 1 second on each proxy in an array.
How could I do this better?
How could I do this in F#?

public static Result RequestWithRetry(string url, string[] proxies, string username, string password)
{
    if (proxies == null) throw new ArgumentNullException("null proxies array");
    var client = new WebClient { Credentials = new NetworkCredential(username, password) };
    var result = String.Empty;
    var proxyIndex = 0;

    var policy = Policy
            .Handle<Exception>()
            .WaitAndRetry(new[]
                {
                    TimeSpan.FromSeconds(1)
                }, (exception, timeSpan) => proxyIndex++);

    policy.Execute(() =>
    {                 
        if (proxyIndex >= proxies?.Length) throw new Exception($"Exhausted proxies: {String.Join(", ", proxies)}");

        client.Proxy = new WebProxy(proxies?[proxyIndex]) { UseDefaultCredentials = true };
        result = client.DownloadString(new Uri(url));
    });

    return new Result(value: result, proxy: proxies[proxyIndex]);
}

Answer 1

You may try more functional way with Result<'TOk,'TError> and Async<T>

open System.Net
open System

type Request =
    { Url      : string
      Proxies  : string list
      UserName : string
      Password : string }

let requestWithRetry request =
    let client = 
        new WebClient (
            Credentials = new NetworkCredential(
                request.UserName,
                request.Password))
    let uri = Uri request.Url
    let rec retry = function
        | [] -> Error "Exhausted proxies" |> async.Return
        | (proxy:string)::rest -> async {
            try 
                do client.Proxy <- new WebProxy(proxy, UseDefaultCredentials = true)
                let! response = client.AsyncDownloadString uri
                return Ok (response, proxy)
            with _ ->
                do! Async.Sleep 1000
                return! retry rest
        }
    retry request.Proxies