Custom dynamic response with Suave?

Question

I want to build a simple counter with Suave.

[<EntryPoint>]
let main argv =

  let mutable counter = 0;

  let app =
    choose
      [
        GET
        >=> choose
          [
            path "/" >=> OK "Hello, world. ";
            path "/count" >=> OK (string counter)
          ]
        POST
        >=> choose
          [
            path "/increment"
            >=> (fun context -> async {
              counter <- counter + 1
              return Some context
            })
          ]
      ]

  startWebServer defaultConfig app
  0

However, with my current solution, the count at /count never updates.

I think this is because the WebPart is computed when the app is launched, instead of for each request.

What is the best way to achieve this in Suave?

Answer 1

You are right in the assumption that Webparts are values, so computed once. (See this).

You need to use a closure to get what you want:

path "/count" >=> (fun ctx ->
    async {
        let c = counter in return! OK (string c) ctx
    })