Question

here is my code :

let go thriller =
    work.ListPos.Clear()
    thriller
    work.Thread.Start()

member X.Start() =
    work.ListPos.Add <| new Packet( connector.Authorize("admin","1") ) |> go

So it doesn’t work. I used wrapper with parameter like this :

let inconnection thriller   = 
    using <| new SqlConnection(insql)  <| fun X -> X.Open(); thriller X;

inconnection <| fun X ->

I can’t use it this way without parameters because I got no X (parameter) but how to create lambda without parameters ?

Thank you.

Answer 1

As far as I can see, you’re trying to implement the “hole in the middle” pattern – that is, run some initialization, then run a function specified by the user and then run some cleanup.

As dahlbyk already pointed out, you need to pass your go operation a function of type unit -> unit. This can be written like this:

let go thriller =
    work.ListPos.Clear()
    thriller()
    work.Thread.Start()

// Use it like this:
go (fun () ->
  work.ListPos.Add(new Packet( connector.Authorize("admin","1"))
)

Aside. Your other example isn’t really idiomatic F# code. The using function can be replaced with a use keyword, which is significantly easier to use:

let inconnection thriller   = 
    use conn = new SqlConnection(insql) 
    conn.Open()
    thriller conn

inconnection (fun conn -> ...)

More tricky option.
Alternatively, you can also use the use keyword for other things than disposing of resources like SQL connections. This could make your go function even nicer (but it depends on your particular case). The idea is that you’ll be able to write:

use u = go()  // work.ListPos.Clear() gets called here
work.ListPos.Add(new Packet(connector.Authorize("Admin", "1")))
// work.Thread.Start() gets called automatically when 'u' variable goes out of scope

To do this, you need to define go as a function that returns IDisposable that performs the cleanup:

let go () =
  work.ListPos.Clear()
  { new IDisposable with
      member x.Dispose() = work.Thread.Start() }