A little nicer

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open System.Diagnostics



let main =
  let w = Stopwatch.StartNew()
  let rec f (lastTime:int64) (accum:int64) : unit =
    System.Threading.Thread.Sleep 1
    let m = w.ElapsedMilliseconds
    match accum + m - lastTime with
    | elapsedTime when elapsedTime > 1000L ->
        printfn "TICK"
        f m (elapsedTime-1000L)
    | _ as elapsedTime -> f m elapsedTime


  f (w.ElapsedMilliseconds) (int64 0)

namespace System

namespace System.Diagnostics

val main : unit

Full name: Script.main

val w : Stopwatch

Multiple items
type Stopwatch =
  new : unit -> Stopwatch
  member Elapsed : TimeSpan
  member ElapsedMilliseconds : int64
  member ElapsedTicks : int64
  member IsRunning : bool
  member Reset : unit -> unit
  member Restart : unit -> unit
  member Start : unit -> unit
  member Stop : unit -> unit
  static val Frequency : int64
  ...

Full name: System.Diagnostics.Stopwatch

--------------------
Stopwatch() : unit

Stopwatch.StartNew() : Stopwatch

val f : (int64 -> int64 -> unit)

val lastTime : int64

Multiple items
val int64 : value:'T -> int64 (requires member op_Explicit)

Full name: Microsoft.FSharp.Core.Operators.int64

--------------------
type int64 = System.Int64

Full name: Microsoft.FSharp.Core.int64

--------------------
type int64<'Measure> = int64

Full name: Microsoft.FSharp.Core.int64<_>

val accum : int64

type unit = Unit

Full name: Microsoft.FSharp.Core.unit

namespace System.Threading

Multiple items
type Thread =
  inherit CriticalFinalizerObject
  new : start:ThreadStart -> Thread + 3 overloads
  member Abort : unit -> unit + 1 overload
  member ApartmentState : ApartmentState with get, set
  member CurrentCulture : CultureInfo with get, set
  member CurrentUICulture : CultureInfo with get, set
  member DisableComObjectEagerCleanup : unit -> unit
  member ExecutionContext : ExecutionContext
  member GetApartmentState : unit -> ApartmentState
  member GetCompressedStack : unit -> CompressedStack
  member GetHashCode : unit -> int
  ...

Full name: System.Threading.Thread

--------------------
System.Threading.Thread(start: System.Threading.ThreadStart) : unit
System.Threading.Thread(start: System.Threading.ParameterizedThreadStart) : unit
System.Threading.Thread(start: System.Threading.ThreadStart, maxStackSize: int) : unit
System.Threading.Thread(start: System.Threading.ParameterizedThreadStart, maxStackSize: int) : unit

System.Threading.Thread.Sleep(timeout: System.TimeSpan) : unit
System.Threading.Thread.Sleep(millisecondsTimeout: int) : unit

val m : int64

property Stopwatch.ElapsedMilliseconds: int64

val elapsedTime : int64

val printfn : format:Printf.TextWriterFormat<'T> -> 'T

Full name: Microsoft.FSharp.Core.ExtraTopLevelOperators.printfn