An accurate timer for F# without using System.Diagnostics

timer.fs

//
// This is in fact a thin wrapper around QueryPerformanceCounter() and QueryPerformanceFrequency().
// When those are not available it falls back to System.DateTime.UtcNow.Ticks.
// This module is based on the C# code of StopWatch.
// See: http://referencesource.microsoft.com/#System/services/monitoring/system/diagnosticts/Stopwatch.cs
//

open System.Runtime.InteropServices
open System.Runtime.Versioning

[<AutoOpen>]
module private TimerInternal =

  [<DllImport("kernel32.dll")>]
  [<ResourceExposure(ResourceScope.None)>]
  extern bool QueryPerformanceCounter (int64& value)

  [<DllImport("kernel32.dll")>]
  [<ResourceExposure(ResourceScope.None)>]
  extern bool QueryPerformanceFrequency (int64& value)

module Timer =

  let ElapsedTicks =
    let mutable frequency = 0L
    if QueryPerformanceFrequency &frequency 
    then let tickFrequency = 10000000L / frequency
         fun () -> let mutable timestamp = 0L
                   QueryPerformanceCounter &timestamp |> ignore
                   timestamp * tickFrequency
    else fun () -> DateTime.UtcNow.Ticks

  let inline ElapsedMilliseconds () = ElapsedTicks () / 10000L
  
  //
  // ElapsedTicks () returns the number of ticks since start: 10,000 tick = 1 millisecond
  // ElapsedMilliseconds () returns the number of milliseconds since start
  //