How to make immutable F# more performant?

Question

I'm wanting to write a big chunk of C# code using immutable F#. It's a device monitor, and the current implementation works by constantly getting data from a serial port and updating member variables based on new data. I'd like to transfer that to F# and get the benefits of immutable records, but my first shot at a proof-of-concept implementation is really slow.

open System
open System.Diagnostics

type DeviceStatus = { RPM         : int;
                      Pressure    : int;
                      Temperature : int }

// I'm assuming my actual implementation, using serial data, would be something like 
// "let rec UpdateStatusWithSerialReadings (status:DeviceStatus) (serialInput:string[])".
// where serialInput is whatever the device streamed out since the previous check: something like
// ["RPM=90","Pres=50","Temp=85","RPM=40","Pres=23", etc.]
// The device streams out different parameters at different intervals, so I can't just wait for them all to arrive and aggregate them all at once.
// I'm just doing a POC here, so want to eliminate noise from parsing etc.
// So this just updates the status's RPM i times and returns the result.
let rec UpdateStatusITimes (status:DeviceStatus) (i:int) = 
    match i with
    | 0 -> status
    | _ -> UpdateStatusITimes {status with RPM = 90} (i - 1)

let initStatus = { RPM = 80 ; Pressure = 100 ; Temperature = 70 }
let stopwatch = new Stopwatch()

stopwatch.Start()
let endStatus = UpdateStatusITimes initStatus 100000000
stopwatch.Stop()

printfn "endStatus.RPM = %A" endStatus.RPM
printfn "stopwatch.ElapsedMilliseconds = %A" stopwatch.ElapsedMilliseconds
Console.ReadLine() |> ignore

This runs in about 1400 ms on my machine, whereas the equivalent C# code (with mutable member variables) runs in around 310 ms. Is there any way to speed this up without losing the immutability? I was hoping that the F# compiler would notice that initStatus and all the intermediate status variables were never reused, and thus simply mutate those records behind the scene, but I guess not.

Answer 1

In the F# community, imperative code and mutable data aren't frowned upon as long as they're not part of your public interface. I.e., using mutable data is fine as long as you encapsulate it and isolate it from the rest of your code. To that end, I suggest something like:

type DeviceStatus =
  { RPM         : int
    Pressure    : int
    Temperature : int }

// one of the rare scenarios in which I prefer explicit classes,
// to avoid writing out all the get/set properties for each field
[<Sealed>]
type private DeviceStatusFacade =
    val mutable RPM         : int
    val mutable Pressure    : int
    val mutable Temperature : int
    new(s) =
        { RPM = s.RPM; Pressure = s.Pressure; Temperature = s.Temperature }
    member x.ToDeviceStatus () =
        { RPM = x.RPM; Pressure = x.Pressure; Temperature = x.Temperature }

let UpdateStatusITimes status i =
    let facade = DeviceStatusFacade(status)
    let rec impl i =
        if i > 0 then
            facade.RPM <- 90
            impl (i - 1)
    impl i
    facade.ToDeviceStatus ()

let initStatus = { RPM = 80; Pressure = 100; Temperature = 70 }
let stopwatch = System.Diagnostics.Stopwatch.StartNew ()
let endStatus = UpdateStatusITimes initStatus 100000000
stopwatch.Stop ()

printfn "endStatus.RPM = %d" endStatus.RPM
printfn "stopwatch.ElapsedMilliseconds = %d" stopwatch.ElapsedMilliseconds
stdin.ReadLine () |> ignore

This way, the public interface is unaffected – UpdateStatusITimes still takes and returns an intrinsically immutable DeviceStatus – but internally UpdateStatusITimes uses a mutable class to eliminate allocation overhead.

EDIT: (In response to comment) This is the style of class I would normally prefer, using a primary constructor and lets + properties rather than vals:

[<Sealed>]
type private DeviceStatusFacade(status) =
    let mutable rpm      = status.RPM
    let mutable pressure = status.Pressure
    let mutable temp     = status.Temperature
    member x.RPM         with get () = rpm      and set n = rpm      <- n
    member x.Pressure    with get () = pressure and set n = pressure <- n
    member x.Temperature with get () = temp     and set n = temp     <- n
    member x.ToDeviceStatus () =
        { RPM = rpm; Pressure = pressure; Temperature = temp }

But for simple facade classes where each property will be a blind getter/setter, I find this a bit tedious.

F# 3+ allows for the following instead, but I still don't find it to be an improvement, personally (unless one dogmatically avoids fields):

[<Sealed>]
type private DeviceStatusFacade(status) =
    member val RPM         = status.RPM with get, set
    member val Pressure    = status.Pressure with get, set
    member val Temperature = status.Temperature with get, set
    member x.ToDeviceStatus () =
        { RPM = x.RPM; Pressure = x.Pressure; Temperature = x.Temperature }