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POHow to make immutable F# more performant?
 

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  1. POHow to make immutable F# more performant?
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    <p>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.</p> <pre><code>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 -&gt; status | _ -&gt; 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() |&gt; ignore </code></pre> <p>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.</p>
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    1. COI'm not sure I understand what this is intended to do, but perhaps you can reduce allocation by only returning the part that changes. In this example, Pressure and Temperature are constant, so why copy them?
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    2. COIt may be easier to help if you provide complete code, since your POC is a bit contrived.
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    3. COAndrey's point about `DateTime` makes me wonder if changing `DeviceStatus` to a struct would help performance.
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