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    <p>In the F# community, imperative code and mutable data aren't frowned upon <em>as long as they're not part of your public interface</em>. 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:</p> <pre><code>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 [&lt;Sealed&gt;] 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 &gt; 0 then facade.RPM &lt;- 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 () |&gt; ignore </code></pre> <p>This way, the public interface is unaffected &ndash; <code>UpdateStatusITimes</code> still takes and returns an intrinsically immutable <code>DeviceStatus</code> &ndash; but internally <code>UpdateStatusITimes</code> uses a mutable class to eliminate allocation overhead.</p> <p><strong>EDIT:</strong> (In response to comment) This is the style of class I would normally prefer, using a primary constructor and <code>let</code>s + properties rather than <code>val</code>s:</p> <pre><code>[&lt;Sealed&gt;] 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 &lt;- n member x.Pressure with get () = pressure and set n = pressure &lt;- n member x.Temperature with get () = temp and set n = temp &lt;- n member x.ToDeviceStatus () = { RPM = rpm; Pressure = pressure; Temperature = temp } </code></pre> <p>But for simple facade classes where each property will be a blind getter/setter, I find this a bit tedious.</p> <p>F# 3+ allows for the following instead, but I still don't find it to be an improvement, personally (unless one dogmatically avoids fields):</p> <pre><code>[&lt;Sealed&gt;] 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 } </code></pre>
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    1. POHow to make immutable F# more performant?
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    1. POHow to make immutable F# more performant?
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    1. COAwesome, this is exactly what I was looking for; someone to state definitively whether it was possible or not to get mutable performance from an immutable structure, and, given that it's not, demonstrate the F# community best practices for this problem. Thanks!
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    2. CO@ildjarn : What do you mean "explicit classes"? What do you "normally" prefer? What's different about this situation to make you want to use explicit classes?
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    3. CO@Dax : Edited to demonstrate.
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