[FIXED] how to write decimal numbers as an atomic write in F#?

Issue

the decimal type takes 128 bits, so it is not naturally an atomic write.

I tried:

Interlocked.Exchange(ref myField, some new value)

but then I saw that decimal is not part of the supported types with Interlocked.Exchange.

I was thinking that doing a lock may be a little bit heavy for this write. Are there any other options?

Solution

As you said, Interlocked.Exchange can only work with 32bit or 64bit values, so it does not support decimal. Aside from using locks, one suggestion from a related C# StackOverflow post is to wrap the value in an object and then use Interlocked.Exchange to swap the object references. In F#, this would look like this:

type Boxed<'T when 'T : struct>(v:'T) = 
  member x.Value = v

let mutable d1 = Boxed(1M)
let d2 = Boxed(2M)

Interlocked.Exchange(&d1, d2)

The question is whether the overhead of an additional instance is greater than the overhead of using lock - I think this will depend on your specific case. If you have just a few decimals that you're working with, the extra objects may not be such a big deal, but you'll probably need to run some tests to find out.

Answered By - Tomas Petricek

Answer Checked By - Mary Flores (PHPFixing Volunteer)