Signals when applying different scale results in inexact result

[Benjamin Milde]

I'm working with an encoding, where the scale (and precision) is hardcoded. I'm using Decimal.Context.with/2 to verify the provided decimal value is within those specs. That works great for precision (using Decimal.apply_context), but Decimal.round doesn't provide any signals if the rounding changed the value. I guess this kinda makes sense given the value was explicitly rounded, but on the other hand there's also no alternative to change the scale.

Edit:

As a workaround I'm currently using this:

decimal
|> Decimal.normalize()
|> tap(fn decimal ->
  if Decimal.scale(decimal) > scale do
    Decimal.Context.get()
    |> Map.update!(:flags, &[:inexact | &1])
    |> Decimal.Context.set()
  end
end)
|> Decimal.round(scale)

[Eric Meadows-Jönsson]

How come you can't use Decimal.apply_context/1?

[Benjamin Milde]

I'd love that, but I don't see a scale setting on Decimal.Context. I'm already using it for the precision part after having adjusted the scale.

[Eric Meadows-Jönsson]

Oh, I see. Then comparing against Decimal.scale/1 is the correct approach. We shouldn't emit signals when Decimal.round/2 rounds.

[Benjamin Milde]

Would you be open to making scale part of Decimal.Context (e.g. given a PR)? E.g. consider 40.01 / 10 = 4.001. Given the context of precision 4 / scale 2, this would also produce a problematic value per the scale, but not per the precision.

[Eric Meadows-Jönsson]

I don't think we should add scale because it's not part of the specification we built this library from.

Flags for precision is required because it's the only way to know we lost data during an operation. Scale can always be checked after the operation using `Decimal.scale/1.