The missing bit is language tooling. The regular floating point API exposed by most languages don’t force handling of NaNs.

The benefit of the option type is not necessarily just the extra value, but also the fact that the API that forces you to handle the None value. It’s the difference between null and Option.

Even if the API was better, I think there’s value in expressing it as Option<FloatGuaranteedToNotBeNaN> which compiles down to using NaNs for the extra value to keep it similar to other Option specialisations and not have to remember about this special primitive type that has option built in.

Yeah. You should be very explicit about it. Certainly not treat it like, “ooh, here are some free bits that I can use to tag things in ad hoc ways (like -1 for missing index)”.

https://internals.rust-lang.org/t/pre-rfc-nonnan-type/8418

> NaN's are already an option type, although implemented in hardware

The compromise with this is that it makes it impossible to represent a non-optional float, which leads to the same issues as null pointers in c++/java/etc.

The impacts of NaN are almost certainly not as bad (in aggregate) as `null`, but it'd still be nice if more languages had ways to guarantee that certain numbers aren't NaN (e.g. with a richer set of number types).

> The impacts of NaN are almost certainly not as bad (in aggregate) as `null`, but it'd still be nice if more languages had ways to guarantee that certain numbers aren't NaN (e.g. with a richer set of number types).

The problem with that is that to guarantee arithmetic does not result in a NaN, you need to guarantee that 0 and infinity are not valid values, and those values can still arise from underflow/overflow of regular computation. Basically, there's no subset of floating-point numbers that forms a closed set under +, -, *, or / that doesn't include NaN. So you can define FiniteF32 (e.g.), but you can't really do anything with it without the result becoming a full-on float.

As far as I'm aware, there's no equivalent to a stack trace with NaN, so finding the origin of a NaN can be extremely tedious.

I've never found it to be particularly difficult.

The extremely difficult problems to find are uninitialized data and threading bugs, mainly because they appear and disappear.

Good points!