I probably should have added "user exposed" or something to my comment. Those extra bits certainly seem to offer compiler optimization possibilities, as apparently SpiderMonkey does with WASM.

I can easily *imagine* a library like NumPy or PyTorch deciding to expose something useful with those 52 unused mantissa bits. But that's some future version, if ever.

On Sat, Sep 12, 2020, 2:16 PM Cade Brown <brown.cade@gmail.com> wrote:
As per tagged nans, check for JavaScript tagged NaN optimization. Essentially, the tag of the NaN (i.e. the mantissa) is interpreted as a pointer. Obviously, this is a very advanced use case, probably not worth Python guaranteeing such behavior.

On Sat, Sep 12, 2020, 8:10 PM David Mertz <mertz@gnosis.cx> wrote:
On Sat, Sep 12, 2020, 2:02 PM Steven D'Aprano <steve@pearwood.info> wrote:
In general though, Python doesn't support generating the full range of NANs with payloads directly.

I've researched this a little bit for discussion in a book I'm writing, and I have not been able to identify ANY widely used programming language or tool that does anything meaningful with the NaN payloads.

It's possible I've missed something, but certainly not the top 20 languages are libraries that might come to mind.
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