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The Secret Life of NaN (2018)

47 points • by prakashqwerty • last Sunday at 10:56 AM • 27 comments • view on HN

Comments

WalterBright • yesterday at 10:15 PM

NaNs are a very underappreciated feature of IEEE-754 floating point. In the D programming language, floats get default initialized to NaN, not to 0.0.

    double y = 0.0; // initialized to 0.0
    double x; // initialized to NaN
The discussion routinely comes up as "why not default initialize to 0.0?" The reason is a routine mistake in programming is forgetting to initialize a variable. With a floating point 0.0, one may never realize that the floating point calculation results are wrong. But with NaN, the result of a floating point computation will be NaN, which is unlikely to go unnoticed.

I don't know of any other programming language with this safety feature.

Also, the D `char` type is initialized to 0xFF, not 0, because Unicode says that 0xFF is an invalid character.

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GMoromisato • yesterday at 10:15 PM

I use nan boxing in GridWhale. It feels like the Infinite Hotel[1]: you can always add another type. Note that these techniques also rely on the fact that we don't use all 64-bits for memory addressing. If we ever do, lots of VMs will break.

For me, the major advantage of nan boxing is that you don't have to allocate a whole class of types (like floats). That saves so much at garbage collection time.

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[1] https://en.wikipedia.org/wiki/Hilbert%27s_paradox_of_the_Gra...

jasperry • yesterday at 11:50 PM

This is super useful, thanks. So if I were implementing a programming language, and wanted to have symbols to specify NaN in source code, I'd really only need quiet NaN, right? Because signaling NaN is supposed to always to raise an exception anyway?

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