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Comment by amelius

2 days ago

I'm curious why we have not-a-number, but not not-a-string, not-a-boolean, not-an-enum, and not-a-mycustomtype.

18 comments

amelius

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dzaima  2 days ago

Because NaNs come from a standardized hardware-supported type, whereas the rest of those are largely language-specific (and you could consider null/nil as a "not-a-*" type for those in applicable languages; and there are languages which disallow NaN floats too, which completes all combinations).

Itanium had a bit for "not a thing" for integers (and perhaps some older hardware from around the time floats started being a thing had similar things), so the idea of hardware support for not-a-* isn't exclusive to floats, but evidently this hasn't caught on; generally it's messy because it needs a bit pattern to yoink, but many types already use all possible ones (whereas floats already needed to chop out some for infinities).

Sharlin  2 days ago

Because IEEE 754 creators wanted to signal non-trapping error conditions for mathematically undefined operations, and they had a plenty of bit patterns to spare. Apparently back in the 70s and 80s in many cases it was preferable for a computation to go through and produce NaNs rather than trapping instantly when executing an undefined operation. I'm not quite sure what the reasoning was exactly.

  • wbl  2 days ago

    In early FP machines the floating point processor could not take a trap at a faulting instruction precisely: it could only go bad things. Furthermore for programmers and hardware it can be very expensive. Rather than go through a loop and filter NaN out of results it becomes trap every time and resume and is a pain.

IncreasePosts  2 days ago

Some common numeric operations can result in non-numbers(eg division by zero - Nan or infinity).

Are there any common string operations with similar behavior?

  • masfuerte  2 days ago

    Out of range substring? Some languages throw an error, others return an empty string. You could return a propagating NaS instead. I don't know what you'd use it for.

mathgradthrow  2 days ago

Nana basically means that floating point arithmetic is predicting that your mathematical expression is an "indeterminate form", as in the thing you learn in calculus.

ok_computer  2 days ago

Because representing infinity is not possible outside of symbolic logic and isn’t encodable in floats. I think it is a simple numerical reason and not a deeper computer reason.

  • Sharlin  2 days ago

    Well, infinity is totally representable with IEEE 754 floats. For example 1.0/0.0 == +inf, -1.0/0.0 == -inf, but 0.0/0.0 == NaN.

    • amelius  2 days ago

      A smart compiler should be able to figure out a better value for 0/0, depending on context.

      For example:

          for i in range(0, 10):
        print(i/0.0)

      In this case it should probably print +inf when i == 0.

      But:

          for i in range(-10, 10):
        print(i/0.0)

      Now it is not clear, but at least we know it's an infinity so perhaps we need a special value +-inf.

      And:

          for i in range(-10, 10):
        print(i/i)

      In this case, the value for 0/0 can be 1.

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