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Comment by zevets | Hacker News Reader

Please do this.

But first: we need to take step-zero and introduce a type "r64": a "f64" that is not nan/inf.

Rust has its uint-thats-not-zero - why not the same for floating point numbers??

You can write your "r64" type today. You would need a perma-unstable compiler-only feature to give your type a huge niche where the missing bit patterns would go, but otherwise there's no problem that I can see, so if you don't care about the niche it's just another crate - there is something similar called noisy_float

I can do it, and I do similar such things in C++ - but the biggest benefit of "safe defaults" is the standardization of such behaviors, and the resultant expectations/ecosystem.

> Rust has its uint-thats-not-zero

Why do we need to single out a specific value. It would be way better if we also could use uint-without-5-and-42. What I would wish for is type attributes that really belong to the type.

    typedef unsigned int __attribute__ ((constraint (X != 5 && X != 42))) my_type;

Those are the unstable attributes that your sibling is talking about.

Yeah of course I can put what I want in my toy compiler. My statement was about standard C. I think that's what Contracts really are and hope this will be included in C.

Oh sure, I wouldn’t call rustc a “toy compiler” but yeah, they’d be cool in C as well.

Proper union types would get you there. If you have them, then each specific integer constant is basically its own type, and e.g. uint8 is just (0|1|2|...|255). So long as your type algebra has an operator that excludes one of the variants from the union to produce a new one, it's trivial to exclude whatever, and it's still easy for the compiler to reason about such types and to provide syntactic sugar for them like 0..255 etc.

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