Comment by eru
2 days ago
That and in-band signalling is error-prone.
Another example that C programmers love: using 0 to mean infinity. Eg when you have a timeout setting, and 10 means ten seconds, but 0 means infinity seconds instead of zero seconds.
In general it's a problem of sentinel values. Some languages let you abstract this - I think in Rust and C++ you can define an "intrusive optional" which is like optional but uses an invalid value pattern to store the not-present state. In memory it's the same as a sentinel value but the compiler forces you to unwrap it like an optional.
I'm not sure how well the 'forcing' works for Rust and C++. You can definitely do this kind of thing in dependently typed languages.
The type is akin to optional<int>. You can't add 5 to it, you have to dereference it first which crashes if it was null (0 or another chosen value).
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Amusingly, Win API of all places generally evades this pattern: 0 timeout means "no waiting", and to wait indefinitely, you use timeout of INFINITE which is actually UINT_MAX or, equivalently, -1 (in Win API, timeouts are unsigned, not time_t).
That’s just a very bad API. You could do the same in Rust? There are nice C APIs that would use a nice struct that describes the timeout you want… eg GCD.
I'm not sure a struct is the right thing here in general. You often want an algebraic datatype.
What Rust calls an enum, but an enum with payloads. In C terms it would be a discriminated union.