3 ms·

Don’t forget rust also has unions. Not using the name “union” for a feature that’s different than unions doesn’t feel “obviously wrong” to me.

sirwhinesalot 3y ago
The right solution was to make union safe with an opt-in way to make it unsafe IMO.
- steveklabnik 3y ago
  In concrete terms, what would that look like?
  - metaltyphoon 3y ago
    Syntax wise Zig does it with union(enum). Union and Enums are both concepts directly related to C and can be used separately too.
- mattstir 3y ago
  The thing is that accessing an untagged union's data is just inherently unsafe. Say you made a union with a `u8` field and a `bool` field. If you had an instance of it and set its value to `5`, that's just not a valid `bool`. Hence the requirement of `unsafe`; you need to tell the compiler that you _know_ it's a valid value at the point you're accessing it.
  - sirwhinesalot 3y ago
    Then make them tagged, that's what rust enums are, tagged unions, except they're called enums instead of unions.
    - mattstir 3y ago
      I mean... yeah, that's exactly why Rust has tagged unions as well... both are useful in the right contexts, but tagged unions are easier to work with in general. That doesn't mean that (untagged) unions should be safe to work with though, because they can't be.
binary132 3y ago
Rust's Union is for C FFI, right? Why not call Enum Union, and the FFI one CUnion or something? Like, is there something meaningful that Union is for other than C interop? Isn't it functionally identical to a tagged union in every other respect?
- steveklabnik 3y ago
  C FFI is a very useful thing, sure, but I don’t think it’s exclusive to that. It’s a nice primitive to have access to. I think Rust would feel weird with tagged unions and not untagged ones, just like having both sum and product types are good.
- tialaramex 3y ago
  > Rust's Union is for C FFI, right? No :) > is there something meaningful that Union is for other than C interop? Look more closely at the Rust standard library, MaybeUninit<T> is a union.

sirwhinesalot 3y ago

The right solution was to make union safe with an opt-in way to make it unsafe IMO.

steveklabnik 3y ago
In concrete terms, what would that look like?
- metaltyphoon 3y ago
  Syntax wise Zig does it with union(enum). Union and Enums are both concepts directly related to C and can be used separately too.
mattstir 3y ago
The thing is that accessing an untagged union's data is just inherently unsafe. Say you made a union with a `u8` field and a `bool` field. If you had an instance of it and set its value to `5`, that's just not a valid `bool`. Hence the requirement of `unsafe`; you need to tell the compiler that you _know_ it's a valid value at the point you're accessing it.
- sirwhinesalot 3y ago
  Then make them tagged, that's what rust enums are, tagged unions, except they're called enums instead of unions.
  - mattstir 3y ago
    I mean... yeah, that's exactly why Rust has tagged unions as well... both are useful in the right contexts, but tagged unions are easier to work with in general. That doesn't mean that (untagged) unions should be safe to work with though, because they can't be.

binary132 3y ago

Rust's Union is for C FFI, right? Why not call Enum Union, and the FFI one CUnion or something? Like, is there something meaningful that Union is for other than C interop? Isn't it functionally identical to a tagged union in every other respect?

steveklabnik 3y ago
C FFI is a very useful thing, sure, but I don’t think it’s exclusive to that. It’s a nice primitive to have access to. I think Rust would feel weird with tagged unions and not untagged ones, just like having both sum and product types are good.
tialaramex 3y ago
> Rust's Union is for C FFI, right? No :) > is there something meaningful that Union is for other than C interop? Look more closely at the Rust standard library, MaybeUninit<T> is a union.