[masklinn]

> That said I wonder why they named it enum.

Because it's a type-safe enumeration? ADTs are pretty much a superset of C-style enums, an ADT with all-dataless constructors is equivalent to a C enum:

    enum Foo { Bar, Baz, Qux, Quux }

And members can converted to integrals:

    > println!("{}", Foo::Qux as u8);
    2

With explicit integrals very much like C:

    > enum Foo { Bar = 5, Baz, Qux, Quux }
    > println!("{}", Foo::Qux as u8);
    7    

Calling it "enum" makes it familiar to C developers[0], "union" would have been confusing for the same (as you can't just write one representation and read the other) and "type" is a tad too generic (and used for type aliases à la typedef).

[0] even more so C++ developers as they have `enum class` which is also type-safe and not implicitly converted to integrals

[nemothekid]

Well you are right, the name does make sense, but I would have thought "union" would have made more sense than "enum" for those of us from the C-family.

OTOH I haven't written a line of Rust of yet, so enum might be more familiar in use.

[masklinn]

> Well you are right, the name does make sense, but I would have thought "union" would have made more sense than "enum" for those of us from the C-family.

The issue with `union` is it might hint at things which are not available (e.g. the union of an int and a float where you can access to the raw data as either).

A Rust enum is really an encoding of the `struct { enum, union }` pattern, but it degenerates to an enum if there's no union part (no data payload), not to a union. So calling it an enum makes more sense.