Comment by continuational

The point is that algebraic data types are common in functional languages. "Maybe" is just an example of an algebraic data type, there's tons more.

If the article says "functional programmers should take a look at Zig", and Zig makes algebraic data types hard, then maybe they shouldn't use it.

If you even say "the annoyingness is a feature, use zig the way it is intended to be used" then that's another signal for functional programmers that they won't be able to use zig the same way they use functional languages.

> if optional were a general sum type you wouldn't be able to make these optimizations easily without extra information

Rust has these optimizations (called "niche optimizations") for all sum types. If a type has any unused or invalid bit patterns, then those can be used for enum discriminants, e.g.:

- References cannot be null, so the zero value is a niche

- References must be aligned properly for the target type, so a reference to a type with alignment 4 has a niche in the bottom 2 bits

- bool only uses two values of the 256 in a byte, so the other 254 form a niche

There's limitations though, in that you still must be able to create and pass around pointers to values contained within enum, and so the representation of a type cannot change just because it's placed within an enum. So, for example, the following enum is one byte in size:

    enum Foo {
        A(bool),
        B
    }

Variant A uses the valid bool values 0 and 1, whereas variant B uses some other bit pattern (maybe 2).

But this enum must be two bytes in size:

    enum Foo {
        A(bool),
        B(bool)
    }
 

...because bool always has bit patterns 0 and 1, so it's not possible for an invalid value for A's fields to hold a valid value for B's fields.

You also can't stuff niches in padding bytes between struct fields, because code that operates on the struct is allowed to clobber the padding.