Switching from C++ to Rust

It depends what you take "unfit" to mean.

Suppose I say that cars without seatbelts are "unfit" to use on public roads. Clearly I can't mean it's impossible to use such cars, people used to do it all the time, but perhaps I mean it's a bad idea to use them, and that's harder to argue with which is why we got laws saying you need seatbelts.

Based on your other replies in this thread, as far as I can tell, you're not here to try and understand something. You're here to fuck around and play games with people over definitions. So, I'm not responding to you. I'm responding to the people following along who might get confused by the mess you're making here. More to the point, the Wikipedia article for "sum type" redirects to "tagged union," which just honestly makes this more of a mess.

A tagged union is a specific kind of data structure. It essentially refers to a pair of information: the first is a tag and the second is the union. The union might represent many different kinds of values. The important bit is that the size of that memory is the size of the largest value. The tag then tells you what kind of value is in that memory, and crucially, tells you how to interpret that memory. Getting this wrong, for example, in a language like C can lead to undefined behavior.

Tagged unions show up in all sorts of places. The classical example is an AST. You usually have one "node" type that can be one of many possible things: a literal, a binary operation, a function call, a type definition, whatever. And for any non-primitive node, it usually has pointers to children. And what is the type of a child? Well, a "node"! It's a recursive data type. It lets you represent children as just more nodes, and those nodes might be any other AST node. (And as you might imagine, an AST can be quite a bit more complicated than this. You often want restrictions on which types of children are allowed in certain contexts, and so you wind up with many different tagged unions.)

The use of a tagged union like this can be conceptually thought of as a sum type. A sum type is not a data structure. A sum type is a concept. It comes from type theory and it is the "dual" of product types. Product types represent the conjunction of values while sum types represent the disjunction of values. In type theory notation, you usually see 'A x B x C' to refer to the product type of A, B and C. Similarly, you usually see 'A + B + C' to refer to the sum type of A, B or C.

A sum type can be represented by just about anything, because it's just a concept. You can take something that is typically used for product types and use it to present an API that gives you a sum type:

    type OptionInt struct {
        exists bool // private
        number int // private
    }
    
    func OptionIntNone() OptionInt {
        return OptionInt{false, 0}
    }
    
    func OptionIntSome(number int) OptionInt {
        return OptionInt{true, number}
    }
    
    func (o OptionInt) IsSome() bool {
        return o.exists
    }
    
    func (o OptionInt) Number() int {
        if !o.exists {
            panic("option is none, no number available")
        }
        return o.number
    }

This is a very contrived and hamstrung example, because it really doesn't give you much. But what it does give you is the API of a sum type. It is a value that can be either missing or present with an integer. Notably, it does not rely on any particular integer sentinel to indicate whether the value is missing or not. It encodes that state separately. So it can fully represent the presence of any integer value.

As you can see, despite it offering an API that represents the sum type concept, it is actually implemented in terms of a product type, or a struct in this case.

In a language like Go, which does not really support either unions or sum types, this sort of API is really hamfisted. In particular, one could object that it is a sum type because it doesn't really represent the "sum typeness" in the type system. Namely, if you get its use wrong, then you don't get a compilation error. That is, you can call the 'Number()' method without checking whether 'IsSome()' returns true or not. In other words, correct use of OptionInt looks like this:

    var opt OptionInt := doSomethingThatMightReturnNone()
    if opt.IsSome() {
        // OK, safe to access Number. It won't panic
        // because we just checked that IsSome() is true.
        fmt.Println(opt.Number())
    }

But nothing in Go stops you from writing this instead:

    var opt OptionInt := doSomethingThatMightReturnNone()
    fmt.Println(opt.Number())

This is why "support for sum types" is critical for making them as useful as they can be. In Rust, that same option type is defined like this:

    enum OptionInt {
        None,
        Some(i32),
    }

That's it. The language gives you the rest. And crucially, instead of splitting out the 'IsSome()' check and the access via 'Number()', they are coupled together to make it impossible to get wrong. So the above code for getting a number out looks like this instead:

    let opt: OptionInt = do_something_that_might_return_none()
    if let OptionInt::Some(number) = opt {
        println!("{}", number);
    }

You can't get at the 'number' without doing the pattern matching. The language won't let you. Now, you can define methods on your sum types that behave like Go's 'Number()' method above and panic when the value isn't what you expect. And indeed, Rust's Option and Result types provide this routine under the names 'unwrap()' and 'expect()'. But the point here isn't just about Option and Result, it's about sum types in general. And you don't have to define those methods that might panic. You can just let the language help guide you along instead.

Sum types are an idea. A concept. A technique. Tagged unions are an implementation strategy.

Notice also that I haven't mentioned exhaustiveness at all here. It isn't required for sum types. Haskell, for example, doesn't mind at all if your pattern matching isn't exhaustive. You actually have to tell it to warn you about it, and even then, it's not a compilation error unless you treat it as one. In Rust, exhaustiveness checking is enabled by default for all sum types. But you can disable it for a particular sum type with the '#[non_exhaustive]' attribute. The benefit of disabling it is that adding a new variant is possibly semver compatible change, where as adding a new variant in an exhaustive sum type is definitely a semver incompatible change.

I know your claim will strike some people as absurd, but I agree. I am honestly flabbergasted that any general purpose language created in the last 20 years doesn't have sum types and pattern matching. To me they are very nearly as fundamental as aggregate types (aka structs or product types).

> Pretend we have the following sum type:

which looks remarkably like a union to me, if i understand your made-up language.