How to write a function that only accepts one enum variant as input?

Question

I have an enum:

enum Group {
    OfTwo {
        first: usize,
        second: usize,
    },
    OfThree {
        one: usize,
        two: usize,
        three: usize,
    },
}

I would like to write a function that only takes as argument the Group::OfTwo variant:

fn proceed_pair(pair: Group::OfTwo) {}

But when I do that, I get the message:

error[E0573]: expected type, found variant `Group::OfTwo`
  --> src/lib.rs:13:23
   |
13 | fn proceed_pair(pair: Group::OfTwo) {}
   |                       ^^^^^^^^^^^^
   |                       |
   |                       not a type
   |                       help: try using the variant's enum: `crate::Group`

Is there a way to achieve this?

Peter Hall · Accepted Answer · 2019-06-02 11:42:56Z

The variants of an enum are values and all have the same type - the enum itself. A function argument is a variable of a given type, and the function body must be valid for any value of that type. So what you want to do will just not work.

However, there is a common pattern for designing enums, which might help here. That is, to use a separate struct to hold the data for each enum variant. For example:

enum Group {
    OfTwo(OfTwo),
    OfThree(OfThree),
}

struct OfTwo { first: usize, second: usize }
struct OfThree { one: usize, two: usize, three: usize }

fn proceed_pair(pair: OfTwo) {

}

Anywhere that you previously matched on the enum like this:

match group {
    Group::OfTwo { first, second } => {}
    Group::OfThree { first, second, third } => {}
}

You would replace with:

match group {
    Group::OfTwo(OfTwo { first, second }) => {}
    Group::OfThree(OfThree { first, second, third }) => {}
}

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