`choice` and `match` for structs

[ebinwalter]

Motivation

I can't help but love Rust's enums. However, sometimes I wish I could add a parameter to every variant of an enum at once, and so I end up with something like

struct Type {
  common_value: i32,
  ty: TypeKind,
}

enum TypeKind {
  Integer,
  Ref(Rc<Type>),
  Array(Rc<Type>, u32),
}

Proposal

I would like to add something like the following to our language.

struct Type {
  choice Integer;
  choice Ref(struct Type *);
  choice Array {
    struct Type *ty;
    int length;
  }
  
  int common_value;
}

int x = match type {
  Integer -> {
    0 + type.common_value
  }
  Ref(t) -> {
    1 + type.common_value
  }
  Array a {
    a.length + type.common_value
  }
};

What would this give us? We could avoid if-else chains with tags on structs, and we could essentially get the Rust-style enum for free by creating a struct with only choices.

[ebinwalter]

Based on the syntax shown in the example, it looks like we'll need some kind of 'block' or 'scope' expression capable of giving back a value.