Consider type annotations as pattern syntax #309

With type annotations potentially being added, I think it would be worth considering whether the patterns could be the same as type annotations. Type annotations would cover primitive literal types 1, "some string", true etc. and objects { tag: "a", data }, arrays etc. It would also allow referencing/reusability through the use of interfaces and type aliases.

This would allow this feature to be adopted quicker as existing type annotations could be used in patterns. It also reduces the work for type checkers to do narrowing in the matcher body.

Additional I think that the matching block should be the same AST as arrow function bodies. It should also copy the return syntax being the result of the body rather than introducing different semantics (or a forth return like keyword).

The result being something like

match (res) {
    when (number) => 5,
    when ({ tag: "number", data }) => data + 2,
    when (InterfaceX) => {
        for (const x of res.items) {
            if (x === res.expect) {
                return x;
            }
        }
        return null
    },
}