SE-0413: Typed throws

[beccadax (Becca Royal-Gordon)]

@nnnnnnnn

Just as in those existing cases, I think it would be equally useful to let authors state the type of the error they expect, and have the compiler tell them if that expectation is incorrect. I think catch is CatError and catch let myError as CatError are probably the best syntax for this. I'm extremely reluctant to propose new syntax, but if those are too closely linked with pattern matching, then maybe something more binding related would be acceptable? This doesn't really look right to me, but has the needed parts:

do {
    try callCats()
} catch let myError: CatError { ... }

An alternative might be to specify the type not in the catch clause's pattern, but as an effect on the do block instead:

do throws(CatError) {
    try callCats()
} catch let myError { ... }

You might even be able to apply the typed throws effect to a catch-less do statement if you simply want to state the allowable error type explicitly.