[Pitch N+1] Typed Throws

[nnnnnnnn (Nate Cook)]

Love it!

@toph42

do {
  try callCat() // throws CatError
  try callKids() // throw KidError
} catch {
  // implicit 'error' variable has type 'any Error'
}

This seems like yet another great reason to also revive the guard-catch proposal. The ceremony that encourages grouping multiple try statements in a do block will be that much more harmful when it forces this error type erasure. This is much worse when the throwing expressions yield a value that you need in a later step:

do {
    let filename = try lookUpFileName()       // throws ConfigurationError
    let data = try readFile(named: filename)  // throws FileSystemError
    let result = try processData(data)        // throws DataProcessingError
} catch {
    // any Error :(
}

(ETA: particularly in embedded Swift, where I assume the above do block will yield a compilation error.)