Typed error use-case example

error-propagation.swift

enum ValueError: Error {
  case notFound
  case typeMismatch
}

struct PropertyError: Error {
  let name: String
  let error: ValueError
}

struct ClassError: Error {
  let name: String
  let error: PropertyError
}

class Foo {
  private func validateValue(_ value: Any) throws { // throws ValueError
    ... some logic
    throw ValueError.typeMismatch
  }
  
  private func validateProperty(name: String, value: Any) throws { // should only throw a PropertyError
    if ... {
      throw PropertyError(name: name, error: .notFound)
    }
    try validateValue(value) // Whoops, I forgot to wrap this, now the property name is lost
  }
  
  func validate(name: String) throws { // should only throw a ClassError
    do {
      for (name, value) in self.properties {
        try validateProperty(name: name, value: value)
      }
    } catch let error as PropertyError {
      throw ClassError(name: name, error: error)
    }

    // Uh oh. Because of the bug earlier the ValueError falls through. It still gets passed
    // to the caller, but now all they know is that there was a typeMismatch or notFound error
    // - they've no idea which property or class it relates to.

    // And note that having an explicit catch-all wouldn't help here either, because I'd still
    // only have the ValueError - I wouldn't be able to convert it to the PropertyError that I
    // need because I don't know which property failed validation.
  }
}