Very unexpected automatic behaviour between StringLiteralConvertible and pattern matching!

Using Swift
1

How is it that Swift allows code like this:

struct Sneaky: StringLiteralConvertible {
  init(stringLiteral value: String) {}
  init(extendedGraphemeClusterLiteral value: String) {}
  init(unicodeScalarLiteral value: String) {}
}

func ~=(sneaky: Sneaky, string: String) -> Bool {
  return false
}

enum NormalEnum: String {
  case Super = "super"
  case Mario = "mario"
}

let value = NormalEnum(rawValue: "super”) // return nil!!!!

It hit completely by surprise today because of of a Regex library:

struct Regex: StringLiteralConvertible {
  init(stringLiteral value: String) {}
  init(extendedGraphemeClusterLiteral value: String) {}
  init(unicodeScalarLiteral value: String) {}

  //...
}

func ~=(regex: Regex, string: String) -> Bool {
  return regex.matches(string)
}

If I was not already a Swift enthusiast, this behaviour would have left me completely dumbfounded.
What can we do about it?

David.

2

How is it that Swift allows code like this:

struct Sneaky: StringLiteralConvertible {
  init(stringLiteral value: String) {}
  init(extendedGraphemeClusterLiteral value: String) {}
  init(unicodeScalarLiteral value: String) {}
}

func ~=(sneaky: Sneaky, string: String) -> Bool {
  return false
}

enum NormalEnum: String {
  case Super = "super"
  case Mario = "mario"
}

let value = NormalEnum(rawValue: "super”) // return nil!!!!

I see two bugs here. When an enum has a raw value type, the compiler generates this initializer:

init(rawValue: String) {
  switch rawValue {
  case "super":
    self = .Super
  ...
  }
}

so uses ~= pattern matching to match the raw value. It would be more sensible to always use `==` comparison in the synthesized initializer. However, I'm surprised too that the type checker favors ~=(Sneaky, String) over ~=(String, String); it should at best be ambiguous. Do you have time to file these two bugs?

-Joe

···

On Jan 5, 2016, at 9:28 AM, David Hart via swift-users <swift-users@swift.org> wrote:

It hit completely by surprise today because of of a Regex library:

struct Regex: StringLiteralConvertible {
  init(stringLiteral value: String) {}
  init(extendedGraphemeClusterLiteral value: String) {}
  init(unicodeScalarLiteral value: String) {}

  //...
}

func ~=(regex: Regex, string: String) -> Bool {
  return regex.matches(string)
}

If I was not already a Swift enthusiast, this behaviour would have left me completely dumbfounded.
What can we do about it?

David.

_______________________________________________
swift-users mailing list
swift-users@swift.org
https://lists.swift.org/mailman/listinfo/swift-users

3

I can file those bugs. Would it be beneficial if I also created failing unit tests?
David.

···

On 06 Jan 2016, at 20:05, Joe Groff <jgroff@apple.com> wrote:

On Jan 5, 2016, at 9:28 AM, David Hart via swift-users <swift-users@swift.org <mailto:swift-users@swift.org>> wrote:

How is it that Swift allows code like this:

struct Sneaky: StringLiteralConvertible {
  init(stringLiteral value: String) {}
  init(extendedGraphemeClusterLiteral value: String) {}
  init(unicodeScalarLiteral value: String) {}
}

func ~=(sneaky: Sneaky, string: String) -> Bool {
  return false
}

enum NormalEnum: String {
  case Super = "super"
  case Mario = "mario"
}

let value = NormalEnum(rawValue: "super”) // return nil!!!!

I see two bugs here. When an enum has a raw value type, the compiler generates this initializer:

init(rawValue: String) {
  switch rawValue {
  case "super":
    self = .Super
  ...
  }
}

so uses ~= pattern matching to match the raw value. It would be more sensible to always use `==` comparison in the synthesized initializer. However, I'm surprised too that the type checker favors ~=(Sneaky, String) over ~=(String, String); it should at best be ambiguous. Do you have time to file these two bugs?

-Joe

It hit completely by surprise today because of of a Regex library:

struct Regex: StringLiteralConvertible {
  init(stringLiteral value: String) {}
  init(extendedGraphemeClusterLiteral value: String) {}
  init(unicodeScalarLiteral value: String) {}

  //...
}

func ~=(regex: Regex, string: String) -> Bool {
  return regex.matches(string)
}

If I was not already a Swift enthusiast, this behaviour would have left me completely dumbfounded.
What can we do about it?

David.

_______________________________________________
swift-users mailing list
swift-users@swift.org <mailto:swift-users@swift.org>
https://lists.swift.org/mailman/listinfo/swift-users

4

Definitely. Thanks for uncovering these issues.

-Joe

···

On Jan 6, 2016, at 2:31 PM, David Hart <david@hartbit.com> wrote:

I can file those bugs. Would it be beneficial if I also created failing unit tests?
David.