If case let, guard case let, for case let

[wowbagger (冀卓疌)]

At the 16:43 mark of WWDC20 session 10189 is this code snippet:

func handleIncomingFile(_ incomingResourceURL: URL, with name: String, from fromID: String) {
    guard
         case let safeFileName = name.lastPathComponent, 
         safeFileName.count > 0,
         safeFileName != "..", safeFileName != "." else { return }

    let destinationFileURL = URL(fileURLWithPath: NSTemporaryDirectory())
                                 .appendingPathComponent(safeFileName)

    // Copy the file into a temporary directory
    try! FileManager.default.copyItem(at: incomingResourceURL, to: destinationFileURL)

}

There is no documentation of guard case let in the official language guide. All resources I found online are dated about 4 years back, written for Swift 2. The best one I found is this write-up that covers if case let, guard case let, and for case let.

Because there were a lot of source breaking changes between Swift 2 and 5, and because these expressions are not documented in the language guide, I wonder if they have been replaced by other expressions?

[Lantua]

It’s still there within grammars. if and guard accept list of conditions and case condition is one of those.

[wowbagger (冀卓疌)]

Thanks!

[Nevin]

@Lantua

It’s still there within grammars. if and guard accept list of conditions and case condition is one of those.

It’s worth noting that just because something is permitted by the grammar, that does not automatically guarantee it is valid in the language:

Where is the real grammar for Swift?

The key thing is that there is an important distinction between syntactic and semantic validity. For example, something like class class Foo { ] is not well-formed syntactically. However, class C { protocol P {} } is valid syntax but doesn't make sense semantically. There is nothing in the language grammar preventing a protocol declaration from being nested inside of a class. Instead, it is the type checker's job to enforce such semantic rules.