Pattern Matching, Part 4: if case, guard case, for case

Now that we’ve revisited the various syntaxes for pattern matching in part 1, part 2 and part 3, let’s finish this blog post series with some advanced syntax using if case let, for case where and all!

Let’s use what we saw in previous articles and apply them all to some advanced expressions.

This post is part of an article series. You can read all the parts here: part 1, part 2, part 3, part 4

if case let

The case let x = y pattern allows you to check if y does match the pattern x.

Writing if case let x = y { … } is strictly equivalent to writing switch y { case let x: … }: it’s just a more compact syntax which is useful when you only want to pattern-match against one case — as opposed to a switch which is more adapted to multiple cases matching.

For example, let’s use an enum similar to the one from the previous articles:

enum Media {
  case book(title: String, author: String, year: Int)
  case movie(title: String, director: String, year: Int)
  case website(urlString: String)
}

Then we can write this¹:

let m = Media.movie(title: "Captain America: Civil War", director: "Russo Brothers", year: 2016)

if case let Media.movie(title, _, _) = m {
  print("This is a movie named \(title)")
}

This is equivalent to the more verbose code:

switch m {
  case let Media.movie(title, _, _):
    print("This is a movie named \(title)")
  default: () // do nothing, but this is mandatory as all switch in Swift must be exhaustive
}

if case let where

We can combine the if case let with a comma (,) – where each condition is separated by , – to create a multi-clause condition:

  if case let Media.movie(_, _, year) = m, year < 1888 {
    print("Something seems wrong: the movie's year is before the first movie ever made.")
  }

That can lead to quite powerful expressions that would otherwise need a complex switch and multiple lines only to test one specific case.

guard case let

Of course, guard case let is similar to if case let. You can use guard case let and guard case let … , … to ensure something matches a pattern and a condition and exit otherwise.

enum NetworkResponse {
  case response(URLResponse, Data)
  case error(Error)
}

func processRequestResponse(_ response: NetworkResponse) {
  guard case let .response(urlResp, data) = response,
    let httpResp = urlResp as? HTTPURLResponse,
    200..<300 ~= httpResp.statusCode else {
      print("Invalid response, can't process")
      return
  }
  print("Processing \(data.count) bytes…")
  /* … */
}

for case

Combining for and case can also let you iterate on a collection conditionally. Using for case … is semantically similar to using a for loop and wrapping its whole body in an if case block: It will only iterate and process the elements that match the pattern.

let mediaList: [Media] = [
  .book(title: "Harry Potter and the Philosopher's Stone", author: "J.K. Rowling", year: 1997),
  .movie(title: "Harry Potter and the Philosopher's Stone", director: "Chris Columbus", year: 2001),
  .book(title: "Harry Potter and the Chamber of Secrets", author: "J.K. Rowling", year: 1999),
  .movie(title: "Harry Potter and the Chamber of Secrets", director: "Chris Columbus", year: 2002),
  .book(title: "Harry Potter and the Prisoner of Azkaban", author: "J.K. Rowling", year: 1999),
  .movie(title: "Harry Potter and the Prisoner of Azkaban", director: "Alfonso Cuarón", year: 2004),
  .movie(title: "J.K. Rowling: A Year in the Life", director: "James Runcie", year: 2007),
  .website(urlString: "https://en.wikipedia.org/wiki/List_of_Harry_Potter-related_topics")
]

print("Movies only:")
for case let Media.movie(title, _, year) in mediaList {
  print(" - \(title) (\(year))")
}
  
/* Output:
Movies only:
 - Harry Potter and the Philosopher's Stone (2001)
 - Harry Potter and the Chamber of Secrets (2002)
 - Harry Potter and the Prisoner of Azkaban (2004)
 - J.K. Rowling: A Year in the Life (2007)
*/

for case where

Adding a where clause to that all can make it even more powerful:

print("Movies by C. Columbus only:")
for case let Media.movie(title, director, year) in mediaList where director == "Chris Columbus" {
  print(" - \(title) (\(year))")
}

/* Output:
Movies by C. Columbus only:
 - Harry Potter and the Philosopher's Stone (2001)
 - Harry Potter and the Chamber of Secrets (2002)
*/

💡 Note: Using for … where without the case pattern matching part is also a valid Swift syntax. For example you can write:

for m in listOfMovies where m.year > 2000 { … }

It is not using pattern matching (no case nor ~=) so that’s a bit out of the scope of this article series, but it’s still totally valid and as useful as the other constructs presented here — as it avoids wrapping the whole body of your for within a big if (or starting it with a guard … else { continue }).

Combining them all

Let’s finish this series with the Grand Finale: combine all that we learned from the beginning (including some syntactic sugar like x? we learned in the previous article):

extension Media {
  var title: String? {
    switch self {
    case let .book(title, _, _): return title
    case let .movie(title, _, _): return title
    default: return nil
    }
  }
  var kind: String {
    // Remember part 1 where we said we can omit the `(…)` associated values in the `case` if we don't care about any of them?
    switch self {
    case .book: return "Book"
    case .movie: return "Movie"
    case .website: return "Web Site"
    }
  }
}

print("All mediums with a title starting with 'Harry Potter'")
for case let (title?, kind) in mediaList.map({ ($0.title, $0.kind) })
  where title.hasPrefix("Harry Potter") {
    print(" - [\(kind)] \(title)")
}

This look might look a little complex, so let’s split it down:

• It uses map to transform the Array<Media> array mediaList into an array of tuples [(String?, String)] containing the title (if any) + the kind of item (as text)
• It only matches if title? matches — which is syntactic sugar to say if .Some(title) matches — the $0.title of each media. This means that it discards any media for which $0.title returns nil (a.k.a. Optional.None) — excluding any WebSite in the process, as those don’t have any title)
• Then it filters the results to only iterate on those for which title.hasPrefix("Harry Potter") is true.

So in the end this will loop on every medium that has a title starting with “Harry Potter”, discarding any medium that don’t have a title — like WebSite — as well as any medium having a title that doesn’t start with "Harry Potter" — excluding the J.K. Rowling documentary from that iteration as well.

The code will thus output this, only listing Harry Potter books and movies:

All medium with a title starting with 'Harry Potter'
 - [Book] Harry Potter and the Philosopher's Stone
 - [Movie] Harry Potter and the Philosopher's Stone
 - [Book] Harry Potter and the Chamber of Secrets
 - [Movie] Harry Potter and the Chamber of Secrets
 - [Book] Harry Potter and the Prisoner of Azkaban
 - [Movie] Harry Potter and the Prisoner of Azkaban

Using neither pattern matching nor any where clause nor syntactic sugar that we learned in this article series, the code might have looked like this instead:

print("All mediums with a title and starting with 'Harry Potter'")
for media in mediaList {
  guard let title = media.title else {
    continue
  }
  guard title.hasPrefix("Harry Potter") else {
    continue
  }
  print(" - [\(media.kind)] \(title)")
}

Some might find it more readable, but you can’t argue that using for case let (title?, kind) in … where … is really powerful and allow you to impress your friends make great use of for loops + pattern matching + where clauses altogether. ✨

Conclusion

This is the end of this “Pattern Matching” series. Hope you enjoyed it and learned some interesting stuff 😉

Next articles will be more focused back on some nice Swifty design patterns and architecture than on Swift syntax and language.

💡 Don’t hesitate to tell me on Twitter if you have any particular subject on Swift you want me blog about and give me some ideas for what to write about next!

Thanks to Frank Manno for updating the code samples of this article to Swift 3!