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Switches and Optionals

Switches provide a nice shortcut for nil testing

I may be straying from our traditional “things you know how how to do in Objective-C” bit – unwrapping is not a thing we needed before Swift – but I can’t help but share this pattern I’ve been using.

As I get more and more accustomed to the places that optionals belong in my Swift code, I keep finding new ways to handle those clunky spots where they feel unwieldy. This is great because I really like the idea of optionals. There are so many ideas in programming that can be thought of as either always having a value or sometimes being nil, so the distinction is apt. Finding ways to handle optionals gracefully makes me even more convinced they’re a great choice for Swift.

One of those spots where optionals were feeling clunky was configuring UITableViewCell object from some state enum which happened to be optional because it was loaded asynchronously. Using if let blocks everywhere was a pain, particularly in this case because they were always immediately followed by a switch statement which was leading down the path to the Pyramid of Doom.

Here’s how it might have looked before:

// somewhere else we've defined our enum as such:
enum Status {
  case Available
  case Unavailable
  case Unknown

// unwrap...
if let status = self.status {
  // and then figure out our status
  switch status {
  case .Available, .Unavailable:
    print("a status")
    print("no status")
} else {
  print("no status")

Fortunately, we can combine these two statements with some interesting syntax and then extend that to deal with optionals in different ways.

We know that optionals are actually an enum type made up of .Some(A) and .None. This represents the cases that we can encounter when we have an optional: either some type or nothing.

We can use this in our switch to check optionals without having to do that same step beforehand. Try this:

switch self.status {
case .Some:
  print("a status")
  print("no status")

Sanity restored to our indentation. I mentioned configuring UITableViewCell instances previously because you need to look at your state in a few different places like cellForRowAtIndexPath: and didSelectCellAtIndexPath:. Trimming these down a level of indentation makes this feel like less of a pain and often you can combine two common states (no state and unknown state) in a single case rather than both the outer if let statement and the inner switch.

Now the extended part: even if you don’t configure your table views this way, you can still use this method when checking multiple optionals for nil. We simply make a switch statement where the only valid case for the inputs is .Some and the rest hit the default case.

Here’s a situation where you have multiple optional inputs to validate and not a lot of code needed to do it[1]:

switch (self.textValidation, self.passwordValidation) {
case (.Some, .Some):
  print("both look good!")
  print("something was nil...")

There are a few more powerful uses for switch along these lines, including conditional cases with where and ignoring inputs with _ but hopefully we’ll get to those in another post.

The Swift switch continues to amaze and I doubt this will be the last time I bring it up on this blog. We’re almost a year into our public understanding of Swift and new ways to solve problems are still being “discovered.” That’s pretty great.

One more great example from @mmertsock. Say you want default-like behavior with an optional but without nesting your switches (one for the nil case, one for a non-nil catch-all case). You can use .Some(_) to match all cases where the switch is non-nil but still has any value!

switch (self.status) {
case .Some(.Available):
  print("status is available")
case .Some(_):
  print("some other non-nil status")
case .None:
  print("status was nil...")

[1] OK, I admit that this won’t be needed for long since Swift 1.2 will let us chain if let optionals but you might still use a switch for this considering how powerful and clear they are over lots of nested ifs and elses.

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