if case let in Swift defined – Donny Wals

In Swift, we are able to use the case key phrase in a number of locations. Mostly, a case is utilized in switched however because you’re right here, you may need seen a case together with an if assertion.

On this publish, we’ll discover completely different locations the place we are able to use the case key phrase to carry out one thing known as sample matching in Swift.

Sample matching is a strong characteristic of Swift that enables us to carry out extremely elegant checks to see if a given kind matches a sure worth.

On this publish, we’ll discover a selected sort of sample matching in Swift; the if case let strategy of sample matching.

Understanding sample matching

The syntax for if case let is considerably complicated. So let’s begin with a fast code pattern that demonstrates how one can write an if assertion that makes an attempt to match an enum case:

enum ShapeType {
  case rectangle, triangle, circle
}

let myShape = ShapeType.rectangle

if case .rectangle = myShape {
  print("myShape is a rectangle")
}

Now, let me begin by saying we didn’t want to make use of the case syntax right here. We may have simply as effectively written the next:

if myShape == .rectangle {
  print("myShape is a rectangle")
}

Nonetheless, I like the sooner instance as a result of it introduces the case syntax in a reasonably clear means.

Now, earlier than I dig in to indicate you the case let syntax I’d like to try the type of sample matching in Swift that’s almost certainly the one you’re most accustomed to:

change myShape {
case .rectangle:
  print("myShape is a rectangle")
case .triangle:
  break
case .circle:
  break
}

A change in programming permits us to jot down an inventory of patterns that we need to evaluate a given worth to. That is way more handy that writing a bunch of if / else statements.

The case key phrase in Swift doesn’t carry out any particular magic. As a substitute, it invokes a particular operator that compares our sample (no matter we write after case) to a worth (the worth we’re switching over in a change).

So… how does that aid you perceive if case let syntax?

Understanding if case let

As soon as you realize that if case .rectangle = myShape invokes a comparability between .rectangle and myShape the next out of the blue makes slightly extra sense:

enum LoadingState {
  case inProgress(Job<String, By no means>)
  case loaded(String)
}

let state = LoadingState.loaded("Howdy, world")

if case .loaded(let string) = state {
  print("Loaded string is (string)")
}

// or

if case let .loaded(string) = state {
  print("Loaded string is (string)")
}

In each comparisons, we evaluate our enum case of .loaded and we assign its related worth to a relentless. I want case .loaded(let string) myself as a result of it appears to be like rather less unusual that case let .loaded(string) however they’re functionally equal.

And in a change, you’d use the identical patterns to match towards which all the time helps me to recollect:

change state {
case .inProgress(let process):
  break
case .loaded(let string):
  print("Loaded string is (string)")
}

Once more, the sample right here is that we evaluate our case to a worth. In a change this appears to be like much more pure than it does in an if assertion however they’re the identical beneath the hood they usually each use the underlying ~= comparator.

That stated, writing if case .loaded(let string) = state while you’re solely desirous about a single case is definitely extra handy than writing a full blown change while you’re solely desirous about a single case.