Pattern matching with switch | Swift

Swift’s switch does more than compare simple values. It can match shapes of data.

Matching enum data

enum LoginResult {
    case success(userID: String)
    case locked(until: Date)
    case failed(reason: String)
}

switch result {
case .success(let userID):
    print("Welcome \(userID)")
case .locked(let date):
    print("Try again after \(date)")
case .failed(let reason):
    print(reason)
}

Each case extracts only the data that exists for that state.

Matching optionals

Optionals are enums under the hood. You can switch over them:

switch nickname {
case .some(let value):
    print(value)
case .none:
    print("No nickname")
}

Most code uses if let or guard let, but seeing this helps explain why optionals fit Swift’s model.

Matching tuples

Tuples group a small number of values without defining a named type:

let response = (status: 404, hasCachedCopy: true)

switch response {
case (200, _):
    print("Fresh data")
case (400..<500, true):
    print("Show cached copy")
case (400..<500, false):
    print("Show error")
default:
    print("Try again later")
}

Use tuples for local grouping. Use structs when data crosses function boundaries or deserves a name.

Adding conditions

Use where when a pattern also needs a condition:

switch articles.count {
case 0:
    print("No articles")
case let count where count < 10:
    print("Small list")
default:
    print("Large list")
}

What to carry forward

pattern matching checks data shape, not just equality
enum cases can extract associated values
optionals can be matched as .some and .none
tuples are useful for small local groupings
where adds a condition to a pattern

Next, you will use protocols to describe shared behavior.