Multi-statement closure type inference

Paul Hudson      @twostraws

SE-0326 dramatically improves Swift’s ability to use parameter and type inference for closures, meaning that many places where we had to specify explicit input and output types can now be removed.

Previously Swift really struggled for any closures that weren’t trivial, but from Swift 5.7 onwards we can now write code like this:

let scores = [100, 80, 85]

let results = scores.map { score in
    if score >= 85 {
        return "\(score)%: Pass"
    } else {
        return "\(score)%: Fail"
    }
}

Prior to Swift 5.7, we needed to specify the return type explicitly, like this:

let oldResults = scores.map { score -> String in
    if score >= 85 {
        return "\(score)%: Pass"
    } else {
        return "\(score)%: Fail"
    }
}

Other changes in Swift 5.7…

• if let shorthand for unwrapping optionals
• Clock, Instant, and Duration
• Regular expressions
• Type inference from default expressions
• Concurrency in top-level code
• Opaque parameter declarations
• Structural opaque result types
• Unlock existentials for all protocols
• Lightweight same-type requirements for primary associated types
• Constrained existential types
• Distributed actor isolation
• buildPartialBlock for result builders
• Implicitly opened existentials
• Unavailable from async attribute

Download all Swift 5.7 changes as a playground Link to Swift 5.7 changes

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