In Swift, functions can be defined with named parameters, for example:
func greet(person: String) -> String { ... } which need to be used when the function is called :
greet(person: "Anna") But it's not allowed to pass named parameters to closures.
What is the reason for such difference between named functions and closures in the language design?
This was decided in SE-0111, Remove type system significance of function argument labels, which cites the following motivation:
Motivation
...
As currently implemented, this feature can lead to surprising behavior:
func sinkBattleship(atX x: Int, y: Int) -> Bool { /* ... */ } func meetsBattingAverage(ofHits hits: Int, forRuns runs: Int) -> Bool { /* ... */ } var battingAveragePredicate : (ofHits: Int, forRuns: Int) -> Bool = meetsBattingAverage battingAveragePredicate = sinkBattleship // sinkBattleship is invoked battingAveragePredicate(ofHits: 1, forRuns: 2)Removing this feature simplifies the type system.
In other words, the argument labels were in a strange limbo, where they were significant enough to impact the inferred type of variables (e.g. battingAveragePredicate infers as (ofHits: Int, forRuns: Int) -> Bool), but were also loose enough that sinkBattleship could be assigned, despite having different argument labels.
The result is a call where the labels are (ofHits: 1, forRuns: 2), but actually behave as a call to (atX: 1, y: 2). This was confusing, and just got torn out entirely.
foo(_:_:)andbar(_:param:)both take 2 params and might have the same type. Closures do not have a name, they are anonymous functions. You cannot declare a variable with labels (e.g.let myClosure(x:y:) = {...}. Parameter labels get erased when using a function or closure as a value. This changed in Swift 3, see github.com/swiftlang/swift-evolution/blob/main/proposals/…