"contextual closure type expects 2 arguments" error when using reduce in Swift 4

Question

The following code compiles in Swift 3

extension Array where Element: Equatable {
    var removeDuplicate: [Element] {
        return reduce([]){ $0.0.contains($0.1) ? $0.0 : $0.0 + [$0.1] }
    }
}

but produces the error

error: contextual closure type '(_, _) -> _' expects 2 arguments, but 1 was used in closure body

in Swift 4. How to convert this code to be compiled in Swift 4?

Answer 1

The closure passed to reduce takes 2 parameters, e.g. $0 and $1 in the shorthand notation:

extension Array where Element: Equatable {
    var removeDuplicate: [Element] {
        return reduce([]) { $0.contains($1) ? $0 : $0 + [$1] }
    }
}

(This compiles in both Swift 3 and 4.)

In Swift 3 you could use single parameter $0, which would the be inferred as a tuple with elements $0.0 and $0.1. This is not possible anymore in Swift 4, as a consequence of SE-0110 Distinguish between single-tuple and multiple-argument function types.

Here is another example demonstrating the change: This

let clo1: (Int, Int) -> Int = { (x, y) in x + y }
let clo2: ((Int, Int)) -> Int = { z in z.0 + z.1 }

both compiles in Swift 3 and 4, but this

let clo3: (Int, Int) -> Int = { z in z.0 + z.1 }

compiles only in Swift 3, not in Swift 4.