How to denote mutable parameters in closures with Swift > 2.2? - swift

Question

Perhaps this is an Xcode 8 beta issue, however, prior to 2.2 the var keyword is allowed to prepend parameters in function signatures:
func (var stringName: String) { ... }
This is has since been deprecated in lieu of there being little benefit over inout
func (stringName: inout String) { ... }
I've attempted the following in a map closure, and though I don't receive a deprecation warning as mildly expected I should, the error was rather a segmentation fault: 11
let demoString = ["hi", "there", "world"].map { (var word) -> String in
let firstChar = word.remove(at: word.startIndex)
}
The error kicks in as soon as I attempt to mutate the (assumedly mutable) word variable.
I've attempted other variation e.g. using inout
let demoString = ["hi", "there", "world"].map { (word: inout String) -> String in
let firstChar = word.remove(at: word.startIndex)
}
But the compiler complains that this erroneously changes the signature of the closure altogether and won't compile.
Obviously, the workaround is simply to copy the variable to a local one within the closure:
let demoString = ["hi", "there", "world"].map { (word) -> String in
let tempWord = word
let firstChar = tempWord.remove(at: tempWord.startIndex)
}
However, I am interested in knowing if this is expected functionality & whether or not there is a way of mutating a parameter passed into a closure directly?

Answer 1

Closures can mutate inout arguments just fine:
var str1 = "Pine"
var str2 = "Juniper"
let closure = { (s1: inout String, s2: inout String) -> Void in
s1 = "Oak"
s2 = "Chestnut"
}
closure(&str1, &str2)
print(str1, str2)
The problem you are facing is Array.map doesn't have a method signature that includes an inout parameter.
The only way around this that I can think of is to extend Array and add the map method yourself:
extension Array {
func map<T>(_ closure: (inout T) -> Void) -> Array<T> {
var arr = [T]()
for i in 0..<self.count {
var temp : T = self[i] as! T
closure(&temp)
arr.append(temp)
}
return arr
}
}
var hi = "hi", there = "there", world = "world"
var demoStrings = [hi, there, world]
var result = demoStrings.map { (word: inout String) in
word.remove(at: word.startIndex)
}
print(result) // ["i", "here", "orld"]

Answer 2

As per SE-0003 var parameters no longer exist in Swift 3.
Fundamentally, you shouldn't be mutating the parameters given from map, anyway. Instead: use non-mutating functions, or make a local mutable copy.
In this case, there's no reason to be using remove(_:) just to get the first character. This would require copying a String's memory (omitting the first character), solely to get the character that was removed. It's quite clunky.
In this case, you can just get the first property (from the Collection protocol) on the characters property of String.
Try this:
let demoStrings = ["hi", "there", "world"]
let firstLetters = demoStrings.map {(word: String) -> String in
return word.characters.first
}
or for short:
let firstLetters = demoStrings.map{ $0.characters.first }