Foundation likes to pass optional errors (versus, say, a unified Result type) to completion handlers. A typical closure uses a (Value?, Error?) -> Void signature,where Value is some sort of data result that varies by operation.

A colleague was struggling to use conditional binding along with casting in his handler. Leaving aside for the moment any rational need to cast to NSError, this is an interesting demonstration of how you perform these two operations synchronously in code.

A cast from Error to NSError is guaranteed to succeed, so you can use the as operator. However, you must phrase the cast just right. Otherwise the Swift compiler emits warnings, as you see in the following screenshot. Here is an example of how you don’t get this job done properly:

As my friend complained in frustration: “But it tells me “did I mean as” and when I switch `as?` to `as` then it complains that error is Error? and isn’t convertible to NSError.”

This attempt followed the normal pattern of conditional casting. Swift automatically lifts double optionals into a single optional result when used this way, but the cast from Error to NSError will always succeed, so you can’t use as? here.

To resolve, use a non-conditional cast to NSError? and then perform conditional binding to unwrap the value:

enum Bad: Error { case luck }

let error: Bad? = Bad.luck

if let error = (error as NSError?) {
    print("Worked")
}

The parens around the non-conditional cast make all the difference. Swift removes its warnings and makes everything work as expected.

Doug Gregor writes:

Hello Swift community,

Swift 3’s SE-0110 eliminated the equivalence between function types that accept a single type and function types that take multiple arguments. However, for various implementation reasons, the implementation of SE-0110 (as well as the elimination of tuple “splat” behavior in SE-0029) was not fully completed.

 

Swift 4 implemented more of SE-0110, which caused a fairly serious usability regression, particularly with closures. Here are a few simple examples involving closures that worked in Swift 3 but do not work in Swift 4:

// #1: Works in Swift 3, error in Swift 4

myDictionary.forEach {

  print(“\($0) -> \($1)”)

}

 

// #2: Works in Swift 3, error in Swift 4

myDictionary.forEach { key, value in

  print(“\(key) -> \(value)”)

}

 

// #3: Works in Swift 3, error in Swift 4

myDictionary.forEach { (key, value) in

  print(“\(key) -> \(value)”)

}

Similar issues occur with passing multi-argument functions where a tuple argument is expected:

// #4: Works in Swift 3, error in Swift 4

_ = zip(array1, array2).map(+)

In all of these cases, it is possible to write a closure that achieves the desired effect, but the result is more verbose and less intuitive:

// Works in both Swift 3 and Swift 4

myDictionary.forEach { element in

  let (key, value) = element

  print(“\(key) -> \(value)”)

}

 

The Swift core team feels that these usability regressions are unacceptable for Swift 4. There are a number of promising solutions that would provide a better model for closures and address the usability regression, but fully designing and implementing those are out of scope for Swift 4.  Therefore, we will “back out” the SE-0110 change regarding function arguments from Swift 4.

 

Specifically, when passing an argument value of function type (including closures) to a parameter of function type, a multi-parameter argument function can be passed to a parameter whose function type accepts a single tuple (whose tuple elements match the parameter types of the argument function). Practically speaking, all of the examples #1-#4 will be accepted in both Swift 3 and Swift 4.

 

We will revisit the design in this area post-Swift 4.

 

– Doug

As you’ve probably heard, Swift 4 now has multiline strings. Rejoice! And thank John Holdsworth. For now you can do stuff like this:

let xml = """
    <?xml version="1.0"?>
    <catalog>
    <book id="bk101" empty="">
        <author>\\(author)</author>
        <title>XML Developer's Guide</title>
        <genre>Computer</genre>
        <price>44.95</price>
        <publish_date>2000-10-01</publish_date>
        <description>An in-depth look at creating applications with XML.</description>
    </book>
    </catalog>
"""

It’s super handy, allowing you to incorporate newline and individual " characters without having to escape them. (You do have to escape the backslash, as in the preceding example).

One of the things you might want to do with a big hefty string is to count the number of words, and maybe find out which word occurs the most. So here’s another multi-line string, one pulled from a lorem ipsum generator:

let lipsum = """
    Lorem ipsum dolor sit amet, consectetur adipiscing elit. Curabitur vitae hendrerit orci. Suspendisse porta ante sed commodo tincidunt.

    Etiam vitae nunc est. Vestibulum et molestie tortor. Ut nec cursus ipsum, id euismod diam. Sed quis imperdiet neque.

    Mauris sit amet sem mattis, egestas ligula ac, fringilla ligula. Nam nec eros posuere, rhoncus neque ut, varius massa.
    """

This particular example occupies 5 lines and includes a lot of text and punctuation. Because you can now treat Strings as collections, you can do stuff like this:

let w = "Hello".filter({ $0 != "l" }) // "Heo"

Similarly, you can use character set membership to select only letters and spaces:

let desiredCharacters = CharacterSet.letters
    .union(CharacterSet(charactersIn: " "))
let workString = lipsum.filter({ character in
    let uniScalars = character.unicodeScalars
    return desiredCharacters
        .contains(uniScalars[uniScalars.startIndex])
})

Unfortunately, Character and CharacterSet are still struggling a bit to get along with each other, which is why I’m doing that nonsense with the unicodeScalars.  Anyway, this gives you a single line string with just letters and spaces, so you can then break the string into words.

// Split along spaces
let words = workString.split(separator: " ")

Dictionary now has a feature that allows you to recognize you’re overwriting an existing key and apply a function to a key’s value each time the key is added. It’s called uniquing, and it lets you do neat things like count the number of times a token appears in a sequence:

// Add to dictionary, with "uniquing"
let baseCounts = zip(words.map(String.init), repeatElement(1, count: .max))
let wordCounts = Dictionary(baseCounts, uniquingKeysWith: +)

This code creates an infinite sequence of the number 1, and applies addition each time a duplicate key is found. You get exactly the same results by applying + 1 closure, although this is uglier and a little wasteful:

let wordCounts = Dictionary(baseCounts, 
    uniquingKeysWith: { (old, _) in old + 1 })

You can then find the word that appears the most

// Find the word that appears most often
var (maxword, maxcount) = ("UNDEFINED", Int.min)
for (word, count) in wordCounts {
    if count > maxcount { (maxword, maxcount) = (word, count) }
}
print("\(maxword) appears \(maxcount) times")
// et appears 8 times (at least it did 
// in my much longer text)

You can use uniqueKeysWithValues to fill up a dictionary by zipping two sequences:

let letterOrders = Dictionary(uniqueKeysWithValues: zip("ABCDEFGHIJKLMNOPQRSTUVWXYZ", 1...))
print(letterOrders)
// ["H": 8, "X": 24, "D": 4, "J": 10, "I": 9, "M": 13, "Z": 26,
//  "S": 19, "A": 1, "C": 3, "N": 14, "Y": 25, "R": 18, "G": 7, 
//  "E": 5, "V": 22, "U": 21, "L": 12, "B": 2, "K": 11, "F": 6, 
//  "O": 15, "W": 23, "T": 20, "P": 16, "Q": 17]

Another thing you might do with updated dictionaries is to build a set or array out of sequence values. This next example collects values for each key:

let invitedFriends: [(String, String)] = [
    ("Rizwan", "John"), ("Rizwan", "Abe"),
    ("Soroush", "Dave"), ("Joe", "Dave"), 
    ("Soroush", "Zev"), ("Soroush", "Erica")]
let invitationLists = Dictionary(
    invitedFriends.map({ ($0.0, [$0.1]) }),
    uniquingKeysWith: { (old: [String], new: [String]) in
        return old + new }
)
print(invitationLists)
// ["Rizwan": ["John", "Abe"], "Soroush": ["Dave", "Zev", "Erica"], "Joe": ["Dave"]]

You can store a tuple of the maximum and minimum values found for each unique key. The value structure has to be established in the initial streams, which can be ugly:

// Create 100 random numbers
let hundredRandom: [(Int, Int)] = (1...100).map({ _ in let value = Int(arc4random_uniform(10000)); return (value, value) })

// Create ten sequences of 1 through 10
let tens = sequence(state: 1, next: { (value: inout Int) -> Int in
    value += 1; return (value % 10) + 1
})

// Build the two together
let values = zip(tens, hundredRandom)
let extremes = Dictionary(values, uniquingKeysWith: { (old: (Int, Int), new: (Int, Int)) in
    return (min(old.0, new.0), max(old.1, new.1))
})
print(extremes)
// [10: (504, 8342), 2: (770, 8874), 4: (164, 7871), 9: (177, 8903), 
//  5: (1707, 9627), 6: (577, 8318), 7: (174, 8818), 3: (2837, 9198),
//  8: (3573, 9432), 1: (474, 8652)]

I probably could have made this a little more elegant but I was running out of time because I had to pick up my kids. If you have improvements for the last few examples, let me know. Sorry about the rush.

p.s. Thanks for the tip about using unicodeScalars on char.

Applying mutableCopy() to an NSObject returns Any, not the version of the type you’re attempting to make mutable, for example, NSMutableArray, NSMutableParagraphStyle, NSMutableAttributedString or whatever.

Nate asks:

Is is acceptable to use as! with mutableCopy() or is there a better way to do this?

// Approach 1: Forced unwrap
let mutableStyle1 = style.mutableCopy() as! NSMutableParagraphStyle

The forced as! cast used in this approach will always succeed (even if using as! makes you want to wash your hands afterwards). But there are other approaches to consider. What do you think of these alternative takes on the question? Here are some other solutions for you to weigh in on.

// Approach 2: Forced unwrap with explanation on failure

/// Very low precedence group
precedencegroup VeryLowPrecedence { lowerThan: FunctionArrowPrecedence }

infix operator !!: VeryLowPrecedence

/// Guaranteed safe unwrap or fatal error with custom error string
public func !! <Wrapped>(value: Wrapped?, complaint: String) -> Wrapped {
    guard let value = value
        else { fatalError(complaint) }
    return value
}

let mutableStyle2 = style.mutableCopy() as? NSMutableParagraphStyle !! "Guaranteed cast to mutable paragraph style failed"

// Approach 3: Guard with explanatory fatal error
guard let mutableStyle3 = style.mutableCopy() as? NSMutableParagraphStyle
    else { fatalError("Guaranteed cast to mutable paragraph style failed") }

// Approach 4: Create then set with current attributes
let mutableStyle4 = NSMutableParagraphStyle()
mutableStyle4.setParagraphStyle(style)

// Approach 5: Protocol to expose typed mutable version
public protocol AvailableMutatingVersion: NSObjectProtocol {
    associatedtype MutableForm
    func mutableCopy() -> Any
    var mutableVersion : MutableForm { get }
}

extension AvailableMutatingVersion {
    public var mutableVersion: MutableForm {
        guard let copy = self.mutableCopy() as? MutableForm
            else { fatalError("Guaranteed mutable copy could not be constructed") }
        return copy
    }
}

extension NSParagraphStyle: AvailableMutatingVersion {
    public typealias MutableForm = NSMutableParagraphStyle
}

let mutableStyle5 = style.mutableVersion

Which approach reigns supreme? Vote now or offer some alternatives…