Archive for the ‘Swift’ Category

Dear Erica: How do I simplify `Swift.print`?

Dear Erica:

In other languages (Python mostly comes to mind, but I think ml and probably Haskell as well) you can essentially create a new name for a function by assigning it to a variable and use that instead.  All type information and usage follows with it.

I use it a lot in Python to have a local variable that’s  already bound to some otherwise nested module call. It saves some lookup time and textual typing. (Gotta be explicit since syntactic typing is relevant in Swift.) In trying to work around the problem of “print” calling  NSDocument.print, I tried declaring a val at the top level:

let Print = Swift.print // for print to console

The compiler seemed happy with the definition but when I tried to use it as in:

Print(“in makeWindowControllers”)

I got:

Hitting Fix did nothing but I wasn’t really expecting much (this is Xcode 9.0 beta). I assumed it wasn’t  telling me to literally put <#String#> in as a second parameter but that the type inference was inferring a type of String for  parameter #2  It didn’t really make any sense (why would a second parameter be required for a variadic argument?) If two, why not 3…?

So I tried the following but to no avail:

Print(“in makeWindowControllers","two”)

One can obviously write a full function which works fine:

func Print(_ items:Any...) -> () {

Print("in makeWindowControllers")

It’s pretty simple but seems unnecessarily heavyweight. I can’t seem to find any way to do a variadic closure. Is this possibly a bug or am I missing something obvious? Is there any reason the simple value declaration shouldn’t work? I know I’ve seen complaints about the lack of an Apply function in Swift. Not sure if this is related. My functional mojo is somewhat lacking…

This is a terrific question, and I’m going to answer it in several parts.

First, the full signature of Swift’s version of print is

public func print<Target>(_ items: Any...,
    separator: String = default, 
    terminator: String = default, 
    to output: inout Target) 
    where Target : TextOutputStream

See all those default items?  They don’t travel nicely to closures.  Second, take careful note of the variadic items, because you can’t pass them along by redirecting them to another function. Your “full function” actually prints an array of the items you pass:

Print("Hello world") // prints ["Hello World"]

The only way around this for 10.12 and earlier is to re-implement print (or a reasonable facsimile). Having tried that this morning, I warn you, it is a doozy. Seriously. You wouldn’t believe how many functions and files go into this one little call.

Fortunately, this name-overlap problem (NSView.print vs Swift.print, etc) is fixed in High Sierra. The 10.13 release notes write:

print() methods in Swift: NSWindow, NSView, NSDocument’s print() instance methods have been renamed to printWindow(), printView(), and printDocument() respectively in Swift 4. This fixes the unexpected experience where adding debug logging to a subclass of one of these instances shows a print panel instead.

You can help the variadics don’t propagate cause (“El viva variadics!“) by filing a radar. Go ahead and dupe, mentioning rdar://problem/12134482.

For the moment, here’s a less capable workaround you can use. It does pretty much what you want with slightly reduced capabilities (and complexity) compared to the built-in Swift.print solution.

public func sprint(
    _ items: Any...,
    separator: String = " ",
    terminator: String = "\n")
    var prefix = ""
    for item in items {
        Swift.print(prefix, terminator: "")
        Swift.print(item, terminator: "")
        prefix = separator

It’s kind of a production-unfriendly workaround for just avoiding Swift.print, but I hope this helps anyway.

Thanks Dave, Tim, Stephen, Caleb

Beta 3 Playground Workarounds

Adding Resources and Sources folders to Playgrounds

Until they’re fixed, you may have to add them by hand.

  1. Right-click/control-click.
  2. Show package contents
  3. You can add new Resources and Sources files at the top level
  4. Alternatively, navigate down to individual pages (which are finally in Beta 3!) by showing their package contents and add them there

Fortunately under Beta 3 you don’t have to manually add pages, as the new page functionality is finally back.

Creating New Playgrounds

They’re no longer listed in the File > New dialog. Instead choose File > New > Playground (Command-Shift-Option-N) or open “Welcome to Xcode” (Command-Shift-1) and click “Get started with a playground”.

Most of the obvious alternatives (like Command-Control-N, which creates new workspaces) are already taken, but if you don’t mind using the menu for that, I think it’s a nicer key binding for “New Playground”. If you want to mess with this open prefs (Command-comma), type playgrounds into the search field, and edit the key binding for “New > Playground”

Don’t forget that new playground page is “Command-Option-N”.

Result types

How do I use result types? When using a Result enum for callbacks, how to access the Error?

The most common Result enumeration looks like this:

enum Result<Value> { 
    case success(Value), failure(Error) }

A Result is used almost exclusively in completion handlers. In synchronous code, it’s more common to use throwing functions rather than Result types:

do {
    let value = try throwingFunction(...)
} catch {
    ... handle error ...

A Result type replaces the (value: Value?, error: Error?) -> Void handler signature used by many Cocoa APIs with this single Swift enumeration. Handling this type requires a slightly different approach than you’d use with thrown error handling.

As a rule, if an error is generated on your behalf, pay attention to it and don’t discard it out of hand. Errors help identify underlying issues that you may be able to resolve. They also provide important information for the developer and end-user of why an operation has failed.

The switch statement provides the simplest approach to handle both result conditions with equal priority:

switch result {
case .failure(let error): 
    // handle error here
case .success(let value): 
    // handle success here

If the error handling code is significantly less detailed than the success code, you might choose to perform early exit instead of using switch. This approach allows you to handle any errors and then move on to processing the returned value at the top scope.

Use an if statement (not a guard statement) to bind error instances. Its primary clause should handle the bound error  and then leave scope.  If the result is success, the if-test will fail. Follow the error check with a guard statement to bind the success value.

if case .failure(let error) = result {
    // handle error
guard case .success(let value) = result 
    else { fatalError("Success value could not be bound") } // this should never happen

// use value

This second approach allows you to promote the typically detailed steps involved in processing a value after extracting it from the Result enumeration. The guard‘s else clause is a little ugly but necessary. Swift doesn’t offer a “forced enumeration unwrap” similar to Optional‘s !.

Breaking the handling down into an if/guard pair is not as elegant as the unified switch statement, but it provides a practical way to promote the importance of the returned value.

Update: If the !! operator is ever adopted, you could extend Result to return a computed var value: Value? member, and then use !! instead of the guard/fatalError combo in the above example to create a streamlined early return / value handling approach:

if case .failure(let error) = result { ... }
let value = result.value !! "Success value could not be bound"

It’s a lot cleaner. See this PR for more details. (Thanks Dave)

Pronouncing “Tuple”

A tuple is a finite ordered list of elements. It is presented as a parentheses-braced, comma-delimited list. In Swift, you can use them as stand-alone heterogenous lists of values or pass them as arguments to functions and closures.

A tuple is pronounced “tupple” (TUH-ple), not “two-pull”. It’s a shortcut pulled from “double, triple, quintuple, sextuple, octuple”, etc. Yes, please note that “quadruple” doesn’t fit with the others and is not used as a basis for speaking the word. Rules about long and short “u”s that apply to other English words are also not relevant to this case.

In Swift, a tuple is analogous to an anonymous struct. Its members are indexed numerically (.0, .1.2, etc). You can also use labels to access members by name:

let point = (x: 5.0, y: 2.3)
print(point.1) // 2.3
print(point.y) // 2.3

Swift 3.0 and later no longer supports splatting, so you cannot decouple a tuple from a function call. You can read more about this in SE-0029, which disallowed the ability. Before the change, you could call a function either with its direct arguments or by passing a tuple:

func foo(a : Int, b : Int) {}
foo(a: 42, b : 17) // still allowed
let x = (a: 1, b: 2)
foo(x) // no longer allowed in Swift 3+

The continued work on SE-0110 is still resolving on how tuples and closures interact.

The word “arity” describes the number of members in a tuple. (It also describes the number of arguments or operands accepted by a function, method, or closure.) The examples above use an arity of 2. You can call this a “2-arity tuple” or the shortcut “2-ary tuple”. Some shorten that further to a “2-tuple”.  All are acceptable.

Some fun facts to finish with:

  • A figurative tuple with an arbitrary arity is an “n-arity” or “n-ary” or “n-tuple”.
  • A tuple with a variable number of arguments is variadic.

Working with optional errors in completion handlers

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?) {

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

More on SE-0110: Important fallout, please read (Updated)

Update: Statement from Austin

…I’d like to appreciate my heartfelt thanks to everyone who reached out to me one way or another. It’s clear to me that the Swift, Apple platform developers, and swift-evolution communities are amazing, and that the people in them are kind, wonderful, generous, passionate, and caring. The Core Team in particular has done an incredible job shepherding the community, befriending people on and off the lists, and leading an open-source project of great technical and social complexity.

After thinking about things, I plan to continue participating in swift-evolution and looking for new ways in which I can serve the Swift and Apple developer communities. I hope to listen more, speak less, be more sensitive to other peoples’ feelings, and offer fair, well-considered feedback.

Often the Swift core team will ask for community help to develop and sponsor a proposal. I’ve worked on several of these. These proposals are generally aimed towards simplifying the compiler, enhancing the language, or addressing technical issues that place stumbling blocks in the effective delivery of compilation.

The reconsideration of SE-0110 should not reflect in any negative way on Austin Zheng. He worked hard on a proposal whose intent was to serve the large Swift developer community. I congratulate Austin for shepherding through this proposal, which can be a long, frustrating process.

The usability regression was unexpected. I applaud the core team for its flexibility in responding to the community’s real concerns when its implementation showed issues.

Today, Austin tweeted:

If my posting of the SE-0110 notice last night contributed to a negative atmosphere, I apologize. I have written to Austin and I hope he will reconsider his decision and rejoin Swift Evolution.

On the SE-0110 Regression (full text)

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


The startling uniquing of Swift 4 dictionaries

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"?>
    <book id="bk101" empty="">
        <title>XML Developer's Guide</title>
        <description>An in-depth look at creating applications with XML.</description>

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

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(, 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...))
// ["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({ ($0.0, [$0.1]) }),
    uniquingKeysWith: { (old: [String], new: [String]) in
        return old + new }
// ["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))
// [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.

Holy War: Mutable copies

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()

// 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…