Archive for the ‘Swift’ Category

Prototyping CoreGraphics in the Playground

No matter how flaky, I love using playgrounds to prototype Core Graphics, SpriteKit, and many other see-as-you-go technologies. They’re fantastic for building out specific custom content with a bare minimum of coding investment. You get a lot of win for very little time.

I was helping someone out the other day, explaining the strokeEnd keypath (versus the path keypath) and building a playground showed it off to perfection.

Admittedly, it helps to have quick helper code on-hand for quick starts. I have playground-specific setup code, handing me a view controller (called vc) and a centered view, ready to start demo-ing in this one.

I also have a couple of pages of code (like the layer(path:) constructor, the animateStrokemethods, and the schedule() utility off page, in the support module. They’re all highly reusable. It’s a pity in-playground debugging is so dreadful. It would be an ideal module-building tool if not for that: build and explore (and ideally build tests) in a single place, without having to be in a fixed workspace lacking the exploration feature. Adding “convert this exploration into a test” would be icing on top.

I’m disappointed that playground-specific visualizations built for teaching and demos don’t transfer to the debugger for real-world production support. I don’t see any reason why a CGPoint instance should get a pretty graphic representation but a CGAffineTransform, for which I have quite a full presentation, does not.

I can use custom mirroring to produce valuable output for dump, and therefore for printing objects in the debugger but not for debug quicklooks. Plus as far as I can tell the custom NSObject-only quicklooks haven’t been updated in years and there’s no hint of extending this to structs and enums.

By the way, what’s the deal with all the API audits? How long are these going to go on? If you thought updating the app delegate was a minor nuisance, you haven’t seen what’s happened to all the constants and Core Graphics APIs. This update is huge and disruptive…

Unwinding a modal segue on a single view controller

For whatever reason, my unwind segues for modal presentations weren’t working when my presenting source was my navigation’s root controller. This appears to be a common issue if you hunt around for “modal unwind doesn’t work”. A lot of the advice simply went: “You just can’t do that.”

After spending far too much time dealing with, I discovered that the autocomplete version, namely:

@IBAction func unwind(for unwindSegue: UIStoryboardSegue, towards subsequentVC: UIViewController)

doesn’t work. Instead, what you need is:

@IBAction func unwindFor(_ unwindSegue: UIStoryboardSegue, towards subsequentVC: UIViewController)

If you notice, the difference is unwind(for vs unwindFor(. I figured this out by implementing

func canPerformUnwindSegueAction(_ action: Selector, from fromViewController: UIViewController, withSender sender: Any) -> Bool

and looking at the selector it expected to perform (specifically unwindFor:towards:). Moving the For outside fixed the problem and granted me a working unwind, allowing me to fetch state data from the modal controller and pass it back to the source.

I assume this issue arose because of the automatic translation of Cocoa APIs into Swift and I wonder if there are other issues that might arise from the same. Have you encountered such things? What are your thoughts?


Tuple Initialization

I’ve been wanting  tuple-initializable in Swift for Core Graphics, where it’s a drag to always use long and cumbersome initializers when I’m not building production code:

let point: CGPoint = (100, 50)

Yeah, it is better form to use labeled initializers but I’m anyone using CGPoint understands the correspondence between (x, y) coordinates. And I don’t want to just build CGPoint.init(_ x:, _ y:) extensions. I like the tuple form.

Right now, the closest you can get is the silly:

let point = (CGPoint.init as (CGFloat, CGFloat) -> CGPoint)(100, 50)

And that’s (pardon the pun) pointless.

It would be pretty cool to allow memberwise tuples with or without labels in place of  initializers when the tuple field types could be unambiguously determined:

let point: CGPoint = (x: 100, y: 50) 
// instead of: let point = CGPoint(x: 100, y: 50)
let person: Person = ("Mary", nil, "Santos")
// instead of: let person = Person(first: "Mary", middle: nil, last: "Santos")

It may be ugly but it would be hella useful in Playgrounds.

Straw Poll: Unwrapping solutions

Swift should be able to mitigate two issues related to forced unwrapping. First, it’s used as a bandaid by developers new to the language who want to make their code compile. Second, developers should be able to provide code-level annotation support for why a guaranteed wrap cannot fail and provide runtime diagnostics in any “Black Swan” scenario where they do. These items are discussed further in this proposal.

Please let me know which of the following designs you prefer. Each contains a link to a code solution. (The first item in the list (“Unwrap or Die”) links to a full proposal so scroll down to the design section.) The other two offer alternate designs. You can assume !! can be redesigned to support both throwing and Never solutions just as easily as fail or ??. The proposal goes into detail as to why that was not the original design, as doing so fundamentally changes nil-coalescing semantics.

Thank you in advance for your feedback and/or participation in the survey.

link to poll

Creating a low-cost ADD/ADHD refocusing band

My middle child recently had an evaluation regarding her processing and retention in reading. She has diagnosed ADD, and aside from the specific results of her tests, the specialist recommended we look for and purchase a device that would buzz her wrist at regular intervals when doing homework and reading. The goal is to refocus when one is easily distractable. It’s a bit like Apple’s “stand up and move” reminder.

However, when we headed over to Amazon, we were met by two realities. First, these things cost a lot. Second, they have terrible reviews. It occurred to me that I could probably put together a band with things we already had around the house.

Last summer, I did a little work exploring BLE, the low energy form of Bluetooth that works with iOS. My test platform was the 1st generation Mi Band step tracker, which I purchased for under $20. The second generation is similar and appears to cost under $30. (Here’s a link to the first gen model, which is sold by third parties.)

My build consisted of the following components:

  • I built a basic single-view app and added a single centered segmented control. The control specifies the time-out interval, which is simply a standard Timer.
  • When the Timer activates, it uses my Bluetooth helper type to write a single byte to the band, which causes it to buzz. It’s slightly more complicated than that because the code needs to scan for the device, discover it, stop scanning, and then write to the peripheral, but that’s all covered in my previous posts.
  • To keep the app running longer than 3 or 10 minutes in the background, I resort to the standard “play a silent wav file over and over”. I based my code on this simple github repo, which handles audio interrupts and restarts.
  • I added every background mode I thought potentially applicable: plays audio/video streams, uses CoreBluetooth, provides VoIP services. I could probably have dropped the first one but it does no harm so I kept it.

Testing was, as you’d expect, a bit tedious, especially trying to figure out whether I had beaten the automatic time out (which is why went from my custom code to the github version for keeping alive). I put the pedometer portion on an empty diet coke can, to make it buzz a lot louder without having to wear the band.

In the end for a few hours of my time and under $20 capital investment, I ended up with a handy little tool. My daughter has only been using it a few days so it’s still too early to see whether the refocusing component is actually effective.

If you want to give this a try, I’ve put up gists for the primary view controller and the bluetooth controller. You can grab the background handler from the above github  repo link. I didn’t bother cleaning up any of my code, so it is what it is, which is a working prototype. Don’t forget to add the background modes to your Info.plist.

If you build this yourself (or just intend to try it out), drop me a note or a comment and let me know how it goes.

Swift Stories: Please share yours

I’m looking for people who have intentionally avoided transitioning to Swift or who get frustrated with Swift due to changes in the language or who have fought for Swift adoption at their place of work. If any of these scenarios apply to you, please send me an email at erica at ericasadun dot com.

Please let me know if I can use your name or not and what your personal story is in terms of Swift adoption (or lack thereof). It would really help if you let me know the big picture reasons motivating your choices.

Thank you in advance.

p.s. For those confused by this post, I’m doing a talk about Swift adoption and participating in the Swift Evolution process: “The future of Swift belongs to those who show up”.

Evolve Swift: Part 1 – Building your Tools


If you’ve made the slightly rash but equally rewarding decision to dive into developing the Swift language then make sure you have access to a fast computer with good, fast storage. What you don’t want to do is be running an old Mac mini using an old external hard drive because Apple hasn’t updated the mini line in a few forevers.

If your “About this Mac” panel looks like this, seriously question why you’re trying to do this in the first place. For the sake of uncomplicated narrative, I’m going to assume that just about everyone out there in the real world has a better Mac with better memory and primary disk than I do:

For those interested in the naming from these screen shots: Esopus Spitzenburg., Kiku, and Broxwood Firewhelp. I have others but I think those are the names you might see in screen shots and text examples. Hopefully this will be the first of several posts about the topic, in honor of Paul Cantrell.

Embrace the Wait

Before you can change the compiler, you must get the compiler downloaded, compiled, and working in its most recent form. For me, this is multi-day event. You probably will have slightly fewer roadblocks. Still, keep your day open and focus on other work as your Mac may or may not wind tunnel its way through the process.

Even after you’ve got your toolchain under control and build, you’ll continue to have many many waits in your development process as you’ll want to keep updating your repo to match the current state of Apple’s master and its supporting tools. Each time you do this, you can expect a morning of recompiling before you can start working on anything significant.

During this time, you cannot reasonably explore, test, and build unless you do so on a separate machine or for distinct installs. It’s a big pain in the ass but it’s part and parcel of developing the language.

If you plan to do this casually, and a week  has passed between the last pull from upstream/master and now, don’t expect to be productive until the afternoon at the earliest.

Although Swift’s toolset provides a quick iteration tool (called “Ninja”), that adds fast incremental builds, getting up to date and staying up to date means a large time commitment.

Further, don’t be shocked when your compiler or tools simply fail to build. They do that sometimes. It’s utterly frustrating to be stuck with a broken build for whatever reason. Incorporating workarounds into your toolset is a big part of swifting.

If you can’t embrace the wait, you don’t want to be developing swiftlang.

Selecting Xcode

Ensure that your default Xcode is set correctly. Use xcode-select -p to check which version is selected. If it isn’t your main app, use the utility to establish the right one. I use the production (not beta) version of Xcode:

sudo xcode-select -s /Applications/

Package Managers

Make sure you have a good package manager installed. I use Homebrew. I know there was some kind of controversy about that a while ago but I can’t remember why and it does the job for me.

Update your package manager to the most recent version:

brew update

If you run into any issues, you can do a full remove and re-install. These calls require bash and will misfire in csh/tcsh:

Uninstall Brew (removes all existing packages):

ruby -e "$(curl -fsSL"


ruby -e "$(curl -fsSL"

Get cmake and ninja

Grab a copy of cmake and ninja if you do not already have them installed on your system. CMake provides a cross-platform build/test/package solution that generates appropriate Make files, specialized to your environment. Ninja offers a speedy solution for incremental builds. It’s really fast, so once you’ve built out your libraries, you can go in and ninja swift for a really fast update when you’re doing edit/build/test cycles.

brew install cmake ninja

Go ahead and install sphinx now too. Sphinx “makes it easy to create intelligent and beautiful documentation” :

brew install sphinx


Because of the long, long nature of setup and update process, consider some tools to help keep you on top of tasks, so you know when they finish and can move on to your next steps.

I use a command-line utility called noti to announce when each stage of downloading and compilation completes. Noti is an open source tools that triggers feedback as a command line process finishes.

You can have the utility speak, create a macOS system notification, and more. It’s helpful for when you’re running long tasks in background windows. Thanks goes to Scott Robbins for his recommendation.

To install noti, follow the directions in the repo’s Readme file. If you elect to curl a copy to your system, use bash, not csh (or in my case tcsh), for copy/pasting to your command line. The utility runs fine in tcsh but its escaping assumes bash is running.

Once installed, just prefix any command line command with noti followed by flags. I prefer using noti -b -s, which gives me both a banner alert and spoken text.

% noti --help
  noti [flags] [utility [args...]]

noti tar -cjf music.tar.bz2 Music/

  -t, --title string     Set notification title. Default is utility name.
  -m, --message string   Set notification message. Default is "Done!".
  -b, --banner           Trigger a banner notification. This is enabled by default.
  -s, --speech           Trigger a speech notification.
  -c, --bearychat        Trigger a BearyChat notification.
  -i, --hipchat          Trigger a HipChat notification.
  -p, --pushbullet       Trigger a Pushbullet notification.
  -o, --pushover         Trigger a Pushover notification.
  -u, --pushsafer        Trigger a Pushsafer notification.
  -l, --simplepush       Trigger a Simplepush notification.
  -k, --slack            Trigger a Slack notification.
  -w, --pwatch int       Monitor a process by PID and trigger a notification when the pid disappears. (default -1)
  -f, --file string      Path to noti.yaml configuration file.
      --verbose          Enable verbose mode.
  -v, --version          Print noti version and exit.
  -h, --help             Print noti help and exit.

Fork Swift and Clone

Starting on Github, log into your account and then fork Apple’s Swift repo. Navigate to, and click fork:

My fork uses my erica github account. Adjust the following directions accordingly so they correspond to your fork.

Hop over to a nice fast drive, and create a development directory. Enter the directory (cd swiftdev, for example) and clone the Swift repo:

git clone

If you want to get a notification and time how long this took, your command may look like this:

noti -b -s time git clone

I’ll skip the noti and time references from here on so they don’t clutter up these instructions. I like how time-ing gives you a sense of how long each task takes.

Install Supporting Utilities

Grab copies of the supporting utilities using the built-in update-checkout utility. There’s quite a few supporting folders that need to be built, including llvm, clang, and more. If you’re following these instructions, make sure you’re in the same top level directory as your swift install and not in the swift repo. Otherwise, adjust the path for the update-checkout to drop the initial swift:

% pwd
% ls
% ./swift/utils/update-checkout --clone
Skipping clone of 'swift', directory already exists
Running ``obtain_additional_swift_sources`` with up to 8 processes.
Cloning 'compiler-rt'
Cloning 'llvm'
Cloning 'swift-xcode-playground-support'
Cloning 'swift-corelibs-foundation'
Cloning 'clang'
Cloning 'llbuild'
Cloning 'cmark'
Cloning 'lldb'

Now wait for everything to clone. There’s another way to clone using ssh, but I’ve never done it. I just know that it exists and is documented on Apple’s Swift repo Readme file.


You should now be ready to perform a release build. I’m differing here from what’s recommended on Apple’s page because I got help early on from people who didn’t use that method. I’m sticking with my notes because they’re tried and trusted, not because I know what I’m doing.

% cd swift
% noti -s time utils/build-script -R

Building Swift (and, this first time, all its supporting utilities) is not…swift. So go do something else for a significantly long time. Unlike your initial Swift build and when you perform updates by pulling from Apple’s upstream master, incremental changes can be relatively quick. Getting things done the first time and after each update is long and tedious.


If you’ve built things properly, you’ll find a newly build swift compiler waiting for you in the build folder. This directory is located at the top level of your development folder:

% ls

./                       lldb/
../                      llvm/
.DS_Store                ninja/
Hold/                    swift/ [1]
Notes.txt                swift-corelibs-foundation/
build/ [2]               swift-corelibs-libdispatch/
clang/                   swift-corelibs-xctest/
cmark/                   swift-integration-tests/
compiler-rt/             swift-xcode-playground-support/
llbuild/                 swiftpm/

Your cloned swift repo [1] sits along the build folder [2]. Navigate to build > Ninja-ReleaseAssert > swift-macosx-x86_64 > bin to find the swift executable. Check the modification date and run it with the -version flag to confirm it is the right build:

% ls -l build/Ninja-ReleaseAssert/swift-macosx-x86_64/bin/swift 
-rwxr-xr-x  1 ericasadun  staff  96294356 Apr 30 09:23 build/Ninja-ReleaseAssert/swift-macosx-x86_64/bin/swift*

% build/Ninja-ReleaseAssert/swift-macosx-x86_64/bin/swift -version
Swift version 4.2-dev (LLVM bb2a4e8df7, Clang 56384a48b9, Swift 5f9bf115b3)
Target: x86_64-apple-darwin17.5.0

Go ahead and use it to compile a file. Use the path to this build to ensure you don’t accidentally use the default compiler:

% cd ~/Desktop/
% lns ~/github/apple/swift/build/Ninja-ReleaseAssert/swift-macosx-x86_64/bin/swift swift
Created symbolic link at /Users/ericasadun/Desktop/swift
% echo 'print("Hello World")' > test.swift
% ./swift test.swift
Hello World


Getting your initial toolset built and running is the first step on the path to Swift evolution. Next up: ninja builds, creating branches, stashing differences, and managing test builds.

Playing with Strides

Some stride-related discussions took place on the forums today. I was inspired to play with strides as I took a break between my kids’ appointments. If any of this resonates with you as a potential user for it, let me know.

Index Strides

The following extension allows you to stride over a collection’s indices.

extension BidirectionalCollection where Index: Strideable {
    /// Returns the sequence of collection elements (`self[start]`,
    /// `self[start + stride]`, `self[start + 2 * stride]` ... *last*
    /// where *last* is the last element in the collection whose index
    /// is less than or equal to `end`.
    /// - Note: There is no guarantee that the element at index `end`
    ///   is part of the sequence
    /// - Parameter start: The starting index
    /// - Parameter end: The upper index bound
    /// - Parameter distance: The stride for each successive index
    /// - Returns: A lazy sequence of collection members
    public func stride(from start: Index, through end: Index, by distance: Index.Stride) -> LazyMapSequence<StrideThrough<Index>, Element> {
        return Swift.stride(from: start, through: end, by: distance)
  { self[$0] })
    /// Returns the sequence of collection elements (`self[start]`,
    /// `self[start + stride]`, `self[start + 2 * stride]` ... *last*
    /// where *last* is the last element in the collection whose index
    /// is strictly less than `end`.
    /// - Parameter start: The starting index
    /// - Parameter end: The upper index bound
    /// - Parameter distance: The stride for each successive index
    /// - Returns: A lazy sequence of collection members
    public func stride(from start: Index, to end: Index, by distance: Index.Stride) -> LazyMapSequence<StrideTo<Index>, Element> {
        return Swift.stride(from: start, to: end, by: distance)
  { self[$0] })

Here’s an example that uses this approach to form a sequence that strides through the collection:

let myArray = ["a", "b", "c", "d", "e", "f", "g", "h", "i", "j"]
for value in myArray.stride(from: 0, through: myArray.count, by: 3) {
    print(value) // a, d, g, j

Range Operators

The next snippet uses the division operator to create its stride. This was pitched by “Tony Y” in a forum thread.

extension CountableClosedRange {
    static func / (range: CountableClosedRange, divisor: Bound.Stride) -> StrideThrough<Bound> {
        let distance = range.distance(from: range.startIndex, to: range.endIndex) / divisor // deprecated
        return Swift.stride(from: range.lowerBound, through: range.upperBound, by: distance)

In this example, you just “divide” the range:

for idx in (0 ... 100) / 5 {
    print(idx) // 0, 20, 40, 60, 80, 100
    // yes that's 6 not 5, but isn't that what 
    // you want? 5 subranges between each index?

Because I wrote this fairly quickly, I used the deprecated range.distance function as straight index arithmetic seemed to error:

error: binary operator '-' cannot be applied to two 'ClosedRangeIndex' operands
        let distance = (range.endIndex - range.startIndex) / divisor

As usual, I probably messed up somewhere (or several somewheres) or you might see ways I could improve the code. Let me know!

Update: After some thought, I decided I preferred a sequence of subranges (which should probably be a type and not like I do it here):

extension CountableClosedRange {
    static func / (range: CountableClosedRange, divisor: Bound.Stride) -> UnfoldSequence<CountableClosedRange<Bound>, (CountableClosedRange<Bound>?, Bool)> {
        let distance = range.distance(from: range.startIndex, to: range.endIndex) / divisor
        let rangeStride = Swift.stride(from: range.lowerBound, through: range.upperBound, by: distance)
        var indices = Array(rangeStride)
        guard var current = indices.popLast(), var next = indices.popLast() else {
            fatalError("Guaranteed count failed in index population")
        var done = false
        return sequence(first: current ... next.advanced(by: -1), next: { _ in
            guard !done else { return nil }
            (current, next) = (next, indices.popLast()!)
            if current == next {
                done = true
                return next ... next
            } else if next == range.upperBound && indices.isEmpty  {
                done = true
                return current ... next
            } else {
                return current ... next.advanced(by: -1)

for idx in (0 ... 99) / 5 {
    print(idx) // 0...19, 20...39, 40...59, 60...79, 80...99

for idx in (0 ... 100) / 5 {
    print(idx) // 0...19, 20...39, 40...59, 60...79, 80...99, 100...100

This isn’t a debugged solution. For example, trying to divide 1 … 1 by 1 or 1 … 2 by 1 will both fail.

Forcing Compiler Errors in Swift

Thanks to SE-0196, Swift 4.2 introduces #warning() and #error() compiler directives. These will allow you to incorporate diagnostic messages and emit errors during compilation. Here are some examples from the proposal, which has already been accepted and implemented:

#warning("this is incomplete")

  #error("MY_BUILD_CONFIG and MY_OTHER_BUILD_CONFIG cannot both be set")

The #error example uses conditional compilation flags (set with a -D option) to check whether conflicting configurations have been established for the build.

I’ve already written extensively about my dislike for screaming snake case (THINGS_LIKE_THIS) in Swift. Inevitably, it seems, devs use screaming snake case for their conditional compilation flags, whether MY_BUILD_CONFIGMY_OTHER_BUILD_CONFIG, or DEBUG. Although an industry standard, it feels like a clash with Swift’s aesthetics.

I’ve also written about my proposal for detecting debug conditions without having to supply an explicit DEBUG condition flag, so I’ll also leave that topic to the side for now. You can click on the link for more.

Back to topic, Swift’s newly adopted #error and #warning directives represents a big step up from current practices, which often rely on run-time rather than compile-time feedback.

#if !DEBUG
fatalError("This code is incomplete. Please fix before release.")

The unindented style in this snippet is now Swift default, avoiding minor doom pyramids for conditional compilation blocks. Even unindented, surrounding code with those conditional blocks is vertically expansive and subjectively ugly. To counter this, some coders have come up with in-line ways to force compilation (not run-time) errors with minimal condition blocks and a more succinct point-of-use approach.

Here’s an example I discovered from John Estropia. (He, in turn, cribbed it from one of  his co-workers.) He uses conditional compilation to set a TODO or FIXME (or whatever) typealias then uses it in-line at points where a debug build should compile and release builds should error:

#if DEBUG 
internal typealias TODO<T> = T

print("Remove this later") as TODO

It’s clever. Scoping the TODO typealias to debug builds allows lines annotated with as TODO to throw errors during release builds. This ensures compile-time feedback at all points where a TODO cast is performed:

error: ManyWays.playground:5:31: error: use of undeclared type 'TODO'
print("Remove this later") as TODO

It’s not beautiful but it’s effective. It carries information about the call site location and the message you want to emit. If I were applying this hack, I’d probably build an actual todo function rather than using the casting-gotcha. In the following example, I went with an upper camel case name to make the call look more directive-y and less like a standard global function. However, I drew the line at snake case:

internal enum IssueLevel {

internal func ToDo(_ level: IssueLevel, _ string: String) {}

// The point of use offers a compilation error,
// a note, and a priority level
ToDo(.highImportance, "Remove this later")

// error: ManyWays.playground:13:1: error: use of unresolved identifier 'ToDo'
// ToDo(.highImportance, "Remove this later")

The nicest bit is that toggling from debug compilation to release is completely automatic and centralized to a single #if check.  It’s a fascinating approach if adopted consistently and ensures that all compilation message notes like this must be resolved and removed before release.

Right now, Swift does not support a #message directive, which performs a similar tasks. As many shops treat warnings as errors, they cannot establish a nuanced distinction between the two. If #warning were a thing, you could use #message to issue exactly this kind of “fix me” feedback. A further refinement, #messageOrDie (or something like that, because naming is hard)  could message for debug builds and error for release, going by whether assert statements would or would not fire.

Dave DeLong offers another approach for structural project semantics. His introduces a Fatal type to provide runtime cues for common development outcomes including notImplemented(), unreachable(), and mustOverride(). Nothing says “you need to remember to implement this” better than a spectacular runtime crash that explains itself with full position and function context. Another cue, brilliantly named silenceXcode() allows you to add methods that you never intend to implement and which should error if ever called.

There’s still space in Swift for expanding this metadevelopment support. I wouldn’t mind seeing both approaches added to the language: one for compile time (like #messageOrDie) and another for run time (like Fatal‘s namespaced static error members).

What do you think of these? And what parts of the metadevelopment process (like macros) are still MIA for you in Swift? Let me know. I’m curious to hear what else could be better established to support your development.


Writing Swift: Adventures in Compiler Mods

Ever since Swift adopted the “implement first, propose second” rule, some contributors to the Swift Evolution process have felt sidelined and dropped out of the community. It’s frustrating having ideas for positive language changes and not being able to implement those changes.

Despite expertise in Swift and Objective-C (not to mention any number of other languages), they like me may not be proficient in C++ and Python, the core tools of Swift language implementation. To date, my code contributions to Swift have been extremely low level.

I think I fixed a comment, added a string, and maybe one or two other tiny tweaks. (I did work on the Changelog file a while back but that is written in Common Mark, and does not involve programming in the slightest.)

I’ve wanted to be able to build what I can dream. And I’ve slowly been diving into the compiler in recent months to see what it takes to build something new. With quite a lot of hand holding from John Holdsworth, I implemented a couple of build directives to test whether (1) asserts can fire and (2) a build is optimized.

What is Debug?

Answering “what does ‘debug’ mean?” was a harder question than I initially thought. Coming as I do from Xcode, where ‘debug’ means a scheme I select from within the IDE, it took a bit of thinking and advice to think about ‘debug’ from a platform independent viewpoint. After going back and forth on the Swift Evolution email list and later the forums, the consensus centered on the two tests I mentioned above: assertions and optimization.

For many projects, a typical debug build is unoptimized where asserts can fire. As projects move into the beta process, that mindset changes. Many in-house and beta builds meant for wider use need optimization.

The state of the art uses the custom conditional compilation flags set with -D . This approach decouples the meaning of ‘debug’ (or for most developers 'DEBUG') from anything in-language that can be decoupled from build settings and persist through source code. Assert configurations have their own flag, -assert-config <#value#>.

Introducing these two tests lets you align your not-for-release code to assert states and/or optimization states:

#if !configuration(optimized)
    // code for un-optimized builds only

#if configuration(assertsWillFire)
    // code where assertions can fire

The proof-of-concept implementation coupled with a proposal means I may be able to submit a more substantial and meaningful contribution to the language.

Going Solo

Pushing forward, I wanted to test myself and check whether I could make changes on my own, even if that solo journey was quite small. I started by attempting to build the #exit directive that was being discussed on the forums. This turned out to be a little more complicated than I was ready for.

Among other challenges, #exit may or may not be embedded in an #if build configuration test. Using #exit should allow all material up until that point to be compiled into the build while gracefully excluding all material after. I didn’t know how to check whether the directive was embedded in a condition and how to properly complete the condition (#endif) while discarding remaining text. It was, at my stage of the journey, a step too far.

I put my first attempt to the side and tried something else. I tried to push a scope using with(value) { }, so the material within the scope was native to value. That too proved too difficult without assistance although I am beginning to understand how Swift creates and manages its scope. It was a programming failure but a learning success.

Two projects abandoned, I knew I had to pick something very easy to work with. Although I would have loved to have picked up and run with Dave DeLong’s context pitch (which is discussed here) , I recognized that I needed to bite off something smaller first. So I decided to add a #dogcow token that produces the string value `”????????”` in source. How difficult could that be, right?

About five hours and edits to twenty-one files later, I had it working. Kind of. Because I ran into one of the many frequent headdesk situations that plague Swift compiler development. I had focused on my edits to the most recent repo without rebuilding the supporting tool suite.

Ninja Builds

A ninja build is a quick way to build just the compiler component. But at some point you can’t ninja your way into an entire working toolchain. I couldn’t test my changes until I rebuilt everything, a process that can take many many hours on my Mac:

% ./swift ~/Desktop/test.swift
:0: error: module file was created by an older version of the compiler; rebuild 'Swift' and try again: /Volumes/MusicAndData/BookWriting/LiveGithub/Apple/appleswift/build/Ninja-ReleaseAssert/swift-macosx-x86_64/lib/swift/macosx/x86_64/Swift.swiftmodule


Building the compiler is not a quick thing. Even a ninja build is a non-trivial investment of time. So if you want to be completely honest, the total coding and build time a lot longer than just five hours. A lot longer.

Make sure you’ve updated your repo, incorporated the latest changes for swift and all of its support tools, and built them all before working on your branches. It will save you a lot of frustration.

Be aware that in the time it takes to create even a small project, you’ll probably be out of date with master. Hopefully this won’t affect your changes, and the files you think you’re patching are still the right files you should be patching.

Designing My Changes

The best way to add anything to the Swift compiler is to find some construct that has already been contributed and look through pull releases to discover what parts of the language they had touched.

That’s how I got started with my assertions/optimization revision. I looked at the recent canImport()pull, and targeted the seven files that involved. In the end, I only  needed to modify four files in total, excluding tests. It was a fairly “clean” and simple update.

To add #dogcow, again excluding tests, I had to change nearly two dozen files, most of them written in C++, a few using Python and Swift’s own gyb (aka “generate your boilerplate”) macros.

I’ve put up a gist that details my notes as I performed file edits. (I did have to make some further changes once I started testing.) Each group consists of a file name followed by my changes, with some context around them to make it easier to return to those parts of the file.

That’s followed by any relevant grep results that encouraged me to edit the file in question plus error messages from the compiler, of which there were a few, as I made several typos and forgot at times to add semicolons to the ends of lines. (Damn you semicolons! shakes fist) I put ### markers near the errors to make them easier to find in the notes.

As you walk through my notes, you’ll notice that I had to create a token (pound_dogcow), which is a kind of MagicIdentifierLiteralExpr expression. By inserting a simple token without arguments and returning a string, I cut down on my need to parse additional components or produce a complicated return value.

(Sorry Dave! I’ll get there I hope… After all, I know where each of the five components of Context live: file, line, column, function, and dsohandle. I just don’t know how to build and export the struct so that it gets put into place and can be consumed by the Swift user.)

As a string, my #dogcow can be used as a literal, so I conformed it to KnownProtocolKind::ExpressibleByStringLiteral. It needed to be serialized and deserialized, emit its custom string, support code completion, and more. Scrolling down my file, you’ll see the scope of notes including searches, comments, and edits for this one change.


One of the most interesting things that happened during this exercise was when I made an actual logic error, not a syntax error, so Swift compiled but my program failed:

Assertion failed: (isString() && "Magic identifier literal has non-string encoding"), function setStringEncoding, file /Users/ericasadun/github/Apple/appleswift/swift/include/swift/AST/Expr.h, line 1052.

For the longest time I was convinced (wrongly) that because I was using Unicode, that I had somehow screwed up the string encoding. This was actually a coding mistake, an actual bug, and had nothing to do with the way my string was built nor the fact that I used emojis. It took a while to track down because my head was in the wrong place.

Notice I have DogCow returning true here. I accidentally swapped the two lines so it was originally returning false, falling into the Line/Column/DSOHandle case.

 bool isString() const {
   switch (getKind()) {
    case File:
    case Function:
    case DogCow: // it's a string!
      return true;
    // it should not have been down here
    case Line:
    case Column:
    case DSOHandle:
      return false;
  llvm_unreachable("bad Kind");


Once compiled, I used a few simple calls to test my work. Here’s the source code I used. I accidentally added an extra space in the assignment test. You can see in the screenshot as well:

// String interpolation and default argument
func hello(_ greetedParty: String = #dogcow) {
    print("Hello \(greetedParty)")


// Use in assignment
let aDogCow = #dogcow
print("The value is ", aDogCow)

// Use directly in statement

Lessons Learned

Having built a working version of the compiler incorporating my solo changes, no matter how trivial and yes it was extremely trivial, has been a big confidence builder. Exploring the process from consuming tokens to emitting intermediate language representations has enlightening.

  • I learned to update everything and build from scratch before starting my work. Because if you don’t, you’ll end up doing it later and wasting that time.
  • I learned how to track down similar modifications and use them as a template for exploring what parts of the compiler each change touched.
  • I learned that some errors would not be in the compilation but in the testing, as one tends to forget things like “just because it built doesn’t mean it will compile correctly” when one is very very focused on getting things to run and extremely new to the process.

I have now worked on two (technically three) compiler modification projects. Each has  taught me something new. If you’d like, take a peek at some explorations I’ve pushed to my forked repo:

The DogCow changes are clean, in the style of something that I might actually do a pull request for. The optimization checks are not. They retain all my little in-line notes I use for searching through text files to find what I’ve changed.

The early debug checks represent the time before I could get all the compiler tools built on my system. I was basically programming in my head at that point, guessing what would work or not, before the conversation on Swift forums moved me to my current design.

My guesswork was wrong. I focused on using a trio of built-in functions (like _isDebugAssertConfiguration) mentioned on-list. This turned out to be a non-viable solution. I needed to follow the example set by canImport to set my flags.

Finally, a word to the wise: Don’t ./utils/build-script -R in one terminal window and ninja swift in another at the same time. Oops.