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Building an iPhone App with ChatGPT

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I’ve been asked to speak at the 2024 German Perl/Raku Workshop in Frankfurt and the theme of this year’s workshop is Generational AI. I’ve been using Copilot for Vim and doing work with Python and the Llama-2 large language model, so I figured I’d have no problem contributing, but first I had better brush up on my AI background. The ensuing rabbit hole was a nightmare, but worth it.

I’ve been working with ChatGPT 4 long enough and seen enough articles about people building full applications with it that I thought it was time to give it a try, but I wanted an app I could use, not some toy for a demo.

My Shopping App

When I go shopping, I use the Notes application on my iPhone. I’ve long hated it. I add items that I want to buy to a list. My list is long and when I search for an item and I don’t see it, I add it again and I have duplicates. I frequently find I’ve added “rice” multiple times.

I wanted a new app, but the App store was a joke. Asking a shopping list application not to track me across the Internet? Having to view ads if I don’t pay for the privilege of making a shopping list? The opportunity to send my grocery list to my friends and family?

I wanted a dead simple application to handle my grocery list, so I asked ChatGPT to build one for me. It was my first time building an app with ChatGPT and it took me about two hours. Much of that was my learning curve. Getting used to this means I can make these apps even faster.

XCode project of my Shopping App
XCode project of my Shopping App

It’s worth noting that while I’ve done Android development with Kotlin, I know nothing about the iOS ecosystem or the Swift programming language. The first time I thought about developing apps for the iPhone was back in the early days, when Objective-C was king and memory management was still manual. I took one look at the code and my wife had to stop me from jabbing a fork in my eye. I’ve never looked back. Until now.

Why Not Copilot?

You might be wondering why I didn’t just use Copilot? First, Copilot integration with XCode is incredibly painful and after a long time fighting with it, XCode finally had Copilot, but I couldn’t get it to make any suggestions, so I just gave up. It wasn’t the point of what I was trying to do.

Second, Copilot’s just not a good fit for this (yet). Putting aside—for the moment—the problems with hallucinations, you can think of Copilot as autocomplete for methods. Where do I put those methods in an XCode application? I had no clue. ChatGPT, however, is autocomplete for ideas. It’s “top down” instead of “bottom up” and that’s exactly what I needed.

My First Prompt

Since I’ve been doing a deep dive into this area, I had a pretty good idea of what to expect. The clearer and more structured your prompt is to ChatGPT, the better your output (note: this is all ChatGPT 4. Earlier versions are far less capable). So I approached this like a product owner, detailing the exact requirements for the application that the customer (me) would need.

I would like to create a small shopping list app in Swift for the iPhone. I have a newer version of XCode.

  1. The app should allow me to enter and delete food items to buy.
  2. Each entry should have a checkbox to the left. Unchecked items should appear at the top, checked items at the bottom.
  3. At the top of the screen should be a large title which says “Shopping.”
  4. At the bottom of the screen should be two buttons. One says “Alpha” and the other says “Frequency.”
  5. If I click on the button which says “Alpha”, all items should be sorted first by unchecked and the checked status. A secondary, alphabetical sort for each checked and unchecked section.
  6. If I click on the button which says “Frequency,” all items should be sorted first by unchecked and the checked status. A secondary, sort for the frequency with which I first entered or unchecked each item.
  7. The application needs storage to save the number of times I’ve entered or unchecked each item.
  8. After I close and reopen the app, it should show the list in the last state it was observed.
  9. If I delete an item from the list, it should also be deleted from storage.

That might seem like a lot of work, but the up-front effort pays off.

ChatGPT responded quickly with a detailed outline. I’ve reproduced a shorter version below, but rather than overwhelm the reader, I’ve included the full transcript in this Google document . I’ve turned on commenting on that document, so feel free to add your own.

Creating a shopping list app with the features you’ve described in SwiftUI is a comprehensive project that will give you a good understanding of many SwiftUI concepts and data persistence. Here’s a high-level overview of how you can implement it:

Step 1: Define Your Data Model

First, you need to create a data model for your shopping items. This model should include properties for the item’s name, checked status, and frequency.

import Foundation

struct ShoppingItem: Identifiable, Codable {
    var id = UUID()
    var name: String
    var isChecked: Bool
    var frequency: Int

(After this, I’ll skip the detail)

Step 2: Create a View Model

Step 3: Build the User Interface

Step 4: Implement Data Persistence

Step 5: Handling State and Sort Logic

Step 6: Testing and Refinement


This outline provides a framework to get started. Each step requires detailed implementation, and you’ll need to write Swift code in the view model and ContentView to handle the specific behaviors like sorting and persistence. Remember to test each part of your app as you develop it. Good luck!

It was a clear, comprehensive outline of an approach I could take and I immediately hit my first bug:

That looks great! The ShoppingListViewModel is giving me the error “Cannot find ‘saveItems' in scope”. How do I fix that?

And ChatGPT dutifully explained how to fix it.

From there, it was a constant flow of me asking it to explain subsequent steps in more detail, prompting it for bug fixes, and gradually watching an iPhone app come to life before my eyes.


No Editor Integration

The single biggest problem currently is that ChatGPT has no direct integration with your editor. You need to constantly copy-n-paste the code. For a non-programmer, this could easily be a daunting task, unsure of where and how to do this. So for now, you can try this at home, but if you’re not a software developer, you might struggle. However, you can always ask ChatGPT for more information on how to handle this. But that brings us to the problem of context length.

Context Length

Early versions of ChatGPT had a “context length” of about 2,000 tokens. The context length is how much of the conversation the language model remembers. These aren’t just words, they’re tokens. Consider the sentence “I ain’t done here.” Four words, but five tokens:

  1. “I”
  2. " ain’t”
  3. " done”
  4. " here”
  5. “.”

But some LLMs have much longer tokens!

As a general rule for ChatGPT, you can multiply token length by 75% to get a rough word count, so 2,000 tokens is about 1,500 words it “remembers” in the conversation.

For ChatGPT 4, we’re just over 4,000 tokens in context length. That’s about 3,000 words, or roughly the length of a standard book chapter. For long, extended work building an application, we need to resort to prompt chaining (which can be very problematic because you still lose context) or embedding relevant snippets of code from previous parts of the conversation to help ChatGPT “remember” what it was doing. This requires more attention on the part of the developer and if you get it wrong, ChatGPT will, too.

ChatGPT 4-turbo, which as of this writing, is listed as not ready for production use, has a whopping 128K context length! That’s about 96 thousands words, the length of a decent-sized novel. Also, it’s rumored that 4-turbo is two to three times faster than ChatGPT 4, so it will be much easier to build large applications and have ChatGPT follow along.

That being said, I often work with codebases which have millions of lines of code. Even a 128K context length isn’t going to follow that.

As an aside, people are having fun using ChatGPT to create text adventures , but the 4K context length means ChatGPT will often “forget” previous parts of the story. ChatGPT 4-turbo will be a real game changer (heh) here.

XCode is Buggy

Curiously, one of the biggest challenges wasn’t ChatGPT, it was XCode. I had just updated to the latest version and on several occasions, I found myself staring at what looked like perfectly valid code, but XCode was throwing an error about variables not being in scope, even though they appeared to be. ChatGPT assured me my code was correct, so I’d back out changes and redo them, trying to be sure I hadn’t missed anything. Finally, I used XCode’s “Refactor” menu to rename a variable, and it told me the variable it had renamed wasn’t in scope! I restarted XCode and everything worked just fine after that. I had to restart XCode a few times during this process, which slowed things down a bit.

Mansplaining as a Service

One thing thing that annoyed me about the app was that I would type in the name of an item I wanted to buy and the item would be added, but it would also be left in the text field. I wanted that field to be cleared every time I hit “return.” The code looked like this:

TextField("New item", text: $newItemName, onCommit: {
    if !newItemName.isEmpty {
        let newItem = ShoppingItem(name: newItemName, isChecked: false, frequency: 1)
        if !viewModel.addItem(newItem) {
            duplicateItemName = newItem.name
            showingDuplicateItemAlert = true
            fadeOutBackground = true

            // Start a timer to reset the fade-out effect after 10 seconds
            DispatchQueue.main.asyncAfter(deadline: .now() + 5) {
                fadeOutBackground = false
                duplicateItemName = ""
        else {
            duplicateItemName = ""
        newItemName = ""

So I asked ChatGPT how to fix that. It confidently assured me that I merely needed to add newItemName = "" at the end of the closure and the field would be cleared. And then it said that since I already had that, my code was doing what I wanted.

It was not doing what I wanted. I explained this to ChatGPT several times and it repeated the same incorrect information several times. While StackOverflow is slowing being killed by AI , it gave me the answer that ChatGPT could not. I needed to add this:

.onSubmit {
    newItemName = ""

Also, due to context length issues, ChatGPT kept “forgetting” that I was using the latest version of the Swift programming language and was offering deprecated versions of various code snippets. I finally gave up correcting ChatGPT and made the corrections manually.


You Don’t Need ChatGPT

A finger pointing at the moon. The fingernail is dirty.
Image by DALL-E

When I’ve shared bits of this story before, I’ve had people tell me I can just use a spreadsheet or a pen and paper. I’m pointing at the moon and people are telling me my fingernails are dirty. I don’t want a spreadsheet or a pen and paper. I have my phone in my pocket and I want an app that does exactly what I want and that’s what I now have.

What if I want to write an application that grabs The Astronomy Picture of the Day from NASA and sets that as my phone’s background?

What if I want to write an application that grabs the headlines from SpaceNews , presents them to me in a list, and let’s me click through, but presents a short summary of each article instead (with a link to the full article) instead of the entire text?

Spreadsheets or pen and paper aren’t going to give me those. I have a world of possibilities that weren’t open to me before without months of learning a new platform and programming language.


I’m also constantly reminded about hallucinations, as if I wasn’t already aware of them.

The thing is, hallucinations sort of come in two forms: hard and easy. The hard hallucination is when it writes a lot of plausible text, with many “facts.” It can take a lot of time to wade through all of those and verify them. Then, maybe you need to verify your verifications. Just because Wikipedia says it’s so doesn’t make it so.

The easy hallucination is when you can easily verify it. For code, you run the code. Does it work? Most errors are very easy to spot.

One error that wasn’t easy to spot was that the frequency calculations were off. I had suspected that early on, but since I don’t know Swift, I thought maybe there was something unusual about how it worked. It was manual testing that verified the calculations were off and ChatGPT fixed the issue quickly.

Of course, ChatGPT will also help you write tests for your application. I confess that I didn’t do that, despite my being a huge testing fanatic. I wanted to see how far I could push ChatGPT. For production code, even with ChatGPT, you’ll want tests. ChatGPT will lose context and you’ll need those tests to verify you haven’t broken anything.

However, there’s a thing to consider with “hallucinations” and code. What happens when you write code? You write it, you run your tests, you find the bugs, you fix them. It’s the same thing with language models, just much faster!

“Bad” Code

This one is subtle and junior devs will easily miss it.

Much of our work in building large systems involves structuring code in such a way that we can understand it later. Maintainability is important for humans, but it’s less important for AI which can follow the code in ways that we cannot.

For example, can you see an issue with the following code? It displays two buttons, side-by-side in a horizontal stack.

HStack {
    Button("A to Z") {
        selectedSort = .alpha
    .buttonStyle(CustomButtonStyle(isSelected: selectedSort == .alpha))
    .frame(width: 150) // Set a specific width for the button

    Button("Popular") {
        selectedSort = .frequency
    .buttonStyle(CustomButtonStyle(isSelected: selectedSort == .frequency))
    .frame(width: 150) // Set a specific width for the button

Did you see the problem? It’s not a bug. Many newer developers might not see an issue, but you probably do: there’s a lot of duplicated code. Instead, you want that to look similar to this:

HStack {
    SortButton(title: "A to Z", action: viewModel.sortAlpha, selectedSort: $selectedSort)
    SortButton(title: "Popular", action: viewModel.sortFrequency, selectedSort: $selectedSort)

With the above, you can adjust the SortButton struct and have the behavior apply to both of your sort buttons. Otherwise, you’ll have to manually keep those two buttons in sync.

I also had imports in the code which I didn’t need and a couple of unused methods generated, but how is a junior developer going to know that? From a programmer’s standpoint, this is bad. From a business standpoint? So what? “It works; ship it,” will be the refrain. Bad code confuses humans more than language models.

The Near Future

Programmers will be keep their jobs for the time being (unless you work at StackOverflow), but Copilot and ChatGPT allow us to write code faster. When working on a monolithic application, language models will help, but the massive size means they won’t be able to have enough context, reducing their effectiveness.

One clear use case for these tools is microservices. These are much smaller, self-contained programs. Language models, even with a limited context, will find them more tractable. With well-defined OpenAPI documents, microservices will be quick and easy to develop. I’ve written about microservices before and they do have their challenges, but combined with language models, I can see development teams reaching heretofore unknown levels of productivity.

The Far Future

Eventually ChatGPT is going to have native desktop integration. Given that it already has a speech recognition model , you’ll be able to speak to it. You can ask it to build an application and because it will have full integration, it will be writing and running the code for you, seeing the errors for itself and trying to automatically fix them. Because it will have full access to the code, it can “look” for where the errors are coming from and figure out for itself how to fix them, without you needing to remind it.

I expect it will be a few years before this is “production ready,” but with this, the programmer’s ivory tower will finish its collapse. . Our jobs will be going away. The open question is whether we, like the Luddites, will find ourselves without work and losing our homes, or whether this will open the way for more complex work the AIs can’t do (yet).

Just a few of the preferences available for iTerm 2
Just a few of the preferences available for iTerm 2

As a bonus, you will get exactly the applications you want. No more bewildering array of preference selections. The app is for you, not for the masses. You won’t have to ask the app not to track. You won’t have annoying ads until you pay for a premium service. Need a new feature? “Hey, ChatGPT, I need you to change the novel writing app we created.”

We will finally have “programming for the masses.”

For all those who disagree, Dr. Matt Welsh would like to have a word.

What will the code look like? In the early days of programming, we were setting switches and pushing buttons on huge machines. Later, we were writing Fortran on punch cards. Today, we have easy languages like Python and hard languages like Rust, but consider even very difficult languages such as APL. The following is Conway’s Game of Life written in APL :

life ← {⊃1 ⍵ ∨.∧ 3 4 = +/ +⌿ ¯1 0 1 ∘.⊖ ¯1 0 1 ⌽¨ ⊂⍵}

That’s still far easier than entering ones and zeros by toggling switches on a room-sized beast.

Language models don’t need something that humans can read. They’ll be able to write machine code directly. They might still use an intermediate representation that can be easily translated to the machine code for a particular architecture—presumably using a translator they’ve written. Or not. It’s hard to predict the future.

Programs could potentially run faster and more reliably, but many people are not going to feel comfortable with code they can’t verify, but this is where we’re heading.

I also expect many applications will simply go away, being replaced by a single language model which can handle this for us. “Hey, ChatGPT! Send an email to mom, wishing her a happy birthday.” And ChatGPT will do that, probably matching your conversational style.


The programming profession, as we practice is today, is dying. We may not see it yet, but it is. And for all the cries of “programming language X is dying,” the truth is that all programming languages are dying. We face an existential threat, but I see many programmers denying this, insisting that they’re still better than the language models. It’s true, they often are, but they’re slower than the models and the models are getting better. More to the point, they’re getting better faster than we are.

It’s possible there will still be a role for programmers, though. In the 80s, I was tutoring college students in the C programming language. I met many brilliant people who simply couldn’t think about programming the way programmers do. Despite their intelligence, they couldn’t understand how to structure programs, but lacking this skill, they had other skills that programmers often don’t.

This might spill over to prompt engineering. Note how detailed my original prompt was. If you go back to that prompt, note the last item on the list about deleting items from storage when I remove them from my list. If I didn’t have that, I’d eventually risk filling up my storage over time, but not know why. It’s little details like that which will keep programmers in business for a while...until the AI learn to anticipate situations like this and ask us how to address it.

“Prompt engineering” might be renamed “programming” in the future. However, as language models increase in power, I expect that even this strength might go away. If language models can ever incorporate system 2 thinking , all bets are off. That might also be what leads us to artificial general intelligence .

The AI boom today is like the internet boom in the 90s. There will be “bust” periods as many AI companies offer products with no value. There will be detractors who call AI “just a fad.” And AI will continue regardless. I’m both excited and concerned by the future.

You can check out the github project for my code here .

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Copyright © 2018-2024 by Curtis “Ovid” Poe.