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Watch the video review I hardly ever think about my Mac Mini, but it serves a vital role for my family as our home-theater mixer, Plex server, ScanSnap server, Apple Photos backup, and Backblaze host for our NAS.1 Almost every port on the back is in use, and it runs 24/7, reliably, in total silence. Until last week, I thought it would be the last Mac Mini that Apple ever made. And when rumors started swirling about an imminent Mac Mini update, I assumed the worst: if it came at all, it would be a tiny box with a slow, ultra-low-power processor and almost zero ports, optimizing for small size instead of versatility. I don’t think this was an unreasonable fear after the 2014 Mac Mini update, which made many key aspects much worse without making anything much better. It seemed clear then, and for the following four years that it went without an update, that Apple held the Mac Mini and its customers in very low regard. Not anymore. The 2018 Mac Mini is real, and it’s spectacular. It...
over a year ago

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Retreating to Safety

Ten years ago, Apple’s Phil Schiller surprised Apple enthusiasts and developers by walking out on stage at John Gruber’s The Talk Show Live WWDC event and giving an open, human, honest interview to a somewhat jaded community. I wrote this in response: Both Apple and Phil Schiller himself took a huge risk in doing this. That they agreed at all is a noteworthy gift to this community of long-time enthusiasts, many of whom have felt under-appreciated as the company has grown. […] Phil’s appearance on the show was warm, genuine, informative, and entertaining. It was human. And humanizing the company and its decisions, especially to developers — remember, developer relations is all under Phil — might be worth the PR risk. This started a ten-year run of interviews by Apple executives on The Talk Show every year at WWDC that proved to be great, surprisingly safe PR for Apple. No executive ever said something they shouldn’t have (they’re pros), no sensational or negative news stories ever resulted from them, and Apple’s enthusiastic fans and developers felt seen, heard, and appreciated. *     *     * For unspecified reasons, Apple has declined to participate this year, ending what had become a beloved tradition in our community — and I can’t help but suspect that it won’t come back. (A lot has changed in the meantime.) Maybe Apple has good reasons. Maybe not. We’ll see what their WWDC PR strategy looks like in a couple of weeks. In the absence of any other information, it’s easy to assume that Apple no longer wants its executives to be interviewed in a human, unscripted, unedited context that may contain hard questions, and that Apple no longer feels it necessary to show their appreciation to our community and developers in this way. I hope that’s either not the case, or it doesn’t stay the case for long. This will be the first WWDC I’m not attending since 2009 (excluding the remote 2020 one, of course). Given my realizations about my relationship with Apple and how they view developers, I’ve decided that it’s best for me to take a break this year, gain some perspective, and decide what my future relationship should look like. Maybe Apple’s leaders are doing that, too.

a month ago 14 votes
Ten years of Overcast: A new foundation

Today, on the tenth anniversary of Overcast 1.0, I’m happy to launch a complete rewrite and redesign of most of the iOS app, built to carry Overcast into the next decade — and hopefully beyond. Like podcasts better than blog posts? Listen to ATP #596 for more! What’s new Much faster, more responsive, more reliable, and more accessible. Modern design, optimized for easily-reached controls on today’s phone sizes. Improvements throughout, such as undoing large seeks, new playlist-priority options, easier navigation, and more. What’s not Most features. Overcast is still Overcast! The audio engine. It’s the best part of Overcast, and still leads the industry in sound quality, silence skipping, and volume normalization. (More soon!) The business. I’m still a one-person operation, with no funding or external ownership, serving only my customers. My principles. I always want to make the best podcast app, and I’ll never disrespect your time, attention, or privacy. What’s gone Streaming. Most big podcasts now use dynamic ad insertion, which causes bugs and problems for streaming playback.1 Downloading episodes completely before they begin playback is much more reliable. Tapping a non-downloading episode will now open the playback screen, download it, then start playback. It works similarly to the way streaming did before, but playback begins after the download completes, not after a portion of it is buffered. On today’s fast networks, this usually only takes a few extra seconds. And in the near future, I’ll be adding smarter options and more control over selective downloading of episodes to further improve the experience for people who don’t automatically download every episode. What’s next The last few missing features from the old app, such as Shortcuts support, storage management, and OPML. These are absent now, but will return soon. More options for downloading and deleting episodes. Upgrading the Apple Watch app to the new, faster sync engine. (The Watch app is currently unchanged from the previous one.) And, of course, more features, including some of your most-requested features over the last decade. Getting this rewrite out the door was a monumental task. Thank you for your patience as I work through this list! Why? Most of Overcast’s core code was 10 years old, which made it cumbersome or impossible to easily move with the times, adopt new iOS functionality, or add new features, especially as one person. That’s why there haven’t been many new features or changes in years. You saw it, and I saw it. I wasn’t able to serve my customers as well as I wanted. For Overcast to have a future, it needed a modern foundation for its second decade. I’ve spent the past 18 months rebuilding most of the app with Swift, SwiftUI, Blackbird, and modern Swift concurrency. Now, development is rapidly accelerating. I’m more responsive, iterating more quickly, and ultimately making the app much better. Thank you all so much for the first decade of Overcast. Here’s to the next one. Dynamic ad insertion (DAI) splices ads into each download, and no two downloads are guaranteed to have the same number or duration of ads. So, for example, if the first half of an episode downloads, then the download fails, and it downloads the second half with another request, the combined audio may jump forward or back at the halfway mark, losing or repeating content. ↩︎

11 months ago 74 votes
The Overcast Redesign: Part One

Overcast’s latest update (2022.2) brings the largest redesign in its nearly-eight-year history, plus many of the most frequently requested features and lots of under-the-hood improvements. I’m pretty proud of this one. For this first and largest phase of the redesign, I focused on the home screen, playlist screen, typography, and spacing. (I plan to revamp the now-playing and individual-podcast screens in a later update.) The home screen is radically different: Home screen, before (left) and after (right). Playlists now have strong visual identities for nicer and easier navigation. Each playlist has a customizable color, and a custom icon can be selected from over 3,000 SF Symbols to match modern iOS design and the other icons within Overcast. And playlists can be manually reordered with drag-and-drop. Recently played and newly published episodes can now be displayed on the home screen for quick access, much like the widget and CarPlay experience. Podcasts can now be pinned to the top of the home-screen list. Pinned podcasts can also be manually reordered with drag-and-drop. I’ve also rethought the old stacked “Podcasts” and “Played Podcasts” sections to better match people’s needs and expectations. Now, the toggle atop the podcast list switches between three modes: podcasts with current episodes, all followed podcasts, and inactive podcasts (those that you don’t follow and therefore won’t get any more episodes from, or haven’t posted a new episode in a long time). The playlist screen’s structure remains mostly the same, while refining the design for the modern era: Playlist screen, before (left) and after (right). Here, it’s more apparent that I’ve replaced the system San Francisco font with an alternate variant, San Francisco Rounded, to increase legibility and better match the personality of the app. I’ve also added highly demanded features: By far, Overcast’s most-requested feature is a Mark as Played feature. That’s now available as a checkmark button on episode rows, as well as a left-side swipe action. The second-most-requested feature is a way to view all starred episodes. Special playlists for Starred, Downloaded, and In Progress can now be created. The light and dark themes now each have a customizable tint color from the modern iOS UI-color palette, including these favorites from beta testers: And throughout the app, I’ve made tons of tweaks and bug fixes, including: Notifications and background downloads are now much more reliable. Episode downloads can now be individually deleted or re-downloaded. Links can now be opened in Safari. (under Nitpicky Details) Performance is now significantly better with very large playlists and collections. Fixed bugs with episode-duration detection, CarPlay lists, Mac-app sharing, and much more. So much is better in this update that I can’t even remember it all. Thank you so much to everyone who helped me beta-test this massive update. As always, Overcast is free in the App Store. Go get it!

over a year ago 44 votes
Ten years after we lost Steve Jobs

Losing Steve affected me more than it probably should have, given that I never met him or had any correspondence with him. But losing him was devastating — not just to my world, but the world. He was a sort of virtual father figure: I was always hoping that maybe Steve would notice something I did. We all wanted his attention and approval, and that drove us to do better work — even those of us who never worked at Apple. Nobody replaced him in this role. Nobody can. But as an outsider who had no personal relationship with him to mourn, it has been most depressing to consider how much of his work the world missed out on. He wasn’t taken from us after a long, complete life — he was taken in his prime. He had so much more to offer the world.

over a year ago 44 votes
The future of the App Store

After the dust settles from the developer class-action settlement, the South Korean law, the JFTC announcement, and the Apple v. Epic decision, I think the most likely long-term outcome isn’t very different from the status quo — and that’s a good thing. Allowing external purchases Here’s what I think we’ll end up with: Apple will still require apps to use their IAP system for any qualifying purchases that occur in the apps themselves. All app types will be allowed to link out to a browser for other purchase methods. Most apps will be required to also offer IAP side-by-side with any external methods.1 Only “Reader apps” will be exempt from this requirement.2 Apple will have many rules regarding the display, descriptions, and behavior of external purchases, many of which will be unpublished and ever-changing. App Review will be extremely harsh, inconsistent, capricious, petty, and punitive with their enforcement.3 Apple won’t require price-matching between IAP and external purchases. These few but important corrections reduce Apple’s worst behavior and should relieve most regulatory pressure. The result won’t look much different than the status quo: Most big media apps (qualifying as “reader” apps) won’t offer IAP, but will finally be allowed to link to their websites from their apps and offer purchases there. Many games will offer both IAP and external purchases, with the external choice offering a discount, bonus gems, extra loot boxes, or other manipulative tricks to optimize the profitability of casino games for children (commissions from which have been the largest portion of Apple’s “services revenue” to date). Most importantly, many products, services, and business models will become possible that previously weren’t, leading to more apps, more competition, and more money going to more places. External purchase methods will evolve to be almost as convenient as IAP (especially if Apple Pay is permitted in this context), and payment processors will reduce the burden of manual credit-card entry with shared credentials available across multiple apps. The payment-fraud doomsday scenarios argued by Apple and many fans mostly won’t happen, in part because App Review will prevent most obvious cases, but also because parents don’t typically offer their credit cards to untrustworthy children; and for buyers of all ages, most credit cards themselves provide stronger fraud prevention and easier recourse from unwanted charges than the App Store ever has. No side-loading I don’t expect side-loading or alternative app stores to become possible, and I’m relieved, because that is not a future I want for iOS. When evaluating such ideas, I merely ask myself: “What would Facebook do?” Facebook owns four of the top ten apps in the world. If side-loading became possible, Facebook could remove Instagram, WhatsApp, the Facebook app, and Messenger from Apple’s App Store, requiring customers to install these extremely popular apps directly from Facebook via side-loading. And everyone would. Most people use a Facebook-owned app not because it’s a good app, but because it’s a means to an important end in their life. Social pressure, family pressure, and network lock-in prevent most users from seeking meaningful alternatives. People would jump through a few hoops if they had to. Facebook would soon have apps that bypassed App Review installed on the majority of iPhones in the world. Technical limitations of the OS would prevent the most egregious abuses, but there’s a lot they could still do. We don’t need to do much imagining — they already have attempted multiple hacks, workarounds, privacy invasions, and other unscrupulous and technically invasive behavior with their apps over time to surveil user behavior outside of their app and stay running longer in the background than users intend or expect. The OS could evolve over time to reduce some of these vulnerabilities, but technical measures alone cannot address all of them. Without the threat of App Review to keep them in check, Facebook’s apps would become even more monstrous than they already are. As a user and a fan of iOS, I don’t want any part of that. No alternative app stores Alternative app stores would be even worse. Rather than offering individual apps via side-loading, Facebook could offer just one: The Facebook App Store. Instagram, WhatsApp, the Facebook app, and Messenger could all be available exclusively there. The majority of iOS users in the world would soon install it, and Facebook would start using leverage in other areas — apps’ social accounts, stats packages, app-install ads, ad-attribution requirements — to heavily incentivize (and likely strong-arm) a huge number of developers to offer their apps in the Facebook App Store, likely in addition to Apple’s. Maybe I’d be required to add the Facebook SDK to my app in order to be in their store, which they would then use to surveil my users. Maybe I’d need to buy app-install ads to show up in search there at all. Maybe I’d need to pay Facebook to “promote” each app update to reach more than a tiny percentage of my existing customers. And Facebook wouldn’t even be the only app store likely to become a large player on iOS. Amazon would almost certainly bring their garbage “Appstore” to iOS, but at least that one probably wouldn’t go anywhere. Maybe Google would bring the Play Store to iOS and offer a unified SDK to develop a single codebase for iOS and Android, effectively making every app feel like an Android app and further marginalizing native apps when they’re already hurting. Media conglomerates that own many big-name properties, like Disney, might each have their own app stores for their high-profile apps. Running your own store means you can promote all of your own apps as much as you want. What giant corporation would resist? Don’t forget games! Epic and Steam would come to iOS with their own game stores. Maybe Microsoft and Nintendo, too. Maybe you’d need to install seven different app stores on your iPhone just to get the apps and games you already use — and all without App Review to keep them in check. Most developers would probably need to start submitting our apps to multiple app stores, each with its own rules, metadata, technical requirements, capabilities, approval delays, payment processing, stats, crash reports, ads, promotion methods, and user reviews. As a user, a multiple-app-store world sounds like an annoying mess; as a developer, it terrifies me. Apple’s App Store is the devil we know. The most viable alternatives that would crop up would be far worse. Course correction The way Apple runs its business isn’t perfect, but it’s also not a democracy. I loved this part of Judge Yvonne Gonzalez Rogers’ decision in Apple v. Epic, as quoted by Ben Thompson’s excellent article that you should read: Apple has not offered any justification for the actions other than to argue entitlement. Where its actions harm competition and result in supracompetitive pricing and profits, Apple is wrong. I interpret “entitlement” without a negative connotation here — Apple is entitled to run their platform mostly as they wish, with governmental interference only warranted to fix market-scale issues that harm large segments of commerce or society. As a developer, I’d love to see more changes to Apple’s control over iOS. But it’s hard to make larger changes without potentially harming much of what makes iOS great for both users and developers. Judge Gonzalez Rogers got it right: we needed a minor course correction to address the most egregiously anticompetitive behavior, but most of the way Apple runs iOS is best left to Apple. If the South Korean law holds, IAP may not be required — but only in South Korea. With this exception, I expect the rest of these rules to be enforced the same way globally. ↩︎ Apple defines “reader” apps as “[allowing] a user to access previously purchased content or content subscriptions (specifically: magazines, newspapers, books, audio, music, and video).” This includes many apps that Apple’s services compete with, such as Netflix, Spotify, and Kindle, that raise anticompetitive concerns among regulators and legislators when forced to give Apple 30%. ↩︎ App Review has higher-level queues for managerial review of controversial rules or edge cases, typically identifiable from the outside by an app stuck with “In Review” status for days or weeks, and often ending in a phone call from “Bill”. I’d expect any app offering external purchases to have a very high chance of being escalated to a slower, more pain-in-the-ass review process, possibly causing it not to be worthwhile for most small developers to deal with. I have no plans to add external purchases to Overcast for multiple reasons, including this — but mostly because, for my purposes, I’m satisfied with Apple’s IAP system. ↩︎

over a year ago 42 votes

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11 hours ago 3 votes
Get in losers, we're moving to Linux!

I've never seen so many developers curious about leaving the Mac and giving Linux a go. Something has really changed in the last few years. Maybe Linux just got better? Maybe powerful mini PCs made it easier? Maybe Apple just fumbled their relationship with developers one too many times? Maybe it's all of it. But whatever the reason, the vibe shift is noticeable. This is why the future is so hard to predict! People have been joking about "The Year of Linux on the Desktop" since the late 90s. Just like self-driving cars were supposed to be a thing back in 2017. And now, in the year of our Lord 2025, it seems like we're getting both! I also wouldn't underestimate the cultural influence of a few key people. PewDiePie sharing his journey into Arch and Hyprland with his 110 million followers is important. ThePrimeagen moving to Arch and Hyprland is important. Typecraft teaching beginners how to build an Arch and Hyprland setup from scratch is important (and who I just spoke to about Omarchy). Gabe Newell's Steam Deck being built on Arch and pushing Proton to over 20,000 compatible Linux games is important. You'll notice a trend here, which is that Arch Linux, a notoriously "difficult" distribution, is at the center of much of this new engagement. Despite the fact that it's been around since 2003! There's nothing new about Arch, but there's something new about the circles of people it's engaging. I've put Arch at the center of Omarchy too. Originally just because that was what Hyprland recommended. Then, after living with the wonders of 90,000+ packages on the community-driven AUR package repository, for its own sake. It's really good! But while Arch (and Hyprland) are having a moment amongst a new crowd, it's also "just" Linux at its core. And Linux really is the star of the show. The perfect, free, and open alternative that was just sitting around waiting for developers to finally have had enough of the commercial offerings from Apple and Microsoft. Now obviously there's a taste of "new vegan sees vegans everywhere" here. You start talking about Linux, and you'll hear from folks already in the community or those considering the move too. It's easy to confuse what you'd like to be true with what is actually true. And it's definitely true that Linux is still a niche operating system on the desktop. Even among developers. Apple and Microsoft sit on the lion's share of the market share. But the mind share? They've been losing that fast. The window is open for a major shift to happen. First gradually, then suddenly. It feels like morning in Linux land!

5 hours ago 1 votes
All about Svelte 5 snippets

Snippets are a useful addition to Svelte 5. I use them in my Svelte 5 projects like Edna. Snippet basics A snippet is a function that renders html based on its arguments. Here’s how to define and use a snippet: {#snippet hello(name)} <div>Hello {name}!</div> {/snippet} {@render hello("Andrew")} {@render hello("Amy")} You can re-use snippets by exporting them: <script module> export { hello }; </script> {@snippet hello(name)}<div>Hello {name}!</div>{/snippet} Snippets use cases Snippets for less nesting Deeply nested html is hard to read. You can use snippets to extract some parts to make the structure clearer. For example, you can transform: <div> <div class="flex justify-end mt-2"> <button onclick={onclose} class="mr-4 px-4 py-1 border border-black hover:bg-gray-100" >Cancel</button > <button onclick={() => emitRename()} disabled={!canRename} class="px-4 py-1 border border-black hover:bg-gray-50 disabled:text-gray-400 disabled:border-gray-400 disabled:bg-white default:bg-slate-700" >Rename</button > </div> into: {#snippet buttonCancel()} <button onclick={onclose} class="mr-4 px-4 py-1 border border-black hover:bg-gray-100" >Cancel</button > {/snippet} {#snippet buttonRename()}...{/snippet} To make this easier to read: <div> <div class="flex justify-end mt-2"> {@render buttonCancel()} {@render buttonRename()} </div> </div> snippets replace default <slot/> In Svelte 4, if you wanted place some HTML inside the component, you used <slot />. Let’s say you have Overlay.svelte component used like this: <Overlay> <MyDialog></MyDialog> </Overlay> In Svelte 4, you would use <slot /> to render children: <div class="overlay-wrapper"> <slot /> </div> <slot /> would be replaced with <MyDialog></MyDialog>. In Svelte 5 <MyDialog></MyDialog> is passed to Overlay.svelte as children property so you would change Overlay.svelte to: <script> let { children } = $props(); </script> <div class="overlay-wrapper"> {@render children()} </div> children property is created by Svelte compiler so you should avoid naming your own props children. snippets replace named slots A component can have a default slot for rendering children and additional named slots. In Svelte 5 instead of named slots you pass snippets as props. An example of Dialog.svelte: <script> let { title, children } = $props(); </script> <div class="dialog"> <div class="title"> {@render title()} </div> {@render children()} </div> And use: {#snippet title()} <div class="fancy-title">My fancy title</div> {/snippet} <Dialog title={title}> <div>Body of the dialog</div> </Dialog> passing snippets as implicit props You can pass title snippet prop implicitly: <Dialog> {#snippet title()} <div class="fancy-title">My fancy title</div> {/snippet} <div>Body of the dialog</div> </Dialog> Because {snippet title()} is a child or <Dialog>, we don’t have to pass it as explicit title={title} prop. The compiler does it for us. snippets to reduce repetition Here’s part of how I render https://tools.arslexis.io/ {#snippet row(name, url, desc)} <tr> <td class="text-left align-top" ><a class="font-semibold whitespace-nowrap" href={url}>{name}</a> </td> <td class="pl-4 align-top">{@html desc}</td> </tr> {/snippet} {@render row("unzip", "/unzip/", "unzip a file in the browser")} {@render row("wc", "/wc/", "like <tt>wc</tt>, but in the browser")} It saves me copy & paste of the same HTML and makes the structure more readable. snippets for recursive rendering Sometimes you need to render a recursive structure, like nested menus or file tree. In Svelte 4 you could use <svelte:self> but the downside of that is that you create multiple instances of the component. That means that the state is also split among multiple instances. That makes it harder to implement functionality that requires a global view of the structure, like keyboard navigation. With snippets you can render things recursively in a single instance of the component. I used it to implement nested context menus. snippets to customize rendering Let’s say you’re building a Menu component. Each menu item is a <div> with some non-trivial children. To allow the client of Menu customize how items are rendered, you could provide props for things like colors, padding etc. or you could allow ultimate flexibility by accepting an optional menuitem prop that is a snippet that renders the item. You can think of it as a headless UI i.e. you provide the necessary structure and difficult logic like keyboard navigation etc. and allow the client lots of control over how things are rendered. snippets for library of icons Before snippets every SVG Icon I used was a Svelte component. Many icons means many files. Now I have a single Icons.svelte file, like: <script module> export { IconMenu, IconSettings }; </script> {#snippet IconMenu(arg1, arg2, ...)} <svg>... icon svg</svg> {/snippet}} {#snippet IconSettings()} <svg>... icon svg</svg> {/snippet}}

yesterday 2 votes
Logical Quantifiers in Software

I realize that for all I've talked about Logic for Programmers in this newsletter, I never once explained basic logical quantifiers. They're both simple and incredibly useful, so let's do that this week! Sets and quantifiers A set is a collection of unordered, unique elements. {1, 2, 3, …} is a set, as are "every programming language", "every programming language's Wikipedia page", and "every function ever defined in any programming language's standard library". You can put whatever you want in a set, with some very specific limitations to avoid certain paradoxes.2 Once we have a set, we can ask "is something true for all elements of the set" and "is something true for at least one element of the set?" IE, is it true that every programming language has a set collection type in the core language? We would write it like this: # all of them all l in ProgrammingLanguages: HasSetType(l) # at least one some l in ProgrammingLanguages: HasSetType(l) This is the notation I use in the book because it's easy to read, type, and search for. Mathematicians historically had a few different formats; the one I grew up with was ∀x ∈ set: P(x) to mean all x in set, and ∃ to mean some. I use these when writing for just myself, but find them confusing to programmers when communicating. "All" and "some" are respectively referred to as "universal" and "existential" quantifiers. Some cool properties We can simplify expressions with quantifiers, in the same way that we can simplify !(x && y) to !x || !y. First of all, quantifiers are commutative with themselves. some x: some y: P(x,y) is the same as some y: some x: P(x, y). For this reason we can write some x, y: P(x,y) as shorthand. We can even do this when quantifying over different sets, writing some x, x' in X, y in Y instead of some x, x' in X: some y in Y. We can not do this with "alternating quantifiers": all p in Person: some m in Person: Mother(m, p) says that every person has a mother. some m in Person: all p in Person: Mother(m, p) says that someone is every person's mother. Second, existentials distribute over || while universals distribute over &&. "There is some url which returns a 403 or 404" is the same as "there is some url which returns a 403 or some url that returns a 404", and "all PRs pass the linter and the test suites" is the same as "all PRs pass the linter and all PRs pass the test suites". Finally, some and all are duals: some x: P(x) == !(all x: !P(x)), and vice-versa. Intuitively: if some file is malicious, it's not true that all files are benign. All these rules together mean we can manipulate quantifiers almost as easily as we can manipulate regular booleans, putting them in whatever form is easiest to use in programming. Speaking of which, how do we use this in in programming? How we use this in programming First of all, people clearly have a need for directly using quantifiers in code. If we have something of the form: for x in list: if P(x): return true return false That's just some x in list: P(x). And this is a prevalent pattern, as you can see by using GitHub code search. It finds over 500k examples of this pattern in Python alone! That can be simplified via using the language's built-in quantifiers: the Python would be any(P(x) for x in list). (Note this is not quantifying over sets but iterables. But the idea translates cleanly enough.) More generally, quantifiers are a key way we express higher-level properties of software. What does it mean for a list to be sorted in ascending order? That all i, j in 0..<len(l): if i < j then l[i] <= l[j]. When should a ratchet test fail? When some f in functions - exceptions: Uses(f, bad_function). Should the image classifier work upside down? all i in images: classify(i) == classify(rotate(i, 180)). These are the properties we verify with tests and types and MISU and whatnot;1 it helps to be able to make them explicit! One cool use case that'll be in the book's next version: database invariants are universal statements over the set of all records, like all a in accounts: a.balance > 0. That's enforceable with a CHECK constraint. But what about something like all i, i' in intervals: NoOverlap(i, i')? That isn't covered by CHECK, since it spans two rows. Quantifier duality to the rescue! The invariant is equivalent to !(some i, i' in intervals: Overlap(i, i')), so is preserved if the query SELECT COUNT(*) FROM intervals CROSS JOIN intervals … returns 0 rows. This means we can test it via a database trigger.3 There are a lot more use cases for quantifiers, but this is enough to introduce the ideas! Next week's the one year anniversary of the book entering early access, so I'll be writing a bit about that experience and how the book changed. It's crazy how crude v0.1 was compared to the current version. MISU ("make illegal states unrepresentable") means using data representations that rule out invalid values. For example, if you have a location -> Optional(item) lookup and want to make sure that each item is in exactly one location, consider instead changing the map to item -> location. This is a means of implementing the property all i in item, l, l' in location: if ItemIn(i, l) && l != l' then !ItemIn(i, l'). ↩ Specifically, a set can't be an element of itself, which rules out constructing things like "the set of all sets" or "the set of sets that don't contain themselves". ↩ Though note that when you're inserting or updating an interval, you already have that row's fields in the trigger's NEW keyword. So you can just query !(some i in intervals: Overlap(new, i')), which is more efficient. ↩

2 days ago 5 votes
The missing part of Espressif’s reset circuit

In the previous article, we peeked at the reset circuit of ESP-Prog with an oscilloscope, and reproduced it with basic components. We observed that it did not behave quite as expected. In this article, we’ll look into the missing pieces. An incomplete circuit For a hint, we’ll first look a bit more closely at the … Continue reading The missing part of Espressif’s reset circuit → The post The missing part of Espressif’s reset circuit appeared first on Quentin Santos.

2 days ago 2 votes