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Comment by tomovo | Hacker News Reader

While I appreciate all his work on LLVM, Chris Lattner's Swift didn't work out so well for me, so I'm cautious about this.

Swift has some nice features. However, the super slow compilation times and cryptic error messages really erase any gains in productivity for me.

- "The compiler is unable to type-check this expression in reasonable time?" On an M3 Pro? What the hell!?

- To find an error in SwiftUI code I sometimes need to comment everything out block by block to narrow it down and find the culprit. We're getting laughs from Kotlin devs.

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I think Swift is really successful in that there are so many new Apple developers who would use Swift now but wouldn't have used ObjC.

To be fair to Chris, I’ve only seen the message about compiler not being able to type check the expression in swiftui closure hell. I think he left (maybe partly) because of the SwiftUI influence on Swift.

Actually, even very basic code can cause it. The type system of Swift has issues.

https://www.cocoawithlove.com/blog/2016/07/12/type-checker-i...

let a: Double = -(1 + 2) + -(3 + 4) + -(5)

Still fails on a very recent version of Swift, Swift 6.1.2, if my test works.

Chris Lattner mentions something about him being more of an engineer than a mathematician, but a responsible and competent computer science engineer that is designing programming languages with complex type systems, absolutely has to at least be proficient in university-level mathematics and relevant theory, or delegate out and get computer scientists to find and triple-check any relevant aspects of the budding programing language.

Mojo learns a lot from the mistakes in Swift, including this one. Mojo compiles much faster and doesn't have exponential time type checking! :)

So what did you decide to give up on? Overloading functions with the same name, or bidirectional constraint solving? :)

These days though the type checker is not where compile time is mostly spent in Swift; usually it’s the various SIL and LLVM optimization passes. While the front end could take care to generate less redundant IR upfront, this seems like a generally unavoidable issue with “zero cost abstraction” languages, where the obvious implementation strategy is to spit out a ton of IR, inline everything, and then reduce it to nothing by transforming the IR.

Bidirectional constraint solving. It's bad for compile time but even worse for predictable diagnostics. Mojo does contextual resolution, but it works more similar to how C++ resolves initializer lists etc.

> Bidirectional constraint solving. It's bad for compile time but even worse for predictable diagnostics.

That’s really only true if you have overloading though! Without overloading there are no disjunction choices to attempt, and if you also have principal typing it makes the problem of figuring out diagnostics easier, because each expression has a unique most general type in isolation (so your old CSDiag design would actually work in such a language ;-) )

But perhaps a language where you have to rely on generics for everything instead of just overloading a function to take either an Int or a String is a bridge too far for mainstream programmers.

Mojo has overloading, generics and a much more advanced type system than Swift (dependent and linear types etc), and compile time in all phases is very important. The Mojo design seems to be working well - it gives expressive power, good error messages etc.

It feels like a much better design point overall.

I wasn’t trying to start a fight over languages, that would be silly. I also wrote a language once and then moved on from it (to your former language ;-)), so I know the feeling! I wish you luck with your new language, and I wish for many more languages in the future to try different approaches and learn from each other.

My original reply was just to point out that constraint solving, in the abstract, can be a very effective and elegant approach to these problems. There’s always a tradeoff, and it all depends on the combination of other features that go along with it. For example, without bidirectional inference, certain patterns involving closures become more awkward to express. You can have that, without overloading, and it doesn’t lead to intractability.

Sure, I wasn't trying to start a fight either, I was just sharing my experience and opinion on having worked on both. Mojo (and C++) have closures, for example c++ does lambda type inference without a constraint solver.

In my opinion, constraint solving would be a bad design point for Mojo, and I regret Swift using it. I'm not trying to say that constraint solving is bad for all languages and use-cases.

That last paragraph (the personal attack) is completely needless.

Not a personal attack, and the post is completely valid, but it won't bother me much if you succeed in having my posts censored, as Hacker News (and Western media in general) often censors many people's posts for a variety of purposes, some deeply and completely immoral and corrupt, for this was a drive-by comment from a temporary account.

That programming language designers have to be careful about a type system and its type checking's asymptotic time complexity was 100% widely known before Swift was first created. Some people like to diss on mathematics, but this stuff can have severe practical and widespread engineering consequences. I don't expect everyone to master everything, but budding programming language designers should then at least realize that there may be important issues, and for instance mitigate any issues with having one or more experts checking the relevant aspects.

> let a: Double = -(1 + 2) + -(3 + 4) + -(5)

> Still fails on a very recent version of Swift, Swift 6.1.2, if my test works.

FWIW, the situation with this expression (and others like it) has improved recently:

- 6.1 fails to type check in ~4 seconds

- 6.2 fails to type check in ~2 seconds (still bad obviously, but it's doing the same amount of work in less time)

- latest main successfully type checks in 7ms. That's still a bit too slow though, IMO. (edit: it's just first-time deserialization overhead; if you duplicate the expression multiple times, subsequent instances type check in <1ms).

Fwiw, Chris has mentioned both of those as lessons he took from Swift that he'd like to avoid for Mojo.

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