[slavapestov]

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.

[chrislattner]

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.

[slavapestov]

> 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.

[chrislattner]

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.

[slavapestov]

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.

[chrislattner]

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.