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Comment by lovasoa | Hacker News Reader

    let result: Double = -(1 + 1) + -(1 + 1) - 1

Swift 6 takes 6.2 seconds to compile this one line.

That's crazy...

Swift programmers: do you face this kind of issues often?

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That is outrageous. It also cannot be blamed on Hindley Milner. While HM in theory has exponential worst case time, the constructs to trigger it for OCaml and Standard ML are far more complex, carefully constructed and never occur in practice.

OCaml does have +. for floats to make type checking easier, but Standard ML does not. Also, if Swift has operator overloading, why isn't it instantly clear that the RHS is an int, which can be assigned to a double?

This just looks like a performance bug in the type checker, and nothing that is inherent to HM.

People have been complaining about this issue for 10 years. Presumably if there were an easy fix they would have fixed it by now. They painted themselves into a corner somehow with HM, polymorphic literals and function overloading. Haskell doesn’t have function overloading (except for type classes) and OCaml doesn’t have polymorphic literals.

It seems like it’s not only function overloading (which, as you point out, Haskell does too by way of typeclasses) but also the implicit conversion between Int and Double. OCaml, Haskell, and Rust all require you to convert between them explicitly so they don’t need to figure out whether each 1 in the expression is an Int or a Double.

Am I seeing this correctly? Is HM + polymorphic literals + implicit type conversion the cause of Swift’s exploding compile time in such cases?

Ok, but then they should explain this particular case better. The RHS can be figured out just by choosing the overloaded operators.

In C++ terms, if you have int& operator+(const int&x, const int& y) then (1 + 1) is not ambiguous and can be selected fast. Same for unary minus etc.

The Swift devs should then blog about this example and explain step by step what is going on. If the literal "1" can be both an int and a float, that of course would be insane. Is that what you meant by "polymorphic literals"?

Wow, that is crazy. For me (Swift 5.10) that single line is taking 11 seconds. Meanwhile, I have a 16,000 line app (importing AppKit, SceneKit and more) that compiles in 8 seconds.

Following up with compile time of a few variations.

    let result:Double = -(1 + 1) + -(1 + 1) - 1 // 10.829s
    let result:Double = -(1 + 1) + -(1 + 1)     // 1.724s
    let result:Double =  (1 + 1) +  (1 + 1) - 1 // 1.721s
    let result:Double = -(1 + 1)            - 1 // 0.763s
    let result:Int    = -(1 + 1) + -(1 + 1) - 1 // 0.571s

    let result:Double = -(1 + 1) + -(1 + 1) + 1
    //error: the compiler is unable to type-check this expression in reasonable time; try breaking up the expression into distinct sub-expressions

I've been doing a lot of SwiftUI stuff lately and the compilation times are pretty crap even for simple applications. Change one line and it's a minute or two of compilation again.

Then there's this fucker for whenever you make a programming error involving types:

>The compiler is unable to type-check this expression in reasonable time; try breaking up the expression into distinct sub-expressions

Basically, your code has a type error somewhere in it, and you need to figure out where and how it's wrong.

The devx is terrible. I came back to native iOS and SwiftUI. to upgrade some of my old apps after working in flutter for the last six months and I've been astonished at how slow and clunky it is. Runtime performance can also be quite bad if you do things in a straightforward way and optimizing things is not very straightforward.

Xcode is still awful too.

One minute compiling Swift UI code - is that without the worst case line above?

So every edit you need to take a minute to see how it renders?

If you're just doing UI changes, I think you can do pretty fast iterations using just the Xcode preview canvas. And while some light edits actually build in seconds, it does feel like it needs that minute compilation surprisingly often when doing fast iterations.

>do you face this kind of issues often

In my personal experience, no. That doesn't mean it's not a weakness of the language, but in practice you very rarely write a real expression with more than one or two type inferences in it. And when you do, you can just add explicit typing.

Nope this never happened to me but i'm always writing explicit types in every language :]

I don't know Swift, but you are declaring Double while using Int. 6.2 seconds seem pretty quick for something that I would expect to be: never.

They are using integer literals, not Ints. Any type can declare the ability to be representable by any literal. Double can be represented by floating point literals or integer literals (since Double, the type, can represent integers, the category of number).

  let x: Double = 2
  let y = 5 / x

y is a Double with a value of 2.5.

  let x: Int = 2
  let y = 5 / x

y is an Int with a value of 2.

  let x: Int = 2
  let y: Double = 5 / x

Code doesn't compile.

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