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Comment by lmm | Hacker News Reader

> Try it! I certainly have. You may find encoding a typelevel TM or λ-calculus more difficult than it appears...

These languages were stable, well-specified, and heavily studied for years before these cases were found. One person noodling around for a few weeks failing to find an example is not evidence of anything; a language being too obscure to have seen much academic study is not an indicator that it's likely to have a good type system, if anything it's the opposite.

> T() is not possible for a generic T in Kotlin.

T isn't generic in that code, it's just a class.

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  > One person noodling around for a few weeks

Well, I've been trying for about three years, but to be fair I'm a pretty slow programmer so you may have better luck.

  > a language being too obscure to have seen much academic study

Doesn't seem too obscure to me. https://scholar.google.com/scholar?hl=en&as_sdt=0%2C5&q=kotl...

  > T isn't generic in that code, it's just a class.

I think it needs to be a generic type in Kotlin for this to work, because otherwise it will dispatch to a single method. It's tricky to get Kotlin to do much compile time computation. Not saying that it's impossible, but LMK when you've actually tried it. Here's some sample code if you want to try encoding a Boolean logic:

https://github.com/breandan/galoisenne/blob/2e465e7a753f6341...

> Well, I've been trying for about three years, but to be fair I'm a pretty slow programmer so you may have better luck.

Well, your own link proves it, doesn't it? Kotlin has nominal subtyping, contravariance, and generics, therefore its type system is undecidable.

> I think it needs to be a generic type in Kotlin for this to work, because otherwise it will dispatch to a single method.

That's the whole point - the type system has to decide which overload to dispatch it to at compile time, which it can only do by performing an arbitrarily complex computation. I ported that code directly to Kotlin and it doesn't compile but it "should", which, well, I don't know what you were expecting or what you'd accept as proof when we've already got an academic paper that proves the point.

  > Well, your own link proves it, doesn't it?

That code simulates a bounded cellular automaton, which effectively reduces to a regular language, but that's about as far as I've been able to get. If it's possible to implement general recursion, I'm not quite sure how - yet.

  > Kotlin has nominal subtyping, contravariance, and generics, therefore its type system is undecidable.

They're not using Kennedy and Pierce's system, but it's closely related. See Tate (2013) [1], in particular, "In general, since declaration-site variance can easily be encoded into use-site variance..."

  > I ported that code directly to Kotlin

Can you share your translation of Eric's code? Maybe we can get it to work.

[1]: https://fool2013.cs.brown.edu/tate.pdf

> They're not using Kennedy and Pierce's system, but it's closely related. See Tate (2013) [1], in particular, "In general, since declaration-site variance can easily be encoded into use-site variance..."

Well, Kotlin has both declaration-site and use-site variance. So surely that result applies.

> Can you share your translation of Eric's code? Maybe we can get it to work.

Ah I didn't mean that code, I meant the earlier C# one from SO. https://pl.kotl.in/4BrbZAmZG

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