[erszcz]

> Going from a pure string of SQL or JSON to a concrete type without actually executing any runtime code is pretty crazy.

Going from a JSON string to a type for it is actually one of the easier examples of inference I can imagine. JSON is a data description format in which all the base types are syntactically distinguishable, it has no variables, no arrows (i.e. functions), no generics. In the topic of type inference, you can't have a much easier example.

SQL is more complex, indeed, but still doesn't seem too crazy if you have access to table schemas. It's also a matter of whether triggers and stored procedures are taken into account, but I assume they're not.

There's a lot of prior art described in literature as well as practical programming implementations with much crazier, yet successfully working type inference.

[Vaguely2178]

I just want to make sure we're on the same page here. The JSON example I linked to isn't inferring the types of JSON that's already been parsed and deserialized, that would be trivially easy in any language (including TS). If I have an object that's been parsed from JSON, I can just use the typeof operator in TypeScript to infer the type of that object.

The example I linked to is taking a serialized JSON string, and parsing the literal characters in the string (characters like double quotes, commas, whitespace, etc) into a type, purely using type annotations. And the structure of that JSON can be of arbitrary nested depth.

All of this is accomplished using template literal types which allow you to make assertions about the contents of a string. In TypeScript you can assert more than just "this value should be a string". You can make detailed assertions about the structure of strings, and that's what allows these parser demos to exist.

When you combine these template literal types with recursive types, conditional types, and TypeScript's infer keyword you can do some pretty interesting type level programming.

Just to further demonstrate the point, there's an interpreter for the BF programming language, written entirely using TypeScript type annotations [1].

> There's a lot of prior art described in literature as well as practical programming implementations with much crazier, yet successfully working type inference.

Has any of this been demonstrated in Elixir?

[1] https://github.com/sno2/bf

[h0l0cube]

I’m not the GP, but already Elixir can already accomplish compile time text processing with metaprogramming (like it does for inline html templating for ‘heex’ functions and files) and it’s not a huge stretch for it to be able to call a JSON parser on any string knowable at compile time and convert it into a type. That it hasn’t been done yet is probably because no one has deemed to worthwhile to implement. It does sounds cool though.

Metaprogramming is also why generics aren’t really needed in Elixir or Erlang. All specializations boil down to their reified types using macros

[Vaguely2178]

> I’m not the GP, but already Elixir can already accomplish compile time text processing with metaprogramming (like it does for inline html templating for ‘heex’ functions and files)

It's not the compile time text processing that's interesting, it's the fact that this compile time code can yield incredibly detailed types, and the fact that it's all done purely using type annotations. Almost every language that's used to build websites has some sort of server side templating language where you can sprinkle some code into your html templates.

> That it hasn’t been done yet is probably because no one has deemed to worthwhile to implement.

Maybe, or maybe it's less feasible without an advanced type system. The final version of the JSON example we're talking about ended up being 61 lines of code. Without some concrete Elixir code to inspect, this conversation is becoming very abstract.

Honestly even if some of this were possible with metaprogramming, my intuition is that it would be much more verbose and complex. I feel like if I tried to implement all of the features of TypeScript by mucking around with an abstract syntax tree using metaprogramming, I would end up with a difficult to maintain crude approximation of the real thing. I don't think we're giving compiler developers enough credit by saying everything they've worked on can be replaced with a quick macro. José Valim's blog post on static typing ended with an announcement that they've sponsored a PhD student to work on these problems.

After two days of discussion, I think we're reaching a bit of an impasse. Honestly just use what works for you!

[h0l0cube]

> It's not the compile time text processing that's interesting

You've talked right past me. Metaprogramming is not compile time text processing.

> Honestly even if some of this were possible with metaprogramming, my intuition is that it would be much more verbose and complex.

No. It's probably even how the Microsoft team are achieving the typing you're talking about. The special sauce here isn't as special as you think.

> I think we're reaching a bit of an impasse

Not really. I was just saying that what you're talking about is totally possible with metaprogramming. Not making some rhetorical play.

> José Valim's blog post on static typing ended with an announcement that they've sponsored a PhD student to work on these problems.

That's specifically because strict static typing with guards^ is at least difficult, but maybe impossible, and that's what the PhDs are trying to figure out. However, dialyzer isn't as hobbled as you imagine/purport it to be – the crux of my counterargument – and actual experience trumps speculation or casual reading on the topic.

FYI, guards and pattern matching are one of the reasons why Elixir is so damn expressive, and this kind of function typing isn't available/possible in most non-functional languages

^ https://hexdocs.pm/elixir/guards.html

[Vaguely2178]

> You've talked right past me. Metaprogramming is not compile time text processing.

I'm not talking past you. I never said metaprogramming is compile time text processing. You said "Elixir can already accomplish compile time text processing with metaprogramming", and I was just pointing out that the text processing itself is not the most interesting part of the example, it's the resultant types.

> No. It's probably even how the Microsoft team are achieving the typing you're talking about. The special sauce here isn't as special as you think.

But I don't have to reimplement any of this, because Microsoft has already written it. With enough time maybe you could implement dependent typing using metaprogramming for example, but would you then say that Elixir is just as good at dependent typing as Idris, which has that feature built in?

> I was just saying that what you're talking about is totally possible with metaprogramming.

You've graduated from saying "I think someone more savvy with Elixir would know more." to "totally possible"[1]. This does not sound like an argument from experience.

> That's specifically because strict static typing with guards^ is at least difficult, but maybe impossible, and that's what the PhDs are trying to figure out.

A lot of what you're talking about has already been implemented in other languages. This is not untrodden ground. There's no syntactic sugar for guards in JS (which TS just adds type annotations to), but semantically it's very similar to type narrowing in TS [2], which I use daily. This is not something I've read about casually on the internet.

[1] https://news.ycombinator.com/item?id=35944988

[2]https://www.typescriptlang.org/docs/handbook/2/narrowing.htm...

[h0l0cube]

> This is not something I've read about casually on the internet

Was speaking specifically about your opinions on Dialyzer and Elixir. It's very much a cursory Google around and having stronger opinions about the limits of a language than people who spend time with the language.

> You've graduated from saying "I think someone more savvy with Elixir would know more."

It took further reflection on the idea. It helps to start with at least some epistemic humility, once again the crux of this thread. I'm willing to revert to, "It ought to be possible" given that macros boil down to the final generated code (including typespecs) which is then analyzed by Dialyzer

> Elixir is just as good at dependent typing as Idris, which has that feature built in?

Metaprogramming allows features to become 'built in'. e.g., being able to integrate type checking and compiler errors for HTML/XML or other DSLs.

https://hexdocs.pm/eex/EEx.html#function_from_string/5 https://hexdocs.pm/phoenix_live_view/0.17.0/Phoenix.LiveView...