Comment by dieggsy

I think I mostly agree with the thesis here, but I actually liked the point in the title at face value, too. We need both, I think. I guess I'll be another annoying Lisp guy, but:

> At this point, the right question to ask would be, well can you write a static-typing library for Scheme that then automatically checks your code for type errors? And the current answer, for now and for the foreseeable future, is no. No mainstream language today allows you to write a library to extend its type system.

The author seems to provide a counter example themselves(?):

> Racket and Shen provide mechanisms for extending their type systems...

I wonder if this is as clear-cut as the author is making it out to be. Coalton, which is effectively a library (and language) for Common Lisp, seems like it basically does this. Maybe that's not exactly what the author is referring to, because it is essentially a new language on top of Lisp using its meta-programming facilities, as opposed to merely extending the type system. Still, it can be used as a library right alongside Lisp code, so I think it's in the same spirit of of the first question of writing a "static-typing library that automatically checks your code" in a dynamic language.

Standard scheme may or may not be able to do this, but most Scheme implementations have unhygienic macros like CL's too, so I'd assume something similar would be possible. The fact that that these tend to be extensions from implementation designers might align with the article's point though. Also somewhat to the author's point, Coalton does rely strongly on CL's underlying type system, for which there's no real equivalent in Scheme. It also relies on implementation-specific optimizations alongside that.

For what it's worth you can (and indeed people have) written object systems in Scheme, despite the language not having one, though they tend not to be performant, which is likely another point towards using/writing a different language. CL also tends to allow fairly deep extension of its object system through the Meta-object Protocol.

I guess my point is that in my (probably biased) opinion, Lisps, or other languages with very strong meta-programming facilities, are pretty close to the language longed for in "Perhaps one day we'll have such a language." They aren't a silver bullet, of course. CL has no easy way to write performant coroutines/continuations, for example, even given all its extensibility. Scheme has no real type system, etc. etc.

I don't think any of this invalidates the articles points, I'm just not sure I agree with the absolutes.