[fouric]

> A lot of the reason why Lispers may be averse to static types is because of the perceived inflexibility it can induce into the system.

This perceived inflexibility is what my comment was getting at - that for primitive type systems available back in the 80's, yes, the types significantly constrained the programs you could write. With today's type systems, however, you have far more flexibility, especially those with "Any" types that allow you to "punch a hole in the type system", so to speak.

When I tried typed Python a few years ago, I found out that, to my surprise, 99% of the code that I naturally wrote could have static types attached (or inferred) without modification because of the flexibility of Python's type system.

I also learned that types are a property of programs, more than just languages. If a program is ill-typed, then having a dynamically-typed language will not save you - it will just crash at runtime. Static types are limiting when either (1) they prevent you from writing/expressing well-typed programs because of the inexpressiveness of the type system or (2) it's burdensome to actually express the type to the compiler.

Modern languages and tools present massive advances in both of those areas. Type systems are massively more expressive, so the "false negative" area of valid programs that can't be expressed is much, much smaller. And, with type inference and more expressive types, not only do you sometimes not have to express the type in your source code at all (when it's inferred), but when you do, it's often easier.

The "Any" type is really what steals the show. I don't think that there's a lot of value in a fully statically-typed Lisp where you can't have dynamic values at all - but I think there's a lot of value in a Lisp with a Python-like type system where you start out static and can use "unknown", "any", and "object" to selectively add dynamic types when needed.

Because, being a Lisper, you probably think like me, I'll give you the idea that really convinced me that types are positive value (as opposed to "only" being small negative value): they enable you to build large, complex, and alive systems.

Types are a force-multiplier for our limited human brains. With types, you can more easily build large systems, you can more easily refactor, you can start with a live REPL and more easily transition your code into source on disk. Types help you design and build things - which is why we use Lisps, after all!

[dokyun]

I don't disagree with you there. The only thing CL really misses out on is that its type system isn't specced out enough to be as powerful as it could be. Since they're just macros you can write all kinds of crazy types, but non-terminating ones just might not work if your implementation doesn't handle them a certain way. This was actually a disputed issue: https://www.lispworks.com/documentation/HyperSpec/Issues/iss....

Someone had proposed reifying types to be first-class objects, which might be a good thing. I haven't thought about it enough to decide. https://gist.github.com/Bike/e405cc49a64fed0752b524c292bd715...

[tmtvl]

Being able to declare types is the reason why I switched from Scheme to Common Lisp. It's just a shame that there's basically no concept of generic types and 'satisfies' isn't quite good enough to make up for it.