I think this is more familiarity bias and the presence of an enforced type system than a necessity of auto-complete. In the same way that `a.` auto-completes to methods of `a`, `(fun ` could auto-complete to a list of in-scope variables that satisfy `fun`'s type signature. A lot of lisp is untyped though, even in variants that have support for it.

I also think there's some familiarity bias to the OO style. I don't find it particularly natural, though that's subjective. I often know what I want to do, and have to find the object that has the method. E.g. I know I want to read a particular header, but is it on Request, or on Request.Headers, or are headers passed in as a separate object? It feels cleaner to do something like `(get-header "SOME-HEADER" ` and have the IDE tell me I need to pass in `(get 'headers request)` or similar.