[dTal]

And yet Scheme hasn't turned into something like that. There's a limit to how radical a syntax you can achieve using (define-syntax), and there's no provision for modularity. Worse still, syntax definitions are "imperative" - it's really hard to see what they do by looking at them.

I'd like to see a system where you have tiny, composable pieces of syntax written declaratively in a domain specific language that's as terse and general as possible - stuff like "a[n] -> (list-ref a n)" and "a..b -> (iota a b)". You want to make it as easy as possible to port language concepts to the system. Most of the wacky semantics you can get in other languages can be had in Scheme through some sort of library already, they're just usually kind of a pain in the ass to use without the syntax. Then making the language of your dreams is just a matter of enabling all the syntax you like. You'd need some kind of mechanism for handling grammar conflicts.

Of course, it's really hard. "Syntax" masks a lot of complexity. A feature I want, for instance, is to have the whole language behave like a CAS, where instead of bombing out on an undefined symbol it just computes as much as it can symbolically and hands it back to you e.g. "x+2+3" returns "x+5" if x isn't defined. That sort of thing has been done in Scheme of course, but it's hard to imagine a syntax specification language that would let you just turn that on as a single "feature" without massively affecting basic things like expression parsing grammar.