There's not much use I have for fexprs. I just had certain ideas about how they fit in with other concepts and how they could be made compatible with a compilation-favoring workflow.
I consider it basically a mistake to use the same syntax for macro calls and function calls... but not because they're different things. When we want to invoke a function like a macro, we can just conceptualize it as a macro that expands into a function call.
I think it's the same for fexprs. Macro calls can more or less expand into fexpr calls by simply preserving all the code they received under a `quote` in their expansion result. This doesn't quite get us a lexical-scope-respecting version of fexprs unless we also capture the local lexical environment, and that's not necessarily possible depending on the macro system, but it's not much of a stretch:
- Racket's macroexpander keeps track of the set of local variables in scope, but it doesn't quite expose it to user-defined macros.
- Arc's macroexpander `ac` keeps track of the set `env` of Arc local variables in scope, but it doesn't expose it to user-defined macros.
- In Guile, the local environment can be captured using (procedure-environment (lambda () '())).
- Fexpress currently lets compilation-friendly fexprs do this using the `depends-on-env?` field of a `compilation-result?` data structure. If a subexpression depends on the lexical environment this way, (fexpress-clambda ...) forms surrounding that subexpression expand differently so that they create a run-time representation of the lexical scope. This way, no one step in the code has to traverse or build up the whole environment, so the cost is spread out. Since Fexpress's variables are immutable, variable accesses don't even have to go through this run-time environment; they can be Racket variable accesses.
Anyhow, I basically consider fexprs to be one of the things a macro-capable language is theoretically capable of, even if people don't commonly prefer to use that functionality and macro systems don't always quite offer it.
I've been interested in exploring the concept of typed macros in general for the purposes of designing module systems which have both macros and typed API signatures. And I've had typed fexprs on my mind as an optimization approach since the Eight thread way back when, and I expect these two trains of thought to be on their way to the same destination.
With Fexpress, I explored the typed fexpr side, and I kept a really narrow focus on types for optimization rather than API boundary delineation so that I wouldn't make it any more complicated than it had to be. I don't want to be like "oh, by the way, in this complex type system for macros, you can find fexprs if you look for them," because some people might recoil at that. :-p And if I said something like that, but I didn't actually have an fexprs-by-default language ready to show, that would be quite a tease.
The actual languages I build with these ideas will probably not have fexpr calls as the default behavior for calling a local variable. Personally, I prefer not to have any default; it's not too much work to write out `funcall` unless I have a whole DSL's worth of local variables I'm invoking. But if it's a typed language, a lot of the local variables will probably have Lisp-1-style behavior anyway, because a variable of function type could be known to have function-call-like macro behavior and so on. And if some of the types people use turn out to declare that their variables have fexpr-like call behavior, I'll consider that to be an interesting outcome. The types, if they do any kind of soundness enforcement (unlike Fexpress's unsound hints), can keep those fexprs from sneaking into other people's code unless they're ready to receive them.
I know API enforcement wouldn't necessarily be your favorite feature in those designs. :-p But I think that's basically the picture of where Fexpress-style fexprs slot into my long-term goals, basically as one part of the possibility space that macros have more room to explore with the help of a type system.