[mattdeboard]

So, homoiconicity is a trifling, meaningless similarity?

"Sure, it may be homoiconic, use prefix notation, have first-class functions (in additional to all the other usual functional paradigms that aren't unique to lisps) but it's not a lisp."

Big ok to that one. This must be pedantry of the highest caliber, not ignorance.

[jon6]

They are not homoiconic. The underlying datastructure for many schemes, and racket, is not a list. It is a syntax object. Of course you can still do metaprogramming with syntax objects but I wouldn't call it the same thing.

[elibarzilay]

You should be careful here, and not lump together "many Schemes" and "Racket" (or other specific Scheme implementations). The thing is that Scheme standards have traditionally avoided tying the language with a macro system that requires some specific representation for syntax -- giving you only the simple rewrite rules system means that you don't actually need to know that representation.

In Racket, OTOH, there are definitely syntax objects with enough functionality to write code that handles them, and I suspect that you know that. The question is whether this should be considered "homoiconic" or not, but this is a kind of a subjective issue, since at an extreme, I can say that all languages that have strings are homoiconic. Perhaps you need more from the language to make it so, maybe eval, or maybe actually require it to have compile-time procedural macros? In any case, Racket will have all of the features that CL does, so it is arguably at least "as homoiconic" as CL is. But in fact, it has more than just s-expressions: these syntax objects are basically sexprs + a bunch of stuff like source location and lexical context, so in fact they represent more than what lists in CL do. Should I then conclude that Racket is more homoiconic than CL? And this is not a tongue-in-cheek argument: in fact, many CL implementations are aware of the limits of sexprs as good representation for code, and add things like source location via a backdoor, like a hash table that maps pair objects to additional properties. Racket does that in its basic syntax representation so IMO it's fine to indeed consider it more homoiconic. And I also say that for the addition of lexical context information -- that's something that is not only included in the Racket syntax object, it's something that you just cannot get in CL, so if homoiconicity is being able to have a high-level representation of code (unlike raw strings), then this is another point where Racket wins the pissing context.

Finally, it's not that all "many Schemes" are limited as described above -- there are many of them that have their own macro systems with similar syntax values, and that includes Schemes that follow R6RS since that dictates syntax-case which comes with them. It just happens that Racket is has been traditionally running at the front lines, so it's more advanced.

[p4bl0]

It's not really necessary to second you, but I'd like to add that "code as data" is more real in Racket than is CL since code is not just the AST, it's also (as you point out) location and more importantly, context. In this setting Racket' syntax objects are more "code as data" than "code as sexp" as it is in CL will ever be.

[elibarzilay]

Right. Perhaps a better way to summarize this is that:

* Lisp made the first giant step of having code representable as data for meta-programming, and chose sexprs to do so

* Common Lisp came later, and made the important step of requiring this representation, which means that in every CL implementation you're required to have the code as data aspect

* But the flip side of this is that CL hard-wires just sexprs, it forbids an extended type, which means that you can't get anything more than sexprs (without resorting to "extra properties" hash table tricks)

* Meanwhile, Scheme (R5 and others that have only `syntax-rules') took a step back by specifying only rewrite rules which can be implemented in any way an implementation chooses

* But some Scheme implementations did use sexprs, but since they need to encode more information (lexical context) they extended them into syntax values (note that some Scheme low-level macro systems try to present users with a simplified interface where user code sees just the sexprs)

* Later on, Racket took further steps and enriched its syntax values with "more stuff"

* R6RS got closer to this too, by adopting the syntax-case system (but some people had issues with "wrapping" symbols, since you can't do that with the hash table trick)

* And finally, R7RS (the "small" version) is going to take a step back into the R5RS days. (And in the "big" language it looks like they'll adopt one of these systems that try to keep the sexpr illusion.)

[lucian1900]

Homoiconicity doesn't refer to lists, but to syntax being represented in the data structures of the language.