Although I have hardly any experience with racket/class, and I'm not very knowledgeable about how Racket structs are implemented, this got me curious to look at how struct-copy is implemented.
First, your macro appears to be trying to work around the problem that `get-field-names` only makes sense at runtime (when given some particular object at runtime) -- but your macro is at compile-time. That's probably why you tried the nested `(with-syntax #'( (with-syntax (eval-syntax #'( )))))`. > And also, is there another version of field-names that takes a class instead > of an object, because I think that will create another problem when I get to > the last test (with not-really-chum). `class-info` looks promising for that. If only you could get a `class?` at compile time. But I haven't figured out how. Racket struct identifiers have a meaning both at run time (as a constructor) and at compile time (as a transformer binding to struct info): "5.7 Structure Type Transformer Binding The struct form binds the name of a structure type as a transformer binding that records the other identifiers bound to the structure type, the constructor procedure, the predicate procedure, and the field accessor and mutator procedures. This information can be used during the expansion of other expressions via syntax-local-value." As a result, struct-copy can use syntax-local-value to get the compile-time value, and pass it to extract-struct-info, to get a list of field identifiers. But unfortunately a class identifier does not seem to have any syntax-local-value at all (much less one that you could pass to class-info). That's my attempt at figuring it out. Hopefully one of the Racket experts can give you an actual solution (and/or correct any of what I wrote here). ____________________ Racket Users list: http://lists.racket-lang.org/users
