Question

I want to see if I can map Racket structure fields to columns in a DB.

I’ve figured out how to extract accessor functions from structures in PLT scheme using the fourth return value of:

(struct-type-info)

However the returned procedure indexes into the struct using an integer. Is there some way that I can find out what the field names were at point of definition? Looking at the documentation it seems like this information is “forgotten” after the structure is defined and exists only via the generated-accessor functions: (<id>-<field-id> s).

So I can think of two possible solutions:

1. Search the namespace symbols for ones that start with my struct name (yuk);
2. Define a custom define-struct macro that captures the ordered sequence of field-names inside some hash that is keyed by struct name (eek).

Answer 1

The answer depends on what you want to do with this information. The thing is that it’s not kept in the runtime — it’s just like bindings in functions which do not exist at runtime. But they do exist at the syntax level (= compile-time). For example, this silly example will show you the value that is kept at the syntax level that contains the structure shape:

> (define-struct foo (x y))
> (define-syntax x (begin (syntax-local-value #'foo) 1))
> (define-syntax x (begin (printf ">>> ~s\n" (syntax-local-value #'foo)) 1))
>>> #<checked-struct-info>

It’s not showing much, of course, but this should be a good start (you can look for struct-info in the docs and in the code). But this might not be what you’re looking for, since this information exists only at the syntax level. If you want something that is there at runtime, then perhaps you’re better off using alists or hash tables?

UPDATE (I’ve skimmed too quickly over your question before):

To map a struct into a DB table row, you’ll need more things defined: at least hold the DB and the fields it stand for, possibly an open DB connection to store values into or read values from. So it looks to me like the best way to do that is via a macro anyway — this macro would expand to a use of define-struct with everything else that you’d need to keep around.