[racket-dev] struct type properties and field accesses

On 11/13/2011 09:21 PM, Sam Tobin-Hochstadt wrote:
> Many built-in structure type properties (such as `prop:procedure' and
> `prop:set!-transformer') support providing an integer index into the
> structure to reference a particular field that should supply the
> value.  This is a very useful pattern, but unfortunately, it's not one
> that any Racket programs can use.  In particular, the procedure
> calling mechanism and the expander get around the inspector system to
> implement this behavior.
>
> It would be nice if additional structure type properties could do this
> as well.  Here's a suggestion of how it could work.
> `make-struct-type-property' adds an optional argument which specifies
> that an integer value is an index into the structure.  In that case,
> the generated property accessor procedure performs the field lookup
> itself, avoiding the inspector system.  Note that this doesn't reduce
> the power of the inspector system, since the structure type property
> value could be the struct accessor procedure paired with the index,
> and then you could write your own struct property accessor procedure
> that did the dispatch.  This is less convenient for both users and
> definers, and thus it would be nice to build it in.

I think you can do this yourself already using the guard argument of 
make-struct-type-property. The guard function gets a list that "contains 
the values that struct-type-info would return for the new structure type 
if it skipped the immediate current-inspector control check".

Ryan