On Fri, May 24, 2013 at 1:37 PM, Nick Shelley <[email protected]> wrote: > Thanks Jay. > > So with the generics approach I would just create a gen-struct-copy with the > same signature as struct-copy, then the implementation of gen-struct-copy in > each of my structs would just call struct-copy with the correct struct-id. > Is that the right approach?
A generic can't be a macro, so really you'd have some explicit like a "posn-update-x" function in the generic. > Also, is there a way to introspect the struct-id if the struct is > transparent? Something like: > > (define p (3posn 1 2 3)) > (struct-copy (struct-id p) p [x #:parent posn 5]) > > This obviously wouldn't work if p was a posn, but in my use case, the parent > class is abstract so I know I'm getting child classes. I scanned through > some of the struct docs, but couldn't find a way to introspect a struct > type. It's possible to do something like what you want, but I wouldn't recommend it because there's no way to recover the names of the fields at runtime. You can just know how many there are. For example: > > On Fri, May 24, 2013 at 1:16 PM, Jay McCarthy <[email protected]> > wrote: >> >> On Fri, May 24, 2013 at 11:18 AM, Nick Shelley <[email protected]> >> wrote: >> > The struct-copy docs say "The result of struct-expr can be an instance >> > of a >> > sub-type of id, but the resulting copy is an immediate instance of id >> > (not >> > the sub-type)." Why is this? >> >> The technical reason this is the case is that in (struct-copy <struct> >> ... ...) the <struct> binding describes the fields of the structure >> and gives access to the constructor. Thus, in your example code, you >> are saying "copy this posn" and not "copy this thing and modify the >> posn pieces". You could imagine that we could create a link between >> parent and sub-structures, but it would be messy and imperative, in my >> mind. >> >> Alternatively, you could make a version that lists the possible >> children directly: >> >> >> https://github.com/jeapostrophe/exp/blob/cf6822f41c7637d9074638d0e9b4d4d2d7d27d7b/struct-copy-ish.rkt >> >> Alternatively, you could use generics >> >> > For instance, I would hope this would work: >> > >> > (struct posn (x y)) >> > (struct 3d-posn posn (z)) >> > (3d-posn-z (struct-copy posn (3d-posn 1 2 3) [x 5])) >> > >> > My actual use case is that I'm representing some data with structs. I >> > have >> > the common data in a parent struct and the specific data in the child >> > structs. One of the common fields is a unique id (a number I just >> > increment). I'd like to be able to copy a piece of data and just change >> > the >> > unique id in the struct-copy. Instead I have to have a cond or a match >> > that >> > does the same struct-copy but with different struct ids for each sub >> > type. >> > >> > Is there an easier way to do what I'm trying to do? >> > >> > ____________________ >> > Racket Users list: >> > http://lists.racket-lang.org/users >> > >> >> >> >> -- >> Jay McCarthy <[email protected]> >> Assistant Professor / Brigham Young University >> http://faculty.cs.byu.edu/~jay >> >> "The glory of God is Intelligence" - D&C 93 > > -- Jay McCarthy <[email protected]> Assistant Professor / Brigham Young University http://faculty.cs.byu.edu/~jay "The glory of God is Intelligence" - D&C 93 ____________________ Racket Users list: http://lists.racket-lang.org/users
