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?
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. 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 >
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