Jay, should this code go into the struct-copy module? I had to write a struct-open macro recently, and I think I should add it too.
On May 24, 2013, at 3:16 PM, Jay McCarthy 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 > ____________________ > Racket Users list: > http://lists.racket-lang.org/users ____________________ Racket Users list: http://lists.racket-lang.org/users
