This exact code couldn't go in, because struct-copy is written in the kernel language, so bringing in syntax-parse and match is not allowed. The definition is about 150 lines because of all the different error checks and the pain of #%kernel.
Furthermore... this code has some problems like not rebinding all the fields simultaneously, which struct-copy does. BUT the idea of a new racket/struct module with improved version of macros like this would be nice in my mind. In particular, it might be a nice place for a super-struct like [1] macro that gives you keyword field names, control over the names of predicates, etc. What does your struct-open do? 1. https://github.com/jeapostrophe/exp/blob/master/sstruct-tests.rkt On Fri, May 24, 2013 at 2:40 PM, Matthias Felleisen <[email protected]> wrote: > > 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 > -- 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
