[racket] struct-copy with parent type

From: Matthias Felleisen (matthias at ccs.neu.edu)
Date: Fri May 24 16:40:22 EDT 2013

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 <nickmshelley at gmail.com> 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?
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> --
> Jay McCarthy <jay at cs.byu.edu>
> Assistant Professor / Brigham Young University
> http://faculty.cs.byu.edu/~jay
> "The glory of God is Intelligence" - D&C 93
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