[racket] struct-copy with parent type

From: Jay McCarthy (jay.mccarthy at gmail.com)
Date: Fri May 24 15:16:19 EDT 2013

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

Alternatively, you could make a version that lists the possible
children directly:


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

"The glory of God is Intelligence" - D&C 93

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