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?
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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