[plt-scheme] building mutually recursive structs

You want `shared'.

Robby

At Tue, 20 Jul 2004 13:14:58 -0400 (EDT), "David A. Herman" wrote:
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> 
> What's the most Schemely idiom for building structs with graph structure?
> The most Java-like solution would be to make the accessors functions. The
> Haskellish solution would be to use delay and force. The C solution would
> probably be to run two passes, first creating the objects and then filling
> in the back-pointers.
> 
> I actually prefer the latter, because it requires the least extra work of
> the client: once the data structure is created, you just use the normal
> accessors. But I'm not sure how to abstract over the pattern. I.e., I'd
> like a form that does things like:
> 
>   (define-struct thing (parent value children))
>   (letrec-data ([root (make-thing #f 0 (list (make-thing root 1 null)
>                                              (make-thing root 2 null)
>                                              (make-thing root 3 null)))])
>   root)
>   =>
>   (let ([root (make-thing #f 0 (list (make-thing 'hukairs 1 null)
>                                      (make-thing 'hukairs 2 null)
>                                      (make-thing 'hukairs 3 null)))])
>     (set-thing-parent! (car (thing-children root)))
>     (set-thing-parent! (cadr (thing-children root)))
>     (set-thing-parent! (caddr (thing-children root))))
> 
> I suppose I could write a function that traverses all structs and lists
> within structs, but this is slow and ungeneral.
> 
> Dave