[metacopy-devel] Structure sharing and unbound slots

[Jan Moringen]

Hi,

thanks for metacopy, it's very useful.

I have a small problem and associated suggestion to make, though.
Currently the "duplicate-set" behavior actually has two behaviors (I'm
not completely sure about the first one):

      * setting the slot value in the copied object to the result of
        evaluating an arbitrary form
      * setting the slot value in the copied object to the slot value in
        the original object such that they are `eq' and therefore share
        structure

When used in the second mode, there is no check to make sure the slot is
bound in the original object. Currently, this makes it impossible to
have "safe" copying of slots without invoking the deep copy machinery.

My suggestion is to separate the "duplicate-set" behavior into

      * a modified "duplicate-set" behavior (keyword :set) that does not
        access slots in the original object, but assigns the result of
        evaluating a form
      * a new "duplicate-share" behavior (keyword :share) that does a
        "safe" (nop if unbound) structure sharing assignment

The attached patch roughly implements the suggestion.

What do you think?

Kind regards,
Jan
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