Util suggestion: copy-object

[Ambrevar]

I might be missing an existing implementation of copy-object, but I find it useful sometimes.

Shallow copy:

(defmethod copy-object (object &rest slot-overrides)
  (let ((class-sym (class-name (class-of object))))
    (apply #'make-instance class-sym
           (append
            slot-overrides
            (alexandria:mappend
             (lambda (slot)
               (list (intern (symbol-name slot) "KEYWORD")
                     (slot-value object slot)))
             (mopu:slot-names class-sym)))))) 

• I suggest making it a method so that users can specialize the copy on their own specific objects.
• Indeed, the above function assumes that the initarg is the keyword version of the slot name. This is a strong assumption that won't work in all cases.
• The above function copies the slots in the make-instance call so that initialize-instance :after has the slots set the right values already. Without this requirement, we could first call make-instance without initializing any slot and then loop over the slots, which has the benefit of not relying on a guess of the initargs. The downside is that some classes require slot values at make-instance time.
• Maybe add an argument to allow for deep copies, that is, replace slot-value with copy-object so that it copies the slots recursively.

Thoughts?

[ruricolist]

I'll think about this. I don't want to take on the bigger issue of what it means to "copy" something (see Pitman's essay on the subject), but I could see a place for a shallow copy-object function that works for CLOS objects the same way copy-structure works for structs.