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Re: User-defined record types

From: Stefan Monnier
Subject: Re: User-defined record types
Date: Wed, 15 Mar 2017 23:05:13 -0400
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/26.0.50 (gnu/linux)

>> However, there is still EIEIO which may well need some expert guidance
>> to update.
> I can take care of that.  Can you put your code on a branch like
> scratch/record?

OK, it's not working.
I found a problem with cl-defstruct objects, tho:

We're going to have trouble preserving backward compatibility with
pre-existing .elc files.  If you look at the macro expansion of current

    (macroexpand '(cl-defstruct (sm-test) a b))
      (defvar cl-struct-sm-test-tags)
      (define-inline sm-test-p (x) (declare (side-effect-free error-free))
        (inline-letevals (x)
                          (and (vectorp #1=,x) (>= (length #1#) 3)
                               (memq (aref #1# 0) cl-struct-sm-test-tags)
      (put 'sm-test 'cl-deftype-satisfies 'sm-test-p)
      (define-inline sm-test-a #2=(x)
        "Access slot \"a\" of `(sm-test)' struct CL-X."
        #3=(declare (side-effect-free t))
        (inline-letevals #4=(x)
             (or (memq (aref #5=,x 0) cl-struct-sm-test-tags)
                 (signal #6='wrong-type-argument (list 'sm-test . #7=(,x))))
             (aref #8=,x 1)))))
      (define-inline sm-test-b #2# "Access slot \"b\" of `(sm-test)' struct 
CL-X." #3#
        (inline-letevals #4#
             (or (memq (aref #5# 0) cl-struct-sm-test-tags)
                 (signal #6# (list 'sm-test . #7#)))
             (aref #8# 2)))))
      (defalias 'copy-sm-test (function copy-sequence))
      (cl-defsubst make-sm-test
          (&cl-defs (nil . #9=((cl-tag-slot) (a) (b))) &key a b)
        "Constructor for objects of type `sm-test'."
        (declare (side-effect-free t))
        (vector 'cl-struct-sm-test a b))
        (cl-struct-define 'sm-test nil 'cl-structure-object 'nil nil '#9# 
'cl-struct-sm-test-tags 'cl-struct-sm-test 't))

As you can see, the code says it inherits from `cl-structure-object'
(which now is of `record` type), but its constructor `make-sm-test`
creates a vector rather than a record, so (cl-typep 'cl-structure-object)
will fail if it only considers `type-of'.

Even if we could arrange for cl-struct-define to override the
definition of make-sm-test, that wouldn't help with all the places
where make-sm-test has been inlined already.

[ Of course, the reverse could happen as well: a new struct type (using
  `record` in its constructor) could inherit from a parent that's still
  using `vector`.  But I think we can arrange for that to happen only in
  cases we don't care to support.  ]


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