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Re: User-defined record types

From: Stefan Monnier
Subject: Re: User-defined record types
Date: Wed, 15 Mar 2017 10:38:23 -0400
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/26.0.50 (gnu/linux)

>>> - If you don't pass the :type option to cl-defstruct, you get a
>>> cl-struct-FOO type tag in slot 0.
>> That's right.  And this symbol's value is the class object.
>> The same holds for EIEIO.
>> These are the main candidates for use of the new `record` type.
> Yes, at first I tried to make cl-deftype do this by default.  But I ran
> into problems in cl-preloaded.el and cl-generic.el.

I can help with that.

> Right, I just wanted to list all possible variations to gain some
> understanding.  For example, I don't know if it makes a difference if
> the slot 0 type tag has the form cl-struct-FOO as is the default, or
> just FOO as when you specify :type.  But now I know there might be.

The cl-struct-FOO symbol was used instead of just FOO in order to try
and avoid false-positives (i.e. to try and avoid mistakenly recognizing
some random array as being a cl-struct object).

With `record` you don't need that trick any more, so records could have
the FOO symbol in their slot 0.

For EIEIO objects, it would slow things down a bit, because EIEIO
accessors needs to consult the class object, so the time it takes to get
from (aref obj 0) to the actual class object is important: if we put the
actual type symbol, that means that we'd need something like

    (get (aref obj 0) 'cl--class)

whereas the current code uses

    (symbol-value (aref obj 0))

The problem being that the identifier used for the type name might
already be used as a variable as well as as function, so we can't use
the `symbol-value` or `symbol-function` slot.

As mentioned, ideally, we'd want to store the class object directly in
the slot 0.  The downside is that prin1 would then dump the class object
as well, so when reading dumped objects we'd end up creating another
copy of the class object rather than reusing an existing class object.
And this will break cl-generic dispatch which compares class objects
with `eq`.


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