ASDF components are brittle for backwards compatibility

[Marco Antoniotti]

Hi Robert

Isn't the :properties slot already there?  I just used it (a month ago) to
add an ASDF DOCUMENT op to HELambdaP.  Worked like a charm once I got the
hang of it and it seals the general ASDF machinery from random stuff.

Marco


On Thu, Apr 29, 2021 at 6:13 PM Robert Goldman <rpgoldman at sift.info> wrote:

> ASDF checks to make sure all of the initargs are defined when parsing a
> defsystem. This is good for catching errors, but is terrible for
> extensibility. This means that any attempt to add additional metadata will
> be backwards incompatible.
>
> I can think of two ways to fix this:
>
>    1.
>
>    Add a "garbage can" slot to component that will be filled with a
>    property list, and allow programmers to do whatever they want here. This
>    doesn't seem great to me.
>    2.
>
>    Add a new defsystem parsing error class that is
>    unknown-component-property, raise it when an unknown property is
>    encountered and allow the user to continue, discarding the initarg and
>    accompanying value.
>
> The second alternative is what I favor. It isn't great, because it will
> only maintain extensibility going forward, but I think it's the best we can
> do.
>
> I suppose that we could also add an initarg to tell ASDF to continue such
> errors silently. I'd be inclined to suggest that this take an ASDF version
> expression as value, so that the error is quietly ignored only by ASDF
> versions below that. This means that the property will start to be checked
> when it has become authoritative.
>
> Note that for one's own system and component subclasses, the set of
> initargs can be extended without any monkeying around.
>


-- 
Marco Antoniotti, Associate Professor         tel. +39 - 02 64 48 79 01
DISCo, Università Milano Bicocca U14 2043 http://dcb.disco.unimib.it
Viale Sarca 336
http://cdac2021.lakecomoschool.org
I-20126 Milan (MI) ITALY
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