That slot is there, but it is specifically noted as being for ASDF 2 compatibility only, and not to be used. So you are using this at your own risk.

Having a :plist slot might be a good addition, as well (i.e., both of my 2 proposed solutions) as something lighter weight than my second alternative which is aimed at gracefully being able to extend ASDF's canonical metadata properties.

On 29 Apr 2021, at 13:00, Marco Antoniotti wrote:

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@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
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