From my point of view, attachments should be orthogonal to types. Basically, an attachment is any piece of information you want to attach to an object, and lives and dies with that object. That an attachment is used to overload 'keys or '= is just a use of the attachment concept.
For example, we might want to build a reader which keeps track of line numbers. The reader's output is still 'cons and 'sym, etc., but with an attachment. Each 'cons cell has a 'linenumber attachment which we can use. For example, a macro whose syntax has been violated would be able to report the line number where this violation occurs. This is useful if the macro is used often enough and there is a need to locate the line number of the error, or if its syntax is like CL 'loop and you expect it to span several lines.
In all cases, the cons cell produced by this hypothetical reader is a cons cell. Its representation is a cons cell and is only a cons cell. However, we can extract additional data from it. After it passes through the evaluator and is discarded as trash, its attached data can be thrown away.
In any case file-table.arc only cares that settable functions work, and settable functions only care that attachments work. Whether we make attachments orthogonal to types, or separate stuff-with-attachments as types may not really matter so much anyway. This is Lisp, after all.
I agree that attachments should be orthogonal to types - that's why I added the "isa" overloading to settable-fn2. But I don't think annotations should be orthogonal.
The thing is, there's no way we'll be able to add arbitrary attachments to any object and have it continue to behave just as if there were no attachments in Arc. We'd need to either modify the core to give each object a Python/Ruby/Javascript/etc-style implicit table, which I don't think PG is likely to be very fond of (and which I don't think is a good idea besides); or we need to accept that there will be some cases where we won't be able to get attachments without a few compromises.
"Now this is the noble truth of the origin of suffering: it is this attachment which leads to renewed existence, accompanied by delight and lust, seeking delight here and there, that is, attachment to sensual pleasures, attachment to existence, attachment to extermination."
Therefore... buddha pg, please enlighten us and deliver us from attachment! ^^
