Re: meta information about structure types

[Bruno H. <br...@cl...>]

Sam wrote:
> > The user calls either
> >     (class-direct-slots (find-class name))
> > or
> >     (structure-direct-slots name)
> >
> > depending on what he wants.
>
> what if he does not know whether NAME is implemented with a DEFCLASS or
> a DEFSTRUCT?
> you want him to write
>
> (let ((c (find-class name)))
>   (if c
>       (class-direct-slots c)
>       (structure-direct-slots name)))

This is a hypothetical use case. Doesn't occur in practice. (Where would
the 'name' come from?)

> or
>
> (let ((c (class-of object)))
>   (if (class-p c)
>       (class-direct-slots c)
>       (structure-direct-slots (class-name c))))

Yes. The second branch can never be taken. For a good reason: There's
no way to use the structure-* API when all you have is an object.
If an object is given and it might be a list or a vector, you cannot
determine which structure it comes from. (Is #(FOO 3 BAR #\X BAZ) a
FOO, a BAR, or a BAZ?) Therefore when all you have is an object, you
can only rely on the class-* API.

In other cases you can only use the structure-* API. But never both.

Bruno