On Oct 25, 2:27 am, "Marshall T. Vandegrift" <llas...@gmail.com>
wrote:
> Alan Malloy <a...@malloys.org> writes:
> > It seems to me that it would be nice to have macros automatically
> > include, on their result forms, the metadata from their input
> > &form. Of course, macros may wish to add metadata as well, so the two
> > maps should probably be merged. However, there are certainly some
> > problems with this approach: for example if a macro wants to return
> > something that can't suppport metadata (like an Integer), the compiler
> > needs to be careful not to try to include it. So I'm hoping the
> > community can comment on whether this feature would be useful, or
> > whether there are fundamental problems with it that I haven't
> > foreseen. Is there a reason this can't make it into a future version
> > of Clojure?
>
> I think this is an excellent idea. Overall I believe this reduces the
> number of situations in which one needs to be actively aware that a
> particular expression will be subject to macro-expansion. The
> alternative-world where the 99% of well-behaved macros returning IMetas
> manually forward metadata from &form seems like a world with way too
> much boilerplate to me.
>
> Forwarding metadata does preclude macros which use metadata applied to
> &form as parameters and construct new metadata which may not include the
> literal values specified in the user-supplied metadata. I'm not sure
> this is a good idea anyway, but this sort of case -- and any others
> where metadata should not be forwarded/merged -- could easily be
> supported by providing a variation of defmacro with the current
> behavior. Or perhaps by configuring the metadata-forwarding behavior
> via metadata on the macro var -- something like:
>
> (defmacro foo {:forward-meta false} ...) ;; or,
> (defmacro ^:replace-meta foo ...)
>
> If a macro expands to something which doesn't implement IMeta, then I
> believe the compiler needs to error out if metadata is applied to it,
> just as it does applying metadata to non-IMeta literals. To do
> otherwise would be inconsistent, and result in the same silent data-loss
> as macros are yielding today.
>
> This proposal doesn't touch forwarding metadata on forms which become
> macro arguments, obviously. There's still room for inconsistency there,
> but I think that's clearly in the court of individual macro authors to
> implement the correct behavior.
>
> Are there other contexts where metadata should potentially be forwarded?
> I don't know how often this comes up, but:
>
> (= (meta ^:foo (quote foo)) (meta (quote ^:foo foo)))
> ;; => false
I filed a ticket and wrote a patch: it's at
http://dev.clojure.org/jira/browse/CLJ-865
if you're interested. As for your last snippet, I suppose it might
conceivably make sense to do this forwarding but I think it's
preferable to just understand what reader metadata and quoting mean,
since the difference between these is a fairly necessary consequence
of those things.
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