<guid isPermaLink> never ever true

[jidanni]

Gentlemen, as everywhere in the WordPress code there is <guid
isPermaLink="false">, and nowhere is there <guid isPermaLink="true">,
how can one blame RSS readers for not being able to mark "crossposts"
as read, even between two feeds from the same site. See
​http://news.gmane.org/group/gmane.emacs.gnus.general/thread=68335/force_load=t/focus=68343
Surely there are sometimes one can emit a confident "true" value.

[DD32]

The GUID should not be relied upon to be a Permalink to the post, It can be "235234985732459827498524gdhgdg" and still be a guid.

A Guid is supposed to be Globally unique across ALL feeds(despite what that link you supplied suggested). The fact that the permalink flag is set to false shouldn't affect the readers in the least.

However, In the case where pretty-permalinks are NOT in use, and the guis matches the url of the post, It might be a case where it should be set to true.. i'm not sure, but that's not something that can be believed 100% due to guid being able to be set to anything

[jidanni]

Yes, we know the GUID and URL are different. Smart sites purposely
make them look different too.

A Guid is supposed to be Globally unique across ALL feeds

only if you mark it as true.

But you always mark it as false.

Therefore how can a reader that plays by the rules use it across ALL
feeds?

Here WordPress is the authority on if what it is emitting is true or
false, and it always says false... because it doesn't even trust
itself?

In the case where pretty-permalinks are NOT in use

I'm afraid we're talking about two different meanings of 'permalink',
the index.php?... meaning, and the RSS GUID meaning. But we must
remember that GUIDs are not to be confused with URLs...

[DD32]

I'm afraid we're talking about two different meanings of 'permalink', the index.php?... meaning, and the RSS GUID meaning. But we must remember that GUIDs are not to be confused with URLs...

Exactly.

<guid>dfgdjfhgkjsr5dwk5983479583465</guid> is interprated as that its a URL to the post, specifically that its a Permanent Link to the item. (isPermaLink defaults to true)

<guid isPermaLink=false">dfgdjfhgkjsr5dwk5983479583465</guid> is interprated that its a Unique Identifier only, it CANNOT be trusted to be a Permanent Link to the item (even if its in an URL form)

Therefor, You original statement is invalid:

how can one blame RSS readers for not being able to mark "crossposts" as read, even between two feeds from the same site.

Because the GUID's are the same, The RSS reader should understand that they're the same.

I think you may've just mis-understood the use of the isPermaLink attribute.

[DD32]

Ref: ​http://feedvalidator.org/docs/error/InvalidPermalink.html

[jidanni]

OK you win as I see on ​http://cyber.law.harvard.edu/rss/rss.html .

Gee, they had to tie unique message IDs to URLs. Usenet beats that.

I'll tell those aforementioned GMANE chumps...