Back in the 90s when I lived in Emacs, I had this running. I was genuinely surprised at the insights it would enable. For example, I would be composing and email or writing a document and it would surface relevant chunks from emails and documents that I had not considered or remembered.
I have been wanting the equivalent for macOS for years.
In 2004 I tried integrating the RA into OS X via the Input Manager mechanism, a way to load external code into Cocoa apps that'd have access to the contents of any text field, so you could get the text that the user was currently looking at and send it to the RA for queries.
If I recall correctly, I never had enough time to make it very useful, and it wasn't clear how best to get its input sources - Mail.app stored email it could read, but other things like calendar and chats were inaccessible to it.
(NOTE: the RA code isn't there, I think it was never even useful enough for me to share it)
My favorite use of that was the "ISIM", or emacs-style incremental search in text views. That was nice.
But of course, loading arbitrary external code wasn't sustainable, and that mechanism went away, along with my ISIM.
I no longer track OS X development, so I'm not sure if there's a new equivalent way to do this stuff. It'd be neat to hear.
Not the same thing, but something close: if you have _enough_ prior information stored, something like DevonThink can pick out "related fragments" (i.e. while running locally on privately owned data).
> I have been wanting the equivalent for macOS for years.
Is it really a macOS-type thing? Emacs is all about tying together everything one can do on a computer into one integrated whole where anything can talk to anything; macOS seems to be about a well-integrated UI over top of siloed, proprietary components which can only be integrated to the extent that they wish to be integrated.
Compare to Emacs, wherein one can just rewrite and replace a function in a package which does not play well with others.
I have been wanting the equivalent for macOS for years.