Jan Stolarek
Ben,
1. Should this feature be placed behind a LANGUAGE pragma?
No, I don't think it should. I consider it a tiny addition to TypeFamilies that is not worth having its separate pragma. Injective TFs are fully backwards compatible, so no existing code will be broken.
That being said, it does claim new syntax and consequently would be rather difficult to back out if we realize that this implementation isn't the right direction. I don't have a strong opinion here, just playing devil's advocate.
2. Could you perhaps add some text to better motivate the feature in the users guide? Done. Tell me whether you like the new version.
Definitely better. Thanks for the quick turnaround!
BTW do we still have nightly builds of User's Guide?
I have a script that I periodically run to push the documentation from my test builds here: http://downloads.haskell.org/~ghc/master/users-guide// It's updated pretty close to daily. Cheers, - Ben