Injective type families

[Simon Peyton Jones]

|  >>   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.

On a branch we are working on a fundep-like extension of injectivity, which is a rather bigger feature. Personally I think it might be better for there to be a LANGUAGE pragma to signal that we are using it (both the current r -> a form, and the new piece).

Simon