Injective type families

[Andres Löh]

Hi.

>> 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.

I think it should be a new language extension. Forward compatibility
on its own is not a sufficient argument. Aren't e.g. GADTs forward
compatible? Or functional dependencies? Or flexible instances? Yet all
of these also have separate language extensions. If I actually want to
write backward-compatible type family code using GHC-8.0, I'd prefer
to be able to enable TypeFamilies yet not InjectiveTypeFamilies, and
have GHC check that I am in the common subset. Also, I think it would
be nice to be able to track how many packages will make use of
TypeFamilies and InjectiveTypeFamilies separately from each other.

Cheers,
  Andres