- There is a bug in GHC that allows programs to use DataKinds without the -XDataKinds extension.
- The MR fixes the bug.
- But of course that will mean that some programs that previously compiled (exploiting the bug) will now fail to compile (correctly).
It seems like an example of our (GR2) discussion, but with a very different flavour than the (3+4=8) example.
Personally I think we should do it -- with a deprecation cycle saying "You need DataKinds for this program, and you don't have it on, but you will need it in the next release". I don't want us to grandfather bugs into GHC indefinitely -- although you could argue that the status quo does little harm, I suppose.
Any views? We will have some idea of the impact on head.hackage shortly.
And specifically: do you want a GHC proposal for this change?
Simon