On Mon, Mar 24, 2014 at 10:14 AM, Mark Lentczner <mark.lentczner@gmail.com> wrote:

On Fri, Mar 14, 2014 at 2:36 AM, Johan Tibell <johan.tibell@gmail.com> wrote:

I'm quite worried about this change though. I thought the default role for data type was nominal, as that's the safe default. If the default is representational, every package author will need to be aware of this feature and update their packages. That's quite a high cost.

On Fri, Mar 14, 2014 at 6:00 AM, Brandon Allbery <allbery.b@gmail.com> wrote:

Nominal default breaks everything that uses newtype deriving and doesn't have role annotations, doesn't it? Representational default means things behave in 7.8 as they did in earlier GHCs.

Am I reading these pair of statements correctly? It seems to imply to me that every parameterized type that uses a type constraint on a parameter must be reviewed and possibly annotated to work correctly, one way or the other!

No; if the default is representational, everything works as it did in earlier versions, potential bugs/unsafety and all. If the default is immediately switched to nominal, *then* every affected type must be reviewed immediately. My counter-proposal was to have 7.8 default to representational to give library maintainers a release cycle to add the necessary annotations, then switch the default to nominal in 7.10 to get the additional safety.