We need to add role annotations for 7.8

[Mikhail Glushenkov]

Hi Richard,

On 14 March 2014 15:22, Richard Eisenberg <eir at cis.upenn.edu> wrote:
> There has been some thought about making `coerce`'s behavior different with
> -XSafe than without. See, for example, tickets #8745
> (https://ghc.haskell.org/trac/ghc/ticket/8745) and #8827
> (https://ghc.haskell.org/trac/ghc/ticket/8827). In the end, we decided to
> remove this, instead believing authors' role annotations (or lack thereof).

Interesting, it looks like the paper hasn't yet been updated to reflect this.

> Note that 7.8 does not actually give users a new way to break abstractions
> -- GeneralizedNewtypeDeriving has been around for a while. What 7.8 provides
> is a way to break abstractions more easily (`coerce`) and a way to prevent
> this from happening (role annotations).

Isn't this false in the case of -XSafe? In 7.8 one will be able to
coerce 'Map Int a' to 'Map Age a' (provided that there is no role
annotation for Map), which didn't work previously because SafeHaskell
prohibited GND.