(Adding ghc-devs, who will, I hope, have opinions) I'm sympathetic to this. In general our story for pragmas {-# blah #-} is that they affect only optimisation (eg INLINE) or warnings (eg DEPRECATED). You can ignore/leave them out. But here you really can't. Well, I suppose leaving out the annotation does not risk soundness (as in run-time seg-fault); it just risks sets being ordered wrong. I don't have a strong opinion here. You could also imagine other syntaxes e.g. data Set a(role N) = ... I suppose. You are right that we need to get this nailed for 7.8, so we don't end up changing it later. Simon From: Richard Eisenberg [mailto:eir@cis.upenn.edu] Sent: 23 August 2013 05:52 To: Simon Peyton-Jones; Dimitrios Vytiniotis; Stephanie Weirich Subject: concrete syntax for role annotations Hi all, On a Reddit discussion of my recent blog post on roles (http://www.reddit.com/r/haskell/comments/1kgivs/roles_a_new_feature_of_ghc_p...), it was suggested that a better syntax is available for specifying role annotations. Currently, I've implemented

data Set a@N = ...

to mark Set's parameter's role as N. The suggested syntax is

data Set a {-# ROLE N #-} = ...

This suggested syntax is certainly much more verbose, but it's fully backwards-compatible, avoiding the need for preprocessing statements in libraries. These annotations should hopefully be few and far between, and I just think it's worth considering the backwards-compatible syntax before 7.8 is released and we're stuck. What do we think? Should I forward the question to glasgow-haskell-users, perhaps? Richard