(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:e...@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_patch_gnd_type_families/),
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
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