The recommended way to do this (force a Nominal parameter) is
{-# LANGUAGE RoleAnnotations #-} data Set a@N = Set a
The @N annotation forces `a`'s role to be Nominal, as desired, but makes no other change to the type. Note that role annotations can only "increase" roles -- you can't use role annotations to force, say, a type parameter to a GADT to have role R. You can read http://ghc.haskell.org/trac/ghc/wiki/Roles or my recent blog post http://typesandkinds.wordpress.com/2013/08/15/roles-a-new-feature-of-ghc/ for more introduction. To see the roles that GHC assigns to type variables, I've used -ddump-tc. The role annotations are all printed in the output. Richard On Aug 18, 2013, at 1:45 PM, Joachim Breitner wrote:
Hi,
now that roles are in HEAD, I could play around a bit with it. They were introduced to solve the unsoundness of newtype deriving, but there is also the problem of abstraction: If I define a set type based on an ord instance, e.g.
data Set a = Set a -- RHS here just for demonstration
the I don’t want my users to replace a "Set Int" by a "Set (Down Int)", even though the latter is a newtype of the former. This can be prevented by forcing the role of "a" to be Nominal (and not Representational, as it is by default). What I just noticed is that one does not even have to introduce new syntax for it, one can just use:
type family NominalArg x type instance (NominalArg x) = x data Set' a = Set' (NominalArg a)
and get different roles; here the excerpt from --show-iface (is there an easier way to see role annotations):
5b7b2f7c3883ef0d9fc7934ac56c4805 data Set a@R [..] 8e15d783d58c18b8205191ed3fd87e27 data Set' a@N
The type family does not get into the way, e.g.
conv (Set a) = Set' a
works as usual.
(I now also notice that the parser actually supports role annotations... but still a nice, backward-compatible trick here).
Greetings, Joachim
-- Joachim “nomeata” Breitner mail@joachim-breitner.de • http://www.joachim-breitner.de/ Jabber: nomeata@joachim-breitner.de • GPG-Key: 0x4743206C Debian Developer: nomeata@debian.org _______________________________________________ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users