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
> [email protected] • http://www.joachim-breitner.de/
> Jabber: [email protected] • GPG-Key: 0x4743206C
> Debian Developer: [email protected]
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