Marking type constructor arguments as nominal (e.g. Set)

[Joachim Breitner]

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 at R
        [..]
        8e15d783d58c18b8205191ed3fd87e27
          data Set' a at 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 at joachim-breitner.de • http://www.joachim-breitner.de/
  Jabber: nomeata at joachim-breitner.de  • GPG-Key: 0x4743206C
  Debian Developer: nomeata at debian.org
-------------- next part --------------
A non-text attachment was scrubbed...
Name: not available
Type: application/pgp-signature
Size: 198 bytes
Desc: This is a digitally signed message part
URL: <http://www.haskell.org/pipermail/glasgow-haskell-users/attachments/20130818/2db703d6/attachment.pgp>