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

Joachim Breitner mail at
Sun Aug 18 19:45:58 CEST 2013


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):

          data Set a at R
          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).


Joachim “nomeata” Breitner
  mail at joachim-breitner.de
  Jabber: nomeata at  • GPG-Key: 0x4743206C
  Debian Developer: nomeata at
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