Is the non-injectivity not an issue here because the type family application gets immediately simplified?
On Sun, Aug 18, 2013 at 12:45 PM, Joachim Breitner <m...@joachim-breitner.de > 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 > m...@joachim-breitner.de • http://www.joachim-breitner.de/ > Jabber: nome...@joachim-breitner.de • GPG-Key: 0x4743206C > Debian Developer: nome...@debian.org > > _______________________________________________ > Glasgow-haskell-users mailing list > Glasgow-haskell-users@haskell.org > http://www.haskell.org/mailman/listinfo/glasgow-haskell-users > >
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