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

[Richard Eisenberg]

The recommended way to do this (force a Nominal parameter) is

> {-# LANGUAGE RoleAnnotations #-}
> data Set a at 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 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
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