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

Non-injectivity of type families sometimes hinders type inference. Consider

Set' (Set.singleton 'c')

Naively, we only know that (Char ~ NominalArg a). Since type families are not necessarily injective, this usually means we cannot determine the type variable `a' from this constraint. However, since the NominalArg instance is parametric in `a', I suspect GHC might successfully infer (Char ~ a) in this case. … A quick ghci test confirms that GHC does infer (Char ~ a) here. I apologize for not just experimenting in the first place, but maybe this email will save someone slightly more time than is required to read it. :P On Sun, Aug 18, 2013 at 3:37 PM, Joachim Breitner wrote:

Hi,

not sure – where would injectivity be needed?

Greetings, Joachim

Am Sonntag, den 18.08.2013, 15:00 -0500 schrieb Nicolas Frisby:

...

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 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 mail@joachim-breitner.de • http://www.joachim-breitner.de/ Jabber: nomeata@joachim-breitner.de • GPG-Key: 0x4743206C Debian Developer: nomeata@debian.org

_______________________________________________ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users

_______________________________________________ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users

-- Joachim “nomeata” Breitner mail@joachim-breitner.de • http://www.joachim-breitner.de/ Jabber: nomeata@joachim-breitner.de • GPG-Key: 0x4743206C Debian Developer: nomeata@debian.org

_______________________________________________ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users