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
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