Misha Aizatulin wrote:
The question I'd like to ask is whether there is some trickery to
circumvent this problem. In the f function I'd like to be able to hint
to the compiler that I want Show to be derived from cxt which is
attached to the Box, but I see no way of doing that.
An explicit way of pointing at the dictionary is possible using GADT:
data ShowCxt a where
Sh :: Show a => ShowCxt a
f :: forall cxt . (Contains cxt ShowCxt) => Box cxt -> String
f (Box (x :: a)) = case subDict (dict :: cxt a) of Sh -> show x
The above compiles in GHC HEAD (6.6 is unfortunately not enough).
Regards,
Zun.
_______________________________________________
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
