On 9 February 2011 19:33, Alexey Khudyakov <alexey.sklad...@gmail.com> wrote: > On 09.02.2011 20:57, Chris Smith wrote: >> >> On Wed, 2011-02-09 at 18:15 +0100, Cristiano Paris wrote: >>> >>> I've a type problem that I cannot solve and, before I keep banging my >>> head against an unbreakable wall, I'd like to discuss it with the >>> list. >> >> If I'm understanding your high-level goals correctly, then you're going >> about things the wrong way. It looks like in your "Sealed" type, you're >> accumulating a list of type class constraints that are needed by a >> phantom type, in order to access the value. But type classes are open; >> anyone can make any new type an instance of the type class whenever they >> want. >> > > It's possible to have closed type classes. Trick consist in adding > unsatisfiable constraint. For example: > >> -- This type class is not exported >> class Private a >> class Private a => PRead a > > If Private is not exported one cannot add instances to PRead. > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe@haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >
Indeed, this is exactly what I use in explicit-iomodes: http://hackage.haskell.org/packages/archive/explicit-iomodes/0.6.0.2/doc/html/System-IO-ExplicitIOModes.html#t:ReadModes Bas _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
