On 9 February 2011 19:33, Alexey Khudyakov
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.
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Indeed, this is exactly what I use in explicit-iomodes: http://hackage.haskell.org/packages/archive/explicit-iomodes/0.6.0.2/doc/htm... Bas