[Haskell-cafe] Kind-agnostic type classes
fw at deneb.enyo.de
Fri Oct 3 10:09:02 EDT 2008
* Derek Elkins:
> On Fri, 2008-10-03 at 12:22 +0200, Florian Weimer wrote:
>> I'm trying to encode a well-known, informally-specified type system in
>> Haskell. What causes problems for me is that type classes force types
>> to be of a specific kind. The system I'm targeting however assumes that
>> its equivalent of type classes are kind-agnositic.
>> For instance, I've got
>> class Assignable a where
>> assign :: a -> a -> IO ()
>> class Swappable a where
>> swap :: a -> a -> IO ()
>> class CopyConstructible a where
>> copy :: a -> IO a
>> class (Assignable a, CopyConstructible a) => ContainerType a
>> class (Swappable c, Assignable c, CopyConstructible c) => Container c where
>> size :: (Num i, ContainerType t) => c t -> IO i
> instane Container Maybe where...
> What does copy :: Maybe -> IO Maybe mean?
Maybe is not an instance of CopyConstructible, so this shouldn't
compile. On the other hand, for IORef a,
copy = (>>= newIORef) . readIORef
and for mutable arrays,
copy = mapArray id
(if I'm not mistaken).
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