[Haskell-cafe] "Universal" constraints?
lambda.fairy at gmail.com
Sun Jun 21 10:05:01 UTC 2015
On Sun, Jun 21, 2015 at 5:55 PM, Michael Walker <mike at barrucadu.co.uk> wrote:
> Hi all,
> I was wondering about constraints in class declarations. Suppose we have
> something like this:
> class Monad m => Store m where
> type Var m :: * -> *
> new :: a -> m (Var m a)
> read :: Var m a -> m a
> write :: Var m a -> a -> m ()
> Then some reasonable implementations would be IO and ST, using IORefs and
> instance Store IO where
> type Var IO = IORef
> new = newIORef
> read = readIORef
> write = writeIORef
> instance Store (ST s) where
> type Var (ST s) = STRef s
> new = newSTRef
> read = readSTRef
> write = writeSTRef
> Now, both IORefs and STRefs have equality, which doesn't require equality of the
> contained type,
> instance Eq (IORef a) where ...
> instance Eq (STRef s a) where ...
> This corresponds to pointer equality and, because reading the values is a
> monadic action, is as good an Eq instance as we can get. Given this, it would be
> nice to be able to specify that all instances of our Store class have this
> property. Something like,
> class (Monad m, Eq (Var m a)) => Store m where ...
> But we can't write this! The `a` isn't in scope! What I really want to do is to
> be able to write class constraints like this,
> class (forall a. C (T a)) => D x where ...
> Or why not get really crazy:
> class (forall a. C a => D (T a)) => E x where ...
> Is there some extension or combination of extensions that would make this work
> without completely sacrificing type safety? FlexibleContexts allows it (the
> simpler form, at least) for function constraints, but not for class constraints.
> If not, has anyone thought about this sort of thing before?
One solution is to add an equality operator to the class itself:
class Monad m => Store m where
type Var m :: * -> *
eqVar :: Var m a -> Var m a -> Bool
It may not be as satisfactory as subclassing Eq itself, but it should
work in principle.
> Michael Walker (http://www.barrucadu.co.uk)
> Haskell-Cafe mailing list
> Haskell-Cafe at haskell.org
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