State Transformer

[Ashley Yakeley]

At 2002-01-19 08:16, Marcin 'Qrczak' Kowalczyk wrote:

>Here is mine. The same code can work with MVars too, as long as the
>order of operations is consistent with the empty/full state. The
>empty/full state is real for MVars and imagined for IORef/STRef.

Could you simply the interface if you consider (MVar a) to hold a (Maybe 
a)?

class Monad m => Ref m a r | r -> m a where
    newRef      :: a -> m r
    getRef      :: r -> m a
    setRef      :: r -> a -> m ()
    modifyRef   :: r -> (a -> m a) -> m ()

instance Ref (ST s) a (STRef s a) where
    ...

instance Ref IO a (IORef a) where
    ...

instance Ref IO (Maybe a) (MVar a) where
    ...


-- 
Ashley Yakeley, Seattle WA