Strategy.hs

module Strategy where

type Strategy m a = a -> m (Maybe a)

success :: Monad m => Strategy m a
success = return . Just

failure :: Monad m => Strategy m a
failure _ = return Nothing

--These needs "try" or some way to ignore the side-effects
test :: Monad m => Strategy m a -> Strategy m a
test s x = do
  mbx' <- s x
  case mbx' of Just _ -> return (Just x)
               Nothing -> return Nothing
  
neg :: Monad m => Strategy m a -> Strategy m a
neg s x = do
  mbx' <- s x
  case mbx' of Just _ -> return Nothing
               Nothing -> return (Just x)

seqn :: Monad m => Strategy m a -> Strategy m a -> Strategy m a
seqn s1 s2 x = do
  mbx' <- s1 x
  case mbx' of Just x' -> s2 x'
               Nothing -> return Nothing

choice :: Monad m => Strategy m a -> Strategy m a -> Strategy m a
choice s1 s2 x = do
  mbx' <- s1 x
  case mbx' of Just x' -> return (Just x')
               Nothing -> s2 x