42 lines
1.5 KiB
Plaintext
42 lines
1.5 KiB
Plaintext
module Control.Monad.Logic.Class (class MonadLogic, msplit, interleave, fbind, reflect, (>>-)) where
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import Prelude
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import Control.MonadPlus (class MonadPlus, (<|>), empty)
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import Control.Monad.State.Trans (StateT(..), runStateT)
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import Control.Monad.Reader.Trans (ReaderT(..), runReaderT)
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import Data.Tuple.Nested (type (/\), (/\))
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import Data.Maybe (Maybe(..))
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class (MonadPlus m) <= MonadLogic m where
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msplit :: forall a. m a -> m (Maybe (a /\ (m a)))
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interleave :: forall a. m a -> m a -> m a
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fbind :: forall a b m. MonadLogic m => m a -> (a -> m b) -> m b
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fbind m f = do
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r <- msplit m
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case r of
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Nothing -> empty
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Just (a /\ m') -> interleave (f a) (m' `fbind` f)
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reflect :: forall a m. MonadLogic m => Maybe (a /\ m a) -> m a
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reflect Nothing = empty
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reflect (Just (a /\ ma)) = pure a <|> ma
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infixl 1 fbind as >>-
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instance MonadLogic m => MonadLogic (StateT s m) where
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msplit sm = StateT $ \s -> do
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r <- msplit (runStateT sm s)
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case r of
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Nothing -> pure (Nothing /\ s)
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Just ((a /\ s') /\ m) -> pure (Just (a /\ (StateT $ const m)) /\ s')
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interleave m1 m2 = StateT $ \s -> runStateT m1 s <|> runStateT m2 s
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instance MonadLogic m => MonadLogic (ReaderT r m) where
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msplit sm = ReaderT $ \r -> do
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msplit (runReaderT sm r) >>= case _ of
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Nothing -> pure Nothing
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Just (a /\ m) -> pure $ Just $ a /\ (ReaderT $ const m)
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interleave m1 m2 = ReaderT $ \r -> runReaderT m1 r <|> runReaderT m2 r
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