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Feb 9, 2025
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A more composable interface for monads #25

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26 changes: 13 additions & 13 deletions src/Benign.hs
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Original file line number Diff line number Diff line change
Expand Up @@ -46,7 +46,9 @@ module Benign

-- * Monads
EvalM (..),
withAlteringM,
withSettingM,
unsafeSpanBenignM,
)
where

Expand Down Expand Up @@ -369,25 +371,23 @@ instance Eval a => EvalIO (PureEval a) where
-- implement 'withAltering' and 'unsafeSpanBenign'. This is what the (admittedly
-- poorly named) 'EvalM' class lets monad do.
class EvalM m where
withAlteringM :: Eval b => Field a -> (Maybe a -> Maybe a) -> m b -> m (Result b)
unsafeSpanBenignM :: Eval a => IO () -> IO () -> m a -> m (Result a)
spliceEval :: Eval b => (forall a. Eval a => (a -> Result a)) -> m b -> m (Result b)

withAlteringM :: (EvalM m, Eval b) => Field a -> (Maybe a -> Maybe a) -> m b -> m (Result b)
withAlteringM f g = spliceEval (withAltering f g)

withSettingM :: (EvalM m, Eval b) => Field a -> a -> m b -> m (Result b)
withSettingM f a = withAlteringM f (\_ -> Just a)

unsafeSpanBenignM :: (EvalM m, Eval a) => IO () -> IO () -> m a -> m (Result a)
unsafeSpanBenignM before after = spliceEval (unsafeSpanBenign before after)

instance EvalM Identity where
withAlteringM :: forall b a. Eval b => Field a -> (Maybe a -> Maybe a) -> Identity b -> Identity (Result b)
withAlteringM = coerce $ withAltering @b @a
unsafeSpanBenignM :: forall a. Eval a => IO () -> IO () -> Identity a -> Identity (Result a)
unsafeSpanBenignM = coerce $ unsafeSpanBenign @a
spliceEval :: forall b. Eval b => (forall a. Eval a => a -> Result a) -> Identity b -> Identity (Result b)
spliceEval f = coerce $ f @b

instance (EvalM m, Eval s, Result s ~ s) => EvalM (StateT s m) where
withAlteringM f g (StateT thing) = StateT $ \s -> withAlteringM f g (thing s)

unsafeSpanBenignM :: (EvalM m, Eval s, Result s ~ s, Eval a) => IO () -> IO () -> StateT s m a -> StateT s m (Result a)
unsafeSpanBenignM before after (StateT thing) = StateT $ \s -> unsafeSpanBenignM before after (thing s)
spliceEval f (StateT thing) = StateT $ \s -> spliceEval f (thing s)

instance (EvalM m) => EvalM (ReaderT e m) where
withAlteringM f g (ReaderT thing) = ReaderT $ \e -> withAlteringM f g (thing e)

unsafeSpanBenignM before after (ReaderT thing) = ReaderT $ \e -> unsafeSpanBenignM before after (thing e)
spliceEval f (ReaderT thing) = ReaderT $ \e -> spliceEval f (thing e)