module Distribution.Utils.MapAccum (mapAccumM) where
import Distribution.Compat.Prelude
import Prelude ()
newtype StateM s m a = StateM { runStateM :: s -> m (s, a) }
instance Functor m => Functor (StateM s m) where
fmap f (StateM x) = StateM $ \s -> fmap (\(s', a) -> (s', f a)) (x s)
instance
#if __GLASGOW_HASKELL__ < 709
(Functor m, Monad m)
#else
Monad m
#endif
=> Applicative (StateM s m) where
pure x = StateM $ \s -> return (s, x)
StateM f <*> StateM x = StateM $ \s -> do (s', f') <- f s
(s'', x') <- x s'
return (s'', f' x')
mapAccumM ::
#if __GLASGOW_HASKELL__ < 709
(Functor m, Monad m, Traversable t)
#else
(Monad m, Traversable t)
#endif
=> (a -> b -> m (a, c)) -> a -> t b -> m (a, t c)
mapAccumM f s t = runStateM (traverse (\x -> StateM (\s' -> f s' x)) t) s