Cabal/src/Distribution/Utils/MapAccum.hs

   1 {-# LANGUAGE CPP #-}
   2
   3 module Distribution.Utils.MapAccum (mapAccumM) where
   4
   5 import Distribution.Compat.Prelude
   6 import Prelude ()
   7
   8 -- Like StateT but with return tuple swapped
   9 newtype StateM s m a = StateM {runStateM :: s -> m (s, a)}
  10
  11 instance Functor m => Functor (StateM s m) where
  12   fmap f (StateM x) = StateM $ \s -> fmap (\(s', a) -> (s', f a)) (x s)
  13
  14 instance Monad m => Applicative (StateM s m) where
  15   pure x = StateM $ \s -> return (s, x)
  16   StateM f <*> StateM x = StateM $ \s -> do
  17     (s', f') <- f s
  18     (s'', x') <- x s'
  19     return (s'', f' x')
  20
  21 -- | Monadic variant of 'mapAccumL'.
  22 mapAccumM
  23   :: (Monad m, Traversable t)
  24   => (a -> b -> m (a, c))
  25   -> a
  26   -> t b
  27   -> m (a, t c)
  28 mapAccumM f s t = runStateM (traverse (\x -> StateM (\s' -> f s' x)) t) s