add utilities for interacting with HO state

src/Polysemy/HigherOrder.hs

@@ -71,6 +71,34 @@ module Polysemy.HigherOrder
   -- * Retrieving the type parameters of a 'HigherOrder'
   , TypeParamsH(..)
   , getTypeParamsH
+
+  -- * Interact with reified higher-order state
+  , discardStateWith
+  , discardState
+  , discardStateEither
+  , discardStateMaybe
   ) where
 
 import Polysemy.Internal.HigherOrder
+
+-- | Get the value returned by a higher-order action that exposes the reified higher-order state.
+--
+-- This is needed, for example, when you want to execute multiple actions in a 'controlH' block
+-- that depend on the result of earlier actions.
+--
+-- @since 2.0.0.0
+discardStateWith :: Traversable t => b -> (a -> b) -> t a -> b
+discardStateWith alt f = foldr (const . f) alt
+{-# inline discardStateWith #-}
+
+discardState :: Traversable t => a -> t a -> a
+discardState alt = foldr const alt
+{-# inline discardState #-}
+
+discardStateEither :: Traversable t => b -> t a -> Either b a
+discardStateEither alt = discardStateWith (Left alt) Right
+{-# inline discardStateEither #-}
+
+discardStateMaybe :: Traversable t => t a -> Maybe a
+discardStateMaybe = discardStateWith Nothing Just
+{-# inline discardStateMaybe #-}