fromFree

This is the `Free` analogue of `fromF`. I believe it's a valid
(but possibly inefficient) implementation of `toF`.

src/Control/Monad/Free.hs

@@ -31,6 +31,7 @@ module Control.Monad.Free
   , iterM
   , hoistFree
   , foldFree
+  , fromFree
   , toFreeT
   , cutoff
   , unfold
@@ -365,6 +366,11 @@ foldFree :: Monad m => (forall x . f x -> m x) -> Free f a -> m a
 foldFree _ (Pure a)  = return a
 foldFree f (Free as) = f as >>= foldFree f
 
+-- | Convert a 'Free' monad to an arbitrary 'MonadFree' instance.
+fromFree :: (Functor f, MonadFree f m) => Free f a -> m a
+fromFree (Pure x) = return x
+fromFree (Free ff) = wrap (fromFree <$> ff)
+
 -- | Convert a 'Free' monad from "Control.Monad.Free" to a 'FreeT.FreeT' monad
 -- from "Control.Monad.Trans.Free".
 toFreeT :: (Functor f, Monad m) => Free f a -> FreeT.FreeT f m a

src/Control/Monad/Free/Church.hs

@@ -198,6 +198,8 @@ fromF (F m) = m return wrap
 {-# INLINE fromF #-}
 
 -- | Generate a Church-encoded free monad from a 'Free' monad.
+--
+-- @toF = fromFree@
 toF :: Functor f => Free f a -> F f a
 toF xs = F (\kp kf -> go kp kf xs) where
   go kp _  (Pure a) = kp a