Add hoistDeclare

src/Data/OpenApi/Declare.hs

@@ -29,6 +29,8 @@ import Control.Monad.Trans.RWS.Strict as Strict
 import Control.Monad.Trans.Writer.Lazy as Lazy
 import Control.Monad.Trans.Writer.Strict as Strict
 import Data.Functor.Identity
+import Data.Functor.Compose
+import Data.Tuple (swap)
 
 -- | A declare monad transformer parametrized by:
 --
@@ -115,6 +117,14 @@ execDeclareT (DeclareT f) d = fst <$> f d
 undeclareT :: (Monad m, Monoid d) => DeclareT d m a -> m a
 undeclareT = flip evalDeclareT mempty
 
+-- | Lift a declaration-producing computation for a subpart into a
+-- declaration-producing computation for the whole.
+hoistDeclareT :: Functor m => Lens' d d' -> DeclareT d' m a -> DeclareT d m a
+hoistDeclareT l = DeclareT . unconvert . l . convert . runDeclareT
+  where
+  convert f = Compose . fmap swap . f
+  unconvert f = fmap swap . getCompose . f
+
 -- | A declare monad parametrized by @d@ — the output to accumulate (declarations).
 --
 -- This monad is similar to both state and writer monads.