hoistOp - Op is an endo-functor of category of categories

ChangeLog.md

@@ -17,3 +17,4 @@
 - free arrows (concrete and condensity transformed)
 
 ## Unreleased changes
+- hoistOp

src/Control/Category/Free.hs

@@ -31,7 +31,7 @@ module Control.Category.Free
     , fromC
       -- * Oposite category
     , Op (..)
-
+    , hoistOp
       -- * Free interface re-exports
     , FreeAlgebra2 (..)
     , wrapFree2

src/Control/Category/Free/Internal.hs

@@ -23,6 +23,7 @@
 --
 module Control.Category.Free.Internal
   ( Op (..)
+  , hoistOp
   , ListTr (..)
   , Queue (NilQ, ConsQ)
   , emptyQ
@@ -53,6 +54,16 @@ import           Control.Algebra.Free2 ( AlgebraType0
 --
 newtype Op (f :: k -> k -> *) (a :: k) (b :: k) = Op { runOp :: f b a }
 
+-- | 'Op' is an endo-functor of the category of categories.
+--
+hoistOp :: forall (f :: k -> k -> *)
+                  (g :: k -> k -> *)
+                  a b.
+           (forall x y. f x y -> g x y)
+        -> Op f a b
+        -> Op g a b
+hoistOp nat (Op ba) = Op (nat ba)
+
 instance Category f => Category (Op f) where
     id = Op id
     Op f . Op g = Op (g . f)