Prototype Haskell Fix Traversal combinator

Plutarch/CPS/Optics/Fix.hs

@@ -0,0 +1,27 @@
+{-# LANGUAGE PartialTypeSignatures #-}
+module Plutarch.CPS.Optics.Fix(fixt) where
+
+import Data.Fix
+import Control.Lens
+
+import Data.Functor.Base
+import Data.Function
+
+fixt ::
+  (Applicative f) =>
+  (forall ra rb. (ra -> f rb) -> a ra -> f (b rb)) ->
+  Fix a ->
+  f (Fix b)
+fixt f = fmap Fix . (fix (f . dimap unFix (fmap Fix))) . unFix
+
+list' :: Traversal (Fix (ListF a)) (Fix (ListF b)) a b
+list' f = fixt (listf f)
+
+listf ::
+  (Applicative f) =>
+  (a -> f b) ->
+  (ra -> f rb) ->
+  ListF a ra ->
+  f (ListF b rb)
+listf _ _ Nil = pure Nil
+listf f r (Cons a b) = Cons <$> f a <*> r b

plutarch-core.cabal

@@ -97,6 +97,7 @@ library
 
   exposed-modules:
     Plutarch.Core
+    Plutarch.CPS.Optics.Fix
     Plutarch.CPS.Optics.Iso
     Plutarch.CPS.Optics.Lens
     Plutarch.CPS.Optics.Optic
@@ -116,5 +117,8 @@ library
 
   build-depends:
     , base
+    , data-fix
     , generics-sop
+    , lens
     , mtl
+    , recursion-schemes