diff --git a/README.md b/README.md
--- a/README.md
+++ b/README.md
@@ -94,7 +94,7 @@
 ```haskell
 (,) <$> Fold.sum <*> Fold.length = Fold step (Pair 0 0) done
   where
-    step (Pair x y) = Pair (x + n) (y + 1)
+    step (Pair x y) n = Pair (x + n) (y + 1)
 
     done (Pair x y) = (x, y)
 ```
diff --git a/foldl.cabal b/foldl.cabal
--- a/foldl.cabal
+++ b/foldl.cabal
@@ -1,5 +1,5 @@
 Name: foldl
-Version: 1.4.14
+Version: 1.4.15
 Cabal-Version: >=1.10
 Build-Type: Simple
 License: BSD3
@@ -27,7 +27,7 @@
         base         >= 4.11.0.0 && < 5   ,
         bytestring   >= 0.9.2.1  && < 0.12,
         random       >= 1.2      && < 1.3 ,
-        primitive                   < 0.8 ,
+        primitive                   < 0.9 ,
         text         >= 0.11.2.0 && < 2.1 ,
         transformers >= 0.2.0.0  && < 0.7 ,
         vector       >= 0.7      && < 0.14,
@@ -35,8 +35,8 @@
         unordered-containers        < 0.3 ,
         hashable                    < 1.5 ,
         contravariant               < 1.6 ,
-        profunctors  >= 3.2      && < 5.7 ,
-        semigroupoids >= 1.0     && < 5.4 ,
+        profunctors  >= 4.3.2    && < 5.7 ,
+        semigroupoids >= 1.0     && < 6.1 ,
         comonad      >= 4.0      && < 6
     if impl(ghc < 8.0)
         Build-Depends:
diff --git a/src/Control/Foldl.hs b/src/Control/Foldl.hs
--- a/src/Control/Foldl.hs
+++ b/src/Control/Foldl.hs
@@ -41,6 +41,7 @@
 {-# LANGUAGE BangPatterns              #-}
 {-# LANGUAGE ExistentialQuantification #-}
 {-# LANGUAGE FlexibleContexts          #-}
+{-# LANGUAGE MultiParamTypeClasses     #-}
 {-# LANGUAGE RankNTypes                #-}
 {-# LANGUAGE ScopedTypeVariables       #-}
 {-# LANGUAGE Trustworthy               #-}
@@ -164,6 +165,7 @@
 import Data.Semigroupoid (Semigroupoid)
 import Data.Functor.Extend (Extend(..))
 import Data.Profunctor
+import Data.Profunctor.Sieve
 import Data.Sequence ((|>))
 import Data.Vector.Generic (Vector, Mutable)
 import Data.Vector.Generic.Mutable (MVector)
@@ -242,8 +244,19 @@
     rmap = fmap
 
 instance Choice Fold where
-  right' (Fold step begin done) = Fold (liftA2 step) (Right begin) (fmap done)
-  {-# INLINE right' #-}
+    right' (Fold step begin done) = Fold (liftA2 step) (Right begin) (fmap done)
+    {-# INLINE right' #-}
+
+instance Cosieve Fold [] where
+    cosieve = fold
+    {-# INLINE cosieve #-}
+
+instance Costrong Fold where
+    unfirst p = fmap f list
+      where
+        f as = b
+          where (b, d) = fold p [ (a, d) | a <- as ]
+    {-# INLINE unfirst #-}
 
 instance Comonad (Fold a) where
     extract (Fold _ begin done) = done begin