diff --git a/index.js b/index.js
index 3b460119..0442ce5b 100644
--- a/index.js
+++ b/index.js
@@ -867,11 +867,11 @@
   //. ```
   S.join = def('join', {m: [Z.Chain]}, [m(m(a)), m(a)], Z.join);
 
-  //# chainRec :: ChainRec m => TypeRep m -> (a -> Either (m b) (m b)) -> a -> m b
+  //# chainRec :: ChainRec m => TypeRep m -> (a -> m (Either a b)) -> a -> m b
   //.
   //. Performs a [`chain`](#chain)-like computation with constant stack usage.
   //. Similar to [`Z.chainRec`][], but curried and more convenient due to the
-  //. use of the Either type to indicate completion.
+  //. use of the Either type to indicate completion (via a Right).
   //.
   //. ```javascript
   //. > S.chainRec(Array,
@@ -880,17 +880,17 @@
   //. .            '')
   //. ['oo!', 'oo?', 'on!', 'on?', 'no!', 'no?', 'nn!', 'nn?']
   //. ```
+  function chainRec(typeRep, f, x) {
+    function step(next, done, x) {
+      return Z.map(function(e) { return either(next, done, e); }, f(x));
+    }
+    return Z.chainRec(typeRep, step, x);
+  }
   S.chainRec =
   def('chainRec',
       {m: [Z.ChainRec]},
-      [TypeRep($.TypeVariable('m')), Fn(a, $Either(m(b), m(b))), a, m(b)],
-      function(typeRep, f, x) {
-        function step(next, done, x) {
-          var either = f(x);
-          return Z.map(either.isLeft ? next : done, either.value);
-        }
-        return Z.chainRec(typeRep, step, x);
-      });
+      [TypeRep($.TypeVariable('m')), Fn(a, m($Either(a, b))), a, m(b)],
+      chainRec);
 
   //# extend :: Extend w => (w a -> b) -> w a -> w b
   //.
diff --git a/test/chainRec.js b/test/chainRec.js
index 40b66bbc..296e2ef0 100644
--- a/test/chainRec.js
+++ b/test/chainRec.js
@@ -1,5 +1,7 @@
 'use strict';
 
+var Z = require('sanctuary-type-classes');
+
 var S = require('..');
 
 var eq = require('./internal/eq');
@@ -9,18 +11,18 @@ test('chainRec', function() {
 
   eq(typeof S.chainRec, 'function');
   eq(S.chainRec.length, 3);
-  eq(S.chainRec.toString(), 'chainRec :: ChainRec m => TypeRep m -> (a -> Either (m b) (m b)) -> a -> m b');
+  eq(S.chainRec.toString(), 'chainRec :: ChainRec m => TypeRep m -> (a -> m (Either a b)) -> a -> m b');
 
   function permute(s) {
-    return s.length === 2 ? S.Right([s + '!', s + '?'])
-                          : S.Left([s + 'o', s + 'n']);
+    return s.length === 2 ? Z.map(S.Right, [s + '!', s + '?'])
+                          : Z.map(S.Left, [s + 'o', s + 'n']);
   }
 
   eq(S.chainRec(Array, permute, ''), ['oo!', 'oo?', 'on!', 'on?', 'no!', 'no?', 'nn!', 'nn?']);
 
   function stepper(n) {
-    return n === 3000 ? S.Right(function(env) { return n + env.inc; })
-                      : S.Left(function(env) { return n + env.step; });
+    return n === 3000 ? Z.map(S.Right, function(env) { return n + env.inc; })
+                      : Z.map(S.Left, function(env) { return n + env.step; });
   }
 
   eq(S.chainRec(Function, stepper, 0)({step: 2, inc: 100}), 3100);