diff --git a/samples/hof.js b/samples/hof.js
index f0e4401..823ece1 100644
--- a/samples/hof.js
+++ b/samples/hof.js
@@ -3,7 +3,8 @@ var map = require('../pattern')
   , zip_with = require('../pattern')
   , filtera = require('../pattern')
   , maybe = require('../pattern')
-  , _, f, ac, l, l1, l2, cb, errcb
+  , foldl = require('../pattern')
+  , _, f, ac, l, l1, l2, cb, errcb, z
   ;
 
 // error handling for conditional execution of functions
@@ -11,7 +12,9 @@ maybe(Error, _, errcb, cb, function (err, data, errcb, cb) { errcb(err); });
 maybe(_, _, errcb, cb, function(err, data, errcb, cb) { cb(data); });
 
 // map
-map(f, [], ac, cb, 
+// map _ []     = []
+// map f (x:xs) = f x : map f xs
+map(_, [], ac, cb, 
   function map_done(f, l, ac, cb) { return cb(ac); });
 map(f, l, ac, cb, 
   function map_catch_all(f, l, ac, cb) {
@@ -31,7 +34,11 @@ mapa(f, l, ac, cb,
   });
 
 // filter with async f
-filtera(f, [], ac, cb, function filter_done(f, l, ac, cb) { 
+// filter _ [] = []  
+// filter p (x:xs)   
+//     | p x       = x : filter p xs  
+//     | otherwise = filter p xs
+filtera(_, [], ac, cb, function filter_done(f, l, ac, cb) { 
   return cb(null, ac); 
 });
 filtera(f, l, ac, cb, function filter_catch_all(f, l, ac, cb) {
@@ -44,14 +51,26 @@ filtera(f, l, ac, cb, function filter_catch_all(f, l, ac, cb) {
   });
 });
 
-// foldl
+// foldl with async f, no errors possible
+// foldl f z []     = z                  
+// foldl f z (x:xs) = foldl f (f z x) xs
+foldl(f, z, [], cb, function foldl_done(f, z, l, cb) { cb(z); });
+foldl(f, z, l, cb, function foldl_all(f, z, l, cb) {
+  f(z, l.shift(), function foldl_f_async(new_z) {
+    foldl(f, new_z, l, cb);
+  });
+});
 
-zip_with(f, [], l2, ac, cb, function emptyl1(f, l1, l2, ac, cb) { cb(ac); });
-zip_with(f, l1, [], ac, cb, function emptyl2(f, l1, l2, ac, cb) { cb(ac); });
+// zipWith _ [] _ = []  
+// zipWith _ _ [] = []  
+// zipWith f (x:xs) (y:ys) = f x y : zipWith' f xs ys
+zip_with(_, [], _, ac, cb, function emptyl1(f, l1, l2, ac, cb) { cb(ac); });
+zip_with(_, _, [], ac, cb, function emptyl2(f, l1, l2, ac, cb) { cb(ac); });
 zip_with(f, l1, l2, ac, cb, function all(f, l1, l2, ac, cb) {
   ac.push(f(l1.shift(), l2.shift()));
   zip_with(f, l1, l2, ac, cb);
 });
 
 module.exports = 
-  { map: map, map_async: mapa, zip_with: zip_with, filter_async: filtera };
\ No newline at end of file
+  { map: map, map_async: mapa, zip_with: zip_with, filter_async: filtera
+  , foldl: foldl };
\ No newline at end of file
diff --git a/samples/reverse_list_with_reduce.test.js b/samples/reverse_list_with_reduce.test.js
new file mode 100644
index 0000000..431b015
--- /dev/null
+++ b/samples/reverse_list_with_reduce.test.js
@@ -0,0 +1,10 @@
+/* [ 1, 2, 3, 4, 5 ] */
+var reduce = require('./hof').foldl;
+
+function id(z, x, cb) {
+  setTimeout(
+    function() {z.unshift(x); cb(z);}, Math.ceil(Math.random() * 100)); }
+
+function reserve (list, cb) { reduce(id, [], list, function(z){ cb(z); }); }
+
+reserve([5,4,3,2,1], function (list) { console.error(list); });
\ No newline at end of file