diff --git a/set/hashset.go b/set/hashset.go
index 0201119..2d007ef 100644
--- a/set/hashset.go
+++ b/set/hashset.go
@@ -53,6 +53,15 @@ func (s *HashSet) Foreach(f func(interface{})) {
 	}
 }
 
+// Call f for each item in the set, set result as new key
+func (s *HashSet) Map(f func(interface{}) interface{}) Set {
+	n := make(map[interface{}]nothing)
+	for k := range s.hash {
+		n[f(k)] = nothing{}
+	}
+	return &HashSet{n}
+}
+
 // Returns true if this set contains no elements.
 func (s *HashSet) IsEmpty() bool {
 	return len(s.hash) == 0
diff --git a/set/hashset_test.go b/set/hashset_test.go
index 5557f2d..02d72dc 100644
--- a/set/hashset_test.go
+++ b/set/hashset_test.go
@@ -70,6 +70,22 @@ func TestSet_Foreach(t *testing.T) {
 	s.Foreach(f)
 }
 
+func TestHashSet_Map(t *testing.T) {
+	oldSet := NewHashSet(1, 2, 3)
+	f := func(x int) int { return x * x }
+	fWrapper := func(x interface{}) interface{} {
+		xInt := x.(int)
+		return f(xInt)
+	}
+	s := oldSet.Map(fWrapper)
+	if s.Len() != oldSet.Len() {
+		t.Errorf("Length should be %d", oldSet.Len())
+	}
+	if !s.ContainsAll(1, 4, 9) {
+		t.Error("Set should be 1, 4, 9")
+	}
+}
+
 func TestSet_Len(t *testing.T) {
 	s := NewHashSet()
 	if s.Len() != 0 {
diff --git a/set/set.go b/set/set.go
index 9f9649b..5f3247e 100755
--- a/set/set.go
+++ b/set/set.go
@@ -19,6 +19,9 @@ type Set interface {
 	// Call f for each item in the set
 	Foreach(f func(interface{}))
 
+	// Map f for each item
+	Map(f func(interface{}) interface{}) Set
+
 	// Returns true if this set contains no elements.
 	IsEmpty() bool