diff --git a/filter/map.go b/filter/map.go
index 5a56ce1..e56e572 100644
--- a/filter/map.go
+++ b/filter/map.go
@@ -3,7 +3,10 @@
 
 package filter
 
-import "fmt"
+import (
+	"fmt"
+	"sort"
+)
 
 // Println prints ever element of the set.
 func Println[T P](set []T) {
@@ -11,3 +14,13 @@ func Println[T P](set []T) {
 		fmt.Println(i)
 	}
 }
+
+// Keys returns the keys in lexicographically sorted order.
+func Keys[T any](m map[string]T) []string {
+	keys := []string{}
+	for k, _ := range m {
+		keys = append(keys, k)
+		sort.Strings(keys)
+	}
+	return keys
+}
diff --git a/filter/map_test.go b/filter/map_test.go
index 6eb086c..cdbdf10 100644
--- a/filter/map_test.go
+++ b/filter/map_test.go
@@ -3,7 +3,11 @@
 
 package filter_test
 
-import "github.com/rwxrob/bonzai/filter"
+import (
+	"fmt"
+
+	"github.com/rwxrob/bonzai/filter"
+)
 
 func ExamplePrintln() {
 	set := []string{"doe", "ray", "mi"}
@@ -18,3 +22,13 @@ func ExamplePrintln() {
 	// true
 	// true
 }
+
+func ExampleKeys() {
+	m1 := map[string]int{"two": 2, "three": 3, "one": 1}
+	m2 := map[string]string{"two": "two", "three": "three", "one": "one"}
+	fmt.Println(filter.Keys(m1))
+	fmt.Println(filter.Keys(m2))
+	// Output:
+	// [one three two]
+	// [one three two]
+}