diff --git a/urlenc/build.go b/urlenc/build.go
index f09c46b..cced940 100644
--- a/urlenc/build.go
+++ b/urlenc/build.go
@@ -69,3 +69,27 @@ func Join(builders ...Builder) Values {
 	}
 	return ret
 }
+
+type FnArray[V any] struct {
+	Values []V
+	Fn func([]V, int) (string, string)
+}
+
+func (a FnArray[V]) BuildQueryValues() Values {
+	values := Values{}
+	for i := range a.Values {
+		k, v := a.Fn(a.Values, i)
+		values.Add(k, v)
+	}
+	return values
+}
+
+func (a FnArray[V]) String() string {
+	unesc, err := url.QueryUnescape(
+		a.BuildQueryValues().Encode(),
+	)
+	if err != nil {
+		panic(err)
+	}
+	return unesc
+}
diff --git a/urlenc/build_test.go b/urlenc/build_test.go
index bb4d183..ddf93d2 100644
--- a/urlenc/build_test.go
+++ b/urlenc/build_test.go
@@ -2,6 +2,7 @@ package urlenc
 
 import "testing"
 import "log"
+import "fmt"
 
 func TestArray(t *testing.T) {
 	log.Printf(
@@ -25,3 +26,22 @@ func TestJoin(t *testing.T) {
 		).Encode(),
 	)
 }
+
+func TestFnArray(t *testing.T) {
+	log.Printf(
+		"FnArray{...}: %s",
+		FnArray[int64]{
+			[]int64{
+				1, 2, 3, 11, 12, 13,
+			},
+			func(
+				a []int64, i int,
+			) (string, string) {
+				return fmt.Sprintf(
+					"custom_field_%d_value_changed",
+					a[i],
+				), ""
+			},
+		},
+	)
+}