diff --git a/views-zip-enumerate/main.cpp b/views-zip-enumerate/main.cpp
index f74dc38..79aba65 100644
--- a/views-zip-enumerate/main.cpp
+++ b/views-zip-enumerate/main.cpp
@@ -1,12 +1,22 @@
 #include <iostream>
 #include <string>
+#include <string_view>
+#include <functional>
 #include <ranges>
 
+using pair = std::pair<size_t, char>;
+
+void
+default_callback(pair p)
+{
+    std::cout << "Index: " << p.first << ", Character: " << p.second << '\n';
+}
+
 void
-enumerate(std::string_view str)
+enumerate(std::string_view str, std::function<void(pair)> callback = nullptr)
 {
     for (const auto& [index, character] : std::views::zip(std::views::iota(0), str)) {
-        std::cout << index << ": " << character << '\n';
+        callback({index, character});
     }
 }
 
@@ -15,15 +25,14 @@ main(int argc, char* argv[])
 {
     if (argc < 2) {
         std::cerr << "Usage: " << argv[0] << " <string1> <string2> ...\n";
-        enumerate("default string");
+        enumerate("default string", default_callback);
         // make pipeline happy
         return 0;
     }
     for (int i = 1; i < argc; ++i) {
         std::string_view input = argv[i];
         std::cout << "Enumerating string: " << input << '\n';
-        enumerate(input);
-        std::cout << "------------------------\n";
+        enumerate(input, [](pair p) { std::cout << "Index: " << p.first << ", Character: " << p.second << '\n'; });
     }
     return 0;
 }
diff --git a/views-zip-enumerate/tests.sh b/views-zip-enumerate/tests.sh
new file mode 100755
index 0000000..0382f16
--- /dev/null
+++ b/views-zip-enumerate/tests.sh
@@ -0,0 +1,5 @@
+#!/bin/bash
+
+set -ex
+
+./views-zip-enumerate 1234 ABCD