diff --git a/src/libexpr/primops/flake.cc b/src/libexpr/primops/flake.cc
index fdbdc83bc..af7d51834 100644
--- a/src/libexpr/primops/flake.cc
+++ b/src/libexpr/primops/flake.cc
@@ -121,7 +121,7 @@ nlohmann::json flakeEntryToJson(const LockFile::FlakeEntry & entry)
     return json;
 }
 
-void writeLockFile(const LockFile & lockFile, const Path & path)
+std::ostream & operator <<(std::ostream & stream, const LockFile & lockFile)
 {
     nlohmann::json json;
     json["version"] = 1;
@@ -133,8 +133,14 @@ void writeLockFile(const LockFile & lockFile, const Path & path)
     json["inputs"] = nlohmann::json::object();
     for (auto & x : lockFile.flakeEntries)
         json["inputs"][x.first.to_string()] = flakeEntryToJson(x.second);
+    stream << json.dump(4); // '4' = indentation in json file
+    return stream;
+}
+
+void writeLockFile(const LockFile & lockFile, const Path & path)
+{
     createDirs(dirOf(path));
-    writeFile(path, json.dump(4) + "\n"); // '4' = indentation in json file
+    writeFile(path, fmt("%s\n", lockFile));
 }
 
 Path getUserRegistryPath()