nix-util: Add concatMapStrings

src/libutil-tests/strings.cc

@@ -80,6 +80,42 @@ TEST(concatStringsSep, buildSingleString)
     ASSERT_EQ(concatStringsSep(",", strings), "this");
 }
 
+TEST(concatMapStringsSep, empty)
+{
+    Strings strings;
+
+    ASSERT_EQ(concatMapStringsSep(",", strings, [](const std::string & s) { return s; }), "");
+}
+
+TEST(concatMapStringsSep, justOne)
+{
+    Strings strings;
+    strings.push_back("this");
+
+    ASSERT_EQ(concatMapStringsSep(",", strings, [](const std::string & s) { return s; }), "this");
+}
+
+TEST(concatMapStringsSep, two)
+{
+    Strings strings;
+    strings.push_back("this");
+    strings.push_back("that");
+
+    ASSERT_EQ(concatMapStringsSep(",", strings, [](const std::string & s) { return s; }), "this,that");
+}
+
+TEST(concatMapStringsSep, map)
+{
+    std::map<std::string, std::string> strings;
+    strings["this"] = "that";
+    strings["1"] = "one";
+
+    ASSERT_EQ(
+        concatMapStringsSep(
+            ", ", strings, [](const std::pair<std::string, std::string> & s) { return s.first + " -> " + s.second; }),
+        "1 -> one, this -> that");
+}
+
 /* ----------------------------------------------------------------------------
  * dropEmptyInitThenConcatStringsSep
  * --------------------------------------------------------------------------*/

src/libutil/strings.hh

@@ -55,6 +55,20 @@ extern template std::string concatStringsSep(std::string_view, const std::list<s
 extern template std::string concatStringsSep(std::string_view, const std::set<std::string> &);
 extern template std::string concatStringsSep(std::string_view, const std::vector<std::string> &);
 
+/**
+ * Apply a function to the `iterable`'s items and concat them with `separator`.
+ */
+template<class C, class F>
+std::string concatMapStringsSep(std::string_view separator, const C & iterable, F fn)
+{
+    std::vector<std::string> strings;
+    strings.reserve(iterable.size());
+    for (const auto & elem : iterable) {
+        strings.push_back(fn(elem));
+    }
+    return concatStringsSep(separator, strings);
+}
+
 /**
  * Ignore any empty strings at the start of the list, and then concatenate the
  * given strings with a separator between the elements.