strings: Add optionalBracket helper

src/libutil-tests/strings.cc

@@ -494,4 +494,63 @@ TEST(shellSplitString, testUnbalancedQuotes)
     ASSERT_THROW(shellSplitString("foo\"bar\\\""), Error);
 }
 
+/* ----------------------------------------------------------------------------
+ * optionalBracket
+ * --------------------------------------------------------------------------*/
+
+TEST(optionalBracket, emptyContent)
+{
+    ASSERT_EQ(optionalBracket(" (", "", ")"), "");
+}
+
+TEST(optionalBracket, nonEmptyContent)
+{
+    ASSERT_EQ(optionalBracket(" (", "foo", ")"), " (foo)");
+}
+
+TEST(optionalBracket, emptyPrefixAndSuffix)
+{
+    ASSERT_EQ(optionalBracket("", "foo", ""), "foo");
+}
+
+TEST(optionalBracket, emptyContentEmptyBrackets)
+{
+    ASSERT_EQ(optionalBracket("", "", ""), "");
+}
+
+TEST(optionalBracket, complexBrackets)
+{
+    ASSERT_EQ(optionalBracket(" [[[", "content", "]]]"), " [[[content]]]");
+}
+
+TEST(optionalBracket, onlyPrefix)
+{
+    ASSERT_EQ(optionalBracket("prefix", "content", ""), "prefixcontent");
+}
+
+TEST(optionalBracket, onlySuffix)
+{
+    ASSERT_EQ(optionalBracket("", "content", "suffix"), "contentsuffix");
+}
+
+TEST(optionalBracket, optionalWithValue)
+{
+    ASSERT_EQ(optionalBracket(" (", std::optional<std::string>("foo"), ")"), " (foo)");
+}
+
+TEST(optionalBracket, optionalNullopt)
+{
+    ASSERT_EQ(optionalBracket(" (", std::optional<std::string>(std::nullopt), ")"), "");
+}
+
+TEST(optionalBracket, optionalEmptyString)
+{
+    ASSERT_EQ(optionalBracket(" (", std::optional<std::string>(""), ")"), "");
+}
+
+TEST(optionalBracket, optionalStringViewWithValue)
+{
+    ASSERT_EQ(optionalBracket(" (", std::optional<std::string_view>("bar"), ")"), " (bar)");
+}
+
 } // namespace nix

src/libutil/include/nix/util/strings.hh

@@ -3,6 +3,7 @@
 #include "nix/util/types.hh"
 
 #include <list>
+#include <optional>
 #include <set>
 #include <string_view>
 #include <string>
@@ -93,6 +94,44 @@ extern template std::string dropEmptyInitThenConcatStringsSep(std::string_view,
  */
 std::list<std::string> shellSplitString(std::string_view s);
 
+/**
+ * Conditionally wrap a string with prefix and suffix brackets.
+ *
+ * If `content` is empty, returns an empty string.
+ * Otherwise, returns `prefix + content + suffix`.
+ *
+ * Example:
+ *   optionalBracket(" (", "foo", ")") == " (foo)"
+ *   optionalBracket(" (", "", ")") == ""
+ *
+ * Design note: this would have been called `optionalParentheses`, except this
+ * function is more general and more explicit. Parentheses typically *also* need
+ * to be prefixed with a space in order to fit nicely in a piece of natural
+ * language.
+ */
+std::string optionalBracket(std::string_view prefix, std::string_view content, std::string_view suffix);
+
+/**
+ * Overload for optional content.
+ *
+ * If `content` is nullopt or contains an empty string, returns an empty string.
+ * Otherwise, returns `prefix + *content + suffix`.
+ *
+ * Example:
+ *   optionalBracket(" (", std::optional<std::string>("foo"), ")") == " (foo)"
+ *   optionalBracket(" (", std::nullopt, ")") == ""
+ *   optionalBracket(" (", std::optional<std::string>(""), ")") == ""
+ */
+template<typename T>
+    requires std::convertible_to<T, std::string_view>
+std::string optionalBracket(std::string_view prefix, const std::optional<T> & content, std::string_view suffix)
+{
+    if (!content || std::string_view(*content).empty()) {
+        return "";
+    }
+    return optionalBracket(prefix, std::string_view(*content), suffix);
+}
+
 /**
  * Hash implementation that can be used for zero-copy heterogenous lookup from
  * P1690R1[1] in unordered containers.

src/libutil/strings.cc

@@ -138,4 +138,18 @@ std::list<std::string> shellSplitString(std::string_view s)
 
     return result;
 }
+
+std::string optionalBracket(std::string_view prefix, std::string_view content, std::string_view suffix)
+{
+    if (content.empty()) {
+        return "";
+    }
+    std::string result;
+    result.reserve(prefix.size() + content.size() + suffix.size());
+    result.append(prefix);
+    result.append(content);
+    result.append(suffix);
+    return result;
+}
+
 } // namespace nix