Create the spitPrefix function in parser.hh

src/libutil/parser.hh

@@ -0,0 +1,25 @@
+#pragma once
+
+#include <optional>
+
+namespace nix {
+
+// If `separator` is found, we return the portion of the string before the
+// separator, and modify the string argument to contain only the part after the
+// separator. Otherwise, wer return `std::nullopt`, and we leave the argument
+// string alone.
+std::optional<std::string_view> splitPrefix(std::string_view & string, char separator);
+
+std::optional<std::string_view> splitPrefix(std::string_view & string, char separator) {
+    auto sepInstance = string.find(separator);
+
+    if (sepInstance != std::string_view::npos) {
+        auto prefix = string.substr(0, sepInstance);
+        string.remove_prefix(sepInstance+1);
+        return prefix;
+    }
+
+    return std::nullopt;
+}
+
+}