rpc: add named arg helper

Overload the Arg and MaybeArg helpers to allow accessing arguments
by name as well.

Also update the docs to document Arg and MaybeArg separately

src/rpc/util.cpp

@@ -24,6 +24,8 @@
 #include <util/string.h>
 #include <util/translation.h>
 
+#include <algorithm>
+#include <iterator>
 #include <string_view>
 #include <tuple>
 
@@ -728,6 +730,16 @@ std::vector<std::pair<std::string, bool>> RPCHelpMan::GetArgNames() const
     return ret;
 }
 
+size_t RPCHelpMan::GetParamIndex(std::string_view key) const
+{
+    auto it{std::find_if(
+        m_args.begin(), m_args.end(), [&key](const auto& arg) { return arg.GetName() == key;}
+    )};
+
+    CHECK_NONFATAL(it != m_args.end());  // TODO: ideally this is checked at compile time
+    return std::distance(m_args.begin(), it);
+}
+
 std::string RPCHelpMan::ToString() const
 {
     std::string ret;

src/rpc/util.h

@@ -402,18 +402,25 @@ public:
 
     UniValue HandleRequest(const JSONRPCRequest& request) const;
     /**
-     * Helper to get a request argument.
-     * This function only works during m_fun(), i.e. it should only be used in
-     * RPC method implementations. The helper internally checks whether the
-     * user-passed argument isNull() and parses (from JSON) and returns the
-     * user-passed argument, or the default value derived from the RPCArg
-     * documentation, or a falsy value if no default was given.
+     * @brief Helper to get a required or default-valued request argument.
      *
-     * Use Arg<Type>(i) to get the argument or its default value. Otherwise,
-     * use MaybeArg<Type>(i) to get the optional argument or a falsy value.
+     * Use this function when the argument is required or when it has a default value. If the
+     * argument is optional and may not be provided, use MaybeArg instead.
      *
-     * The Type passed to this helper must match the corresponding
-     * RPCArg::Type.
+     * This function only works during m_fun(), i.e., it should only be used in
+     * RPC method implementations. It internally checks whether the user-passed
+     * argument isNull() and parses (from JSON) and returns the user-passed argument,
+     * or the default value derived from the RPCArg documentation.
+     *
+     * There are two overloads of this function:
+     * - Use Arg<Type>(size_t i) to get the argument (or the default value) by index.
+     * - Use Arg<Type>(const std::string& key) to get the argument (or the default value) by key.
+     *
+     * The Type passed to this helper must match the corresponding RPCArg::Type.
+     *
+     * @return The value of the RPC argument (or the default value) cast to type Type.
+     *
+     * @see MaybeArg for handling optional arguments without default values.
      */
     template <typename R>
     auto Arg(size_t i) const
@@ -427,6 +434,34 @@ public:
             return ArgValue<const R&>(i);
         }
     }
+    template<typename R>
+    auto Arg(std::string_view key) const
+    {
+        return Arg<R>(GetParamIndex(key));
+    }
+    /**
+     * @brief Helper to get an optional request argument.
+     *
+     * Use this function when the argument is optional and does not have a default value. If the
+     * argument is required or has a default value, use Arg instead.
+     *
+     * This function only works during m_fun(), i.e., it should only be used in
+     * RPC method implementations. It internally checks whether the user-passed
+     * argument isNull() and parses (from JSON) and returns the user-passed argument,
+     * or a falsy value if no argument was passed.
+     *
+     * There are two overloads of this function:
+     * - Use MaybeArg<Type>(size_t i) to get the optional argument by index.
+     * - Use MaybeArg<Type>(const std::string& key) to get the optional argument by key.
+     *
+     * The Type passed to this helper must match the corresponding RPCArg::Type.
+     *
+     * @return For integral and floating-point types, a std::optional<Type> is returned.
+    *          For other types, a Type* pointer to the argument is returned. If the
+    *          argument is not provided, std::nullopt or a null pointer is returned.
+    *
+     * @see Arg for handling arguments that are required or have a default value.
+     */
     template <typename R>
     auto MaybeArg(size_t i) const
     {
@@ -439,6 +474,11 @@ public:
             return ArgValue<const R*>(i);
         }
     }
+    template<typename R>
+    auto MaybeArg(std::string_view key) const
+    {
+        return MaybeArg<R>(GetParamIndex(key));
+    }
     std::string ToString() const;
     /** Return the named args that need to be converted from string to another JSON type */
     UniValue GetArgMap() const;
@@ -458,6 +498,8 @@ private:
     mutable const JSONRPCRequest* m_req{nullptr}; // A pointer to the request for the duration of m_fun()
     template <typename R>
     R ArgValue(size_t i) const;
+    //! Return positional index of a parameter using its name as key.
+    size_t GetParamIndex(std::string_view key) const;
 };
 
 /**

src/test/rpc_tests.cpp

@@ -633,6 +633,20 @@ BOOST_AUTO_TEST_CASE(rpc_arg_helper)
         };
     CheckRpc(params, UniValue{JSON(R"([5, "hello", null, null, null, null, null])")}, check_positional);
     CheckRpc(params, UniValue{JSON(R"([5, "hello", 4, "test", true, 1.23, "world"])")}, check_positional);
+
+    //! Check that `self.Arg` returns the same value when using index and key
+    RPCHelpMan::RPCMethodImpl check_named = [&](const RPCHelpMan& self, const JSONRPCRequest& request) -> UniValue {
+            BOOST_CHECK_EQUAL(self.Arg<int>(0), self.Arg<int>("req_int"));
+            BOOST_CHECK_EQUAL(self.Arg<std::string>(1), self.Arg<std::string>("req_str"));
+            BOOST_CHECK_EQUAL(self.Arg<uint64_t>(2), self.Arg<uint64_t>("def_uint64_t"));
+            BOOST_CHECK_EQUAL(self.Arg<std::string>(3), self.Arg<std::string>("def_string"));
+            BOOST_CHECK_EQUAL(self.Arg<bool>(4), self.Arg<bool>("def_bool"));
+            BOOST_CHECK(self.MaybeArg<double>(5) == self.MaybeArg<double>("opt_double"));
+            BOOST_CHECK(self.MaybeArg<std::string>(6) == self.MaybeArg<std::string>("opt_string"));
+            return UniValue{};
+        };
+    CheckRpc(params, UniValue{JSON(R"([5, "hello", null, null, null, null, null])")}, check_named);
+    CheckRpc(params, UniValue{JSON(R"([5, "hello", 4, "test", true, 1.23, "world"])")}, check_named);
 }
 
 BOOST_AUTO_TEST_SUITE_END()