diff --git a/src/CMakeLists.txt b/src/CMakeLists.txt
--- a/src/CMakeLists.txt
+++ b/src/CMakeLists.txt
@@ -332,6 +332,7 @@
 	util/moneystr.cpp
 	util/settings.cpp
 	util/strencodings.cpp
+	util/string.cpp
 	util/system.cpp
 	util/threadnames.cpp
 	util/time.cpp
diff --git a/src/Makefile.am b/src/Makefile.am
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -246,6 +246,7 @@
   util/moneystr.h \
   util/system.h \
   util/settings.h \
+  util/string.h \
   util/threadnames.h \
   util/time.h \
   util/translation.h \
@@ -552,6 +553,7 @@
   util/moneystr.cpp \
   util/settings.cpp \
   util/strencodings.cpp \
+  util/string.cpp \
   util/threadnames.cpp \
   util/time.cpp \
   util/url.cpp \
diff --git a/src/rpc/util.cpp b/src/rpc/util.cpp
--- a/src/rpc/util.cpp
+++ b/src/rpc/util.cpp
@@ -8,6 +8,7 @@
 #include <rpc/util.h>
 #include <tinyformat.h>
 #include <util/strencodings.h>
+#include <util/string.h>
 
 #include <univalue.h>
 
@@ -646,13 +647,9 @@
         }
         case Type::OBJ:
         case Type::OBJ_USER_KEYS: {
-            std::string res;
-            for (size_t i = 0; i < m_inner.size();) {
-                res += m_inner[i].ToStringObj(oneline);
-                if (++i < m_inner.size()) {
-                    res += ",";
-                }
-            }
+            const std::string res = Join(m_inner, ",", [&](const RPCArg &i) {
+                return i.ToStringObj(oneline);
+            });
             if (m_type == Type::OBJ) {
                 return "{" + res + "}";
             } else {
diff --git a/src/test/util_tests.cpp b/src/test/util_tests.cpp
--- a/src/test/util_tests.cpp
+++ b/src/test/util_tests.cpp
@@ -10,6 +10,7 @@
 #include <test/util.h>
 #include <util/moneystr.h>
 #include <util/strencodings.h>
+#include <util/string.h>
 #include <util/time.h>
 
 #include <test/util/setup_common.h>
@@ -155,6 +156,20 @@
         "1feae06279a60939e028a8d65c10b73071a6f16719274855feb0fd8a6704");
 }
 
+BOOST_AUTO_TEST_CASE(util_Join) {
+    // Normal version
+    BOOST_CHECK_EQUAL(Join({}, ", "), "");
+    BOOST_CHECK_EQUAL(Join({"foo"}, ", "), "foo");
+    BOOST_CHECK_EQUAL(Join({"foo", "bar"}, ", "), "foo, bar");
+
+    // Version with unary operator
+    const auto op_upper = [](const std::string &s) { return ToUpper(s); };
+    BOOST_CHECK_EQUAL(Join<std::string>({}, ", ", op_upper), "");
+    BOOST_CHECK_EQUAL(Join<std::string>({"foo"}, ", ", op_upper), "FOO");
+    BOOST_CHECK_EQUAL(Join<std::string>({"foo", "bar"}, ", ", op_upper),
+                      "FOO, BAR");
+}
+
 BOOST_AUTO_TEST_CASE(util_FormatParseISO8601DateTime) {
     BOOST_CHECK_EQUAL(FormatISO8601DateTime(1317425777),
                       "2011-09-30T23:36:17Z");
diff --git a/src/util/string.h b/src/util/string.h
new file mode 100644
--- /dev/null
+++ b/src/util/string.h
@@ -0,0 +1,37 @@
+// Copyright (c) 2019 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#ifndef BITCOIN_UTIL_STRING_H
+#define BITCOIN_UTIL_STRING_H
+
+#include <functional>
+#include <string>
+#include <vector>
+
+/**
+ * Join a list of items
+ *
+ * @param list       The list to join
+ * @param separator  The separator
+ * @param unary_op   Apply this operator to each item in the list
+ */
+template <typename T, typename UnaryOp>
+std::string Join(const std::vector<T> &list, const std::string &separator,
+                 UnaryOp unary_op) {
+    std::string ret;
+    for (size_t i = 0; i < list.size(); ++i) {
+        if (i > 0) {
+            ret += separator;
+        }
+        ret += unary_op(list.at(i));
+    }
+    return ret;
+}
+
+inline std::string Join(const std::vector<std::string> &list,
+                        const std::string &separator) {
+    return Join(list, separator, [](const std::string &i) { return i; });
+}
+
+#endif // BITCOIN_UTIL_STRING_H
diff --git a/src/util/string.cpp b/src/util/string.cpp
new file mode 100644
--- /dev/null
+++ b/src/util/string.cpp
@@ -0,0 +1,5 @@
+// Copyright (c) 2019 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#include <util/string.h>