diff --git a/src/libexpr/common-eval-args.cc b/src/libexpr/common-eval-args.cc
index 10c9c16bb..44baadd53 100644
--- a/src/libexpr/common-eval-args.cc
+++ b/src/libexpr/common-eval-args.cc
@@ -10,24 +10,27 @@ namespace nix {
 
 MixEvalArgs::MixEvalArgs()
 {
-    mkFlag()
-        .longName("arg")
-        .description("argument to be passed to Nix functions")
-        .labels({"name", "expr"})
-        .handler([&](std::vector<std::string> ss) { autoArgs[ss[0]] = 'E' + ss[1]; });
+    addFlag({
+        .longName = "arg",
+        .description = "argument to be passed to Nix functions",
+        .labels = {"name", "expr"},
+        .handler = {[&](std::string name, std::string expr) { autoArgs[name] = 'E' + expr; }}
+    });
 
-    mkFlag()
-        .longName("argstr")
-        .description("string-valued argument to be passed to Nix functions")
-        .labels({"name", "string"})
-        .handler([&](std::vector<std::string> ss) { autoArgs[ss[0]] = 'S' + ss[1]; });
+    addFlag({
+        .longName = "argstr",
+        .description = "string-valued argument to be passed to Nix functions",
+        .labels = {"name", "string"},
+        .handler = {[&](std::string name, std::string s) { autoArgs[name] = 'S' + s; }},
+    });
 
-    mkFlag()
-        .shortName('I')
-        .longName("include")
-        .description("add a path to the list of locations used to look up <...> file names")
-        .label("path")
-        .handler([&](std::string s) { searchPath.push_back(s); });
+    addFlag({
+        .longName = "include",
+        .shortName = 'I',
+        .description = "add a path to the list of locations used to look up <...> file names",
+        .labels = {"path"},
+        .handler = {[&](std::string s) { searchPath.push_back(s); }}
+    });
 }
 
 Bindings * MixEvalArgs::getAutoArgs(EvalState & state)
diff --git a/src/libmain/common-args.cc b/src/libmain/common-args.cc
index 9e1d7cee6..51e199ea5 100644
--- a/src/libmain/common-args.cc
+++ b/src/libmain/common-args.cc
@@ -6,43 +6,47 @@ namespace nix {
 MixCommonArgs::MixCommonArgs(const string & programName)
     : programName(programName)
 {
-    mkFlag()
-        .longName("verbose")
-        .shortName('v')
-        .description("increase verbosity level")
-        .handler([]() { verbosity = (Verbosity) (verbosity + 1); });
+    addFlag({
+        .longName = "verbose",
+        .shortName = 'v',
+        .description = "increase verbosity level",
+        .handler = {[]() { verbosity = (Verbosity) (verbosity + 1); }},
+    });
 
-    mkFlag()
-        .longName("quiet")
-        .description("decrease verbosity level")
-        .handler([]() { verbosity = verbosity > lvlError ? (Verbosity) (verbosity - 1) : lvlError; });
+    addFlag({
+        .longName = "quiet",
+        .description = "decrease verbosity level",
+        .handler = {[]() { verbosity = verbosity > lvlError ? (Verbosity) (verbosity - 1) : lvlError; }},
+    });
 
-    mkFlag()
-        .longName("debug")
-        .description("enable debug output")
-        .handler([]() { verbosity = lvlDebug; });
+    addFlag({
+        .longName = "debug",
+        .description = "enable debug output",
+        .handler = {[]() { verbosity = lvlDebug; }},
+    });
 
-    mkFlag()
-        .longName("option")
-        .labels({"name", "value"})
-        .description("set a Nix configuration option (overriding nix.conf)")
-        .arity(2)
-        .handler([](std::vector<std::string> ss) {
+    addFlag({
+        .longName = "option",
+        .description = "set a Nix configuration option (overriding nix.conf)",
+        .labels = {"name", "value"},
+        .handler = {[](std::string name, std::string value) {
             try {
-                globalConfig.set(ss[0], ss[1]);
+                globalConfig.set(name, value);
             } catch (UsageError & e) {
                 warn(e.what());
             }
-        });
+        }},
+    });
 
-    mkFlag()
-        .longName("max-jobs")
-        .shortName('j')
-        .label("jobs")
-        .description("maximum number of parallel builds")
-        .handler([=](std::string s) {
+    addFlag({
+        .longName = "max-jobs",
+        .shortName = 'j',
+        .description = "maximum number of parallel builds",
+        .labels = Strings{"jobs"},
+        .handler = {[=](std::string s) {
             settings.set("max-jobs", s);
-        });
+        }}
+    });
 
     std::string cat = "config";
     globalConfig.convertToArgs(*this, cat);
diff --git a/src/libmain/shared.cc b/src/libmain/shared.cc
index 8586257bf..70d1f0186 100644
--- a/src/libmain/shared.cc
+++ b/src/libmain/shared.cc
@@ -165,28 +165,32 @@ LegacyArgs::LegacyArgs(const std::string & programName,
     std::function<bool(Strings::iterator & arg, const Strings::iterator & end)> parseArg)
     : MixCommonArgs(programName), parseArg(parseArg)
 {
-    mkFlag()
-        .longName("no-build-output")
-        .shortName('Q')
-        .description("do not show build output")
-        .set(&settings.verboseBuild, false);
+    addFlag({
+        .longName = "no-build-output",
+        .shortName = 'Q',
+        .description = "do not show build output",
+        .handler = {&settings.verboseBuild, false},
+    });
 
-    mkFlag()
-        .longName("keep-failed")
-        .shortName('K')
-        .description("keep temporary directories of failed builds")
-        .set(&(bool&) settings.keepFailed, true);
+    addFlag({
+        .longName = "keep-failed",
+        .shortName ='K',
+        .description = "keep temporary directories of failed builds",
+        .handler = {&(bool&) settings.keepFailed, true},
+    });
 
-    mkFlag()
-        .longName("keep-going")
-        .shortName('k')
-        .description("keep going after a build fails")
-        .set(&(bool&) settings.keepGoing, true);
+    addFlag({
+        .longName = "keep-going",
+        .shortName ='k',
+        .description = "keep going after a build fails",
+        .handler = {&(bool&) settings.keepGoing, true},
+    });
 
-    mkFlag()
-        .longName("fallback")
-        .description("build from source if substitution fails")
-        .set(&(bool&) settings.tryFallback, true);
+    addFlag({
+        .longName = "fallback",
+        .description = "build from source if substitution fails",
+        .handler = {&(bool&) settings.tryFallback, true},
+    });
 
     auto intSettingAlias = [&](char shortName, const std::string & longName,
         const std::string & description, const std::string & dest) {
@@ -205,11 +209,12 @@ LegacyArgs::LegacyArgs(const std::string & programName,
     mkFlag(0, "no-gc-warning", "disable warning about not using '--add-root'",
         &gcWarning, false);
 
-    mkFlag()
-        .longName("store")
-        .label("store-uri")
-        .description("URI of the Nix store to use")
-        .dest(&(std::string&) settings.storeUri);
+    addFlag({
+        .longName = "store",
+        .description = "URI of the Nix store to use",
+        .labels = {"store-uri"},
+        .handler = {&(std::string&) settings.storeUri},
+    });
 }
 
 
diff --git a/src/libstore/globals.cc b/src/libstore/globals.cc
index d6ea0318e..a0a2d850e 100644
--- a/src/libstore/globals.cc
+++ b/src/libstore/globals.cc
@@ -167,21 +167,24 @@ template<> void BaseSetting<SandboxMode>::toJSON(JSONPlaceholder & out)
 
 template<> void BaseSetting<SandboxMode>::convertToArg(Args & args, const std::string & category)
 {
-    args.mkFlag()
-        .longName(name)
-        .description("Enable sandboxing.")
-        .handler([=](std::vector<std::string> ss) { override(smEnabled); })
-        .category(category);
-    args.mkFlag()
-        .longName("no-" + name)
-        .description("Disable sandboxing.")
-        .handler([=](std::vector<std::string> ss) { override(smDisabled); })
-        .category(category);
-    args.mkFlag()
-        .longName("relaxed-" + name)
-        .description("Enable sandboxing, but allow builds to disable it.")
-        .handler([=](std::vector<std::string> ss) { override(smRelaxed); })
-        .category(category);
+    args.addFlag({
+        .longName = name,
+        .description = "Enable sandboxing.",
+        .category = category,
+        .handler = {[=]() { override(smEnabled); }}
+    });
+    args.addFlag({
+        .longName = "no-" + name,
+        .description = "Disable sandboxing.",
+        .category = category,
+        .handler = {[=]() { override(smDisabled); }}
+    });
+    args.addFlag({
+        .longName = "relaxed-" + name,
+        .description = "Enable sandboxing, but allow builds to disable it.",
+        .category = category,
+        .handler = {[=]() { override(smRelaxed); }}
+    });
 }
 
 void MaxBuildJobsSetting::set(const std::string & str)
diff --git a/src/libutil/args.cc b/src/libutil/args.cc
index ba15ea571..91fc2f581 100644
--- a/src/libutil/args.cc
+++ b/src/libutil/args.cc
@@ -3,16 +3,14 @@
 
 namespace nix {
 
-Args::FlagMaker Args::mkFlag()
-{
-    return FlagMaker(*this);
-}
-
-Args::FlagMaker::~FlagMaker()
+void Args::addFlag(Flag && flag_)
 {
+    auto flag = std::make_shared<Flag>(std::move(flag_));
+    if (flag->handler.arity != ArityAny)
+        assert(flag->handler.arity == flag->labels.size());
     assert(flag->longName != "");
-    args.longFlags[flag->longName] = flag;
-    if (flag->shortName) args.shortFlags[flag->shortName] = flag;
+    longFlags[flag->longName] = flag;
+    if (flag->shortName) shortFlags[flag->shortName] = flag;
 }
 
 void Args::parseCmdline(const Strings & _cmdline)
@@ -101,15 +99,14 @@ bool Args::processFlag(Strings::iterator & pos, Strings::iterator end)
     auto process = [&](const std::string & name, const Flag & flag) -> bool {
         ++pos;
         std::vector<std::string> args;
-        for (size_t n = 0 ; n < flag.arity; ++n) {
+        for (size_t n = 0 ; n < flag.handler.arity; ++n) {
             if (pos == end) {
-                if (flag.arity == ArityAny) break;
-                throw UsageError(format("flag '%1%' requires %2% argument(s)")
-                    % name % flag.arity);
+                if (flag.handler.arity == ArityAny) break;
+                throw UsageError("flag '%s' requires %d argument(s)", name, flag.handler.arity);
             }
             args.push_back(*pos++);
         }
-        flag.handler(std::move(args));
+        flag.handler.fun(std::move(args));
         return true;
     };
 
@@ -157,17 +154,18 @@ bool Args::processArgs(const Strings & args, bool finish)
     return res;
 }
 
-Args::FlagMaker & Args::FlagMaker::mkHashTypeFlag(HashType * ht)
+Args::Flag Args::Flag::mkHashTypeFlag(std::string && longName, HashType * ht)
 {
-    arity(1);
-    label("type");
-    description("hash algorithm ('md5', 'sha1', 'sha256', or 'sha512')");
-    handler([ht](std::string s) {
-        *ht = parseHashType(s);
-        if (*ht == htUnknown)
-            throw UsageError("unknown hash type '%1%'", s);
-    });
-    return *this;
+    return Flag {
+        .longName = std::move(longName),
+        .description = "hash algorithm ('md5', 'sha1', 'sha256', or 'sha512')",
+        .labels = {"hash-algo"},
+        .handler = {[ht](std::string s) {
+            *ht = parseHashType(s);
+            if (*ht == htUnknown)
+                throw UsageError("unknown hash type '%1%'", s);
+        }}
+    };
 }
 
 Strings argvToStrings(int argc, char * * argv)
diff --git a/src/libutil/args.hh b/src/libutil/args.hh
index 967efbe1c..9b5e316a5 100644
--- a/src/libutil/args.hh
+++ b/src/libutil/args.hh
@@ -32,13 +32,59 @@ protected:
     struct Flag
     {
         typedef std::shared_ptr<Flag> ptr;
+
+        struct Handler
+        {
+            std::function<void(std::vector<std::string>)> fun;
+            size_t arity;
+
+            Handler() {}
+
+            Handler(std::function<void(std::vector<std::string>)> && fun)
+                : fun(std::move(fun))
+                , arity(ArityAny)
+            { }
+
+            Handler(std::function<void()> && handler)
+                : fun([handler{std::move(handler)}](std::vector<std::string>) { handler(); })
+                , arity(0)
+            { }
+
+            Handler(std::function<void(std::string)> && handler)
+                : fun([handler{std::move(handler)}](std::vector<std::string> ss) {
+                    handler(std::move(ss[0]));
+                  })
+                , arity(1)
+            { }
+
+            Handler(std::function<void(std::string, std::string)> && handler)
+                : fun([handler{std::move(handler)}](std::vector<std::string> ss) {
+                    handler(std::move(ss[0]), std::move(ss[1]));
+                  })
+                , arity(2)
+            { }
+
+            template<class T>
+            Handler(T * dest)
+                : fun([=](std::vector<std::string> ss) { *dest = ss[0]; })
+                , arity(1)
+            { }
+
+            template<class T>
+            Handler(T * dest, const T & val)
+                : fun([=](std::vector<std::string> ss) { *dest = val; })
+                , arity(0)
+            { }
+        };
+
         std::string longName;
         char shortName = 0;
         std::string description;
-        Strings labels;
-        size_t arity = 0;
-        std::function<void(std::vector<std::string>)> handler;
         std::string category;
+        Strings labels;
+        Handler handler;
+
+        static Flag mkHashTypeFlag(std::string && longName, HashType * ht);
     };
 
     std::map<std::string, Flag::ptr> longFlags;
@@ -65,49 +111,7 @@ protected:
 
 public:
 
-    class FlagMaker
-    {
-        Args & args;
-        Flag::ptr flag;
-        friend class Args;
-        FlagMaker(Args & args) : args(args), flag(std::make_shared<Flag>()) { }
-    public:
-        ~FlagMaker();
-        FlagMaker & longName(const std::string & s) { flag->longName = s; return *this; }
-        FlagMaker & shortName(char s) { flag->shortName = s; return *this; }
-        FlagMaker & description(const std::string & s) { flag->description = s; return *this; }
-        FlagMaker & label(const std::string & l) { flag->arity = 1; flag->labels = {l}; return *this; }
-        FlagMaker & labels(const Strings & ls) { flag->arity = ls.size(); flag->labels = ls; return *this; }
-        FlagMaker & arity(size_t arity) { flag->arity = arity; return *this; }
-        FlagMaker & handler(std::function<void(std::vector<std::string>)> handler) { flag->handler = handler; return *this; }
-        FlagMaker & handler(std::function<void()> handler) { flag->handler = [handler](std::vector<std::string>) { handler(); }; return *this; }
-        FlagMaker & handler(std::function<void(std::string)> handler) {
-            flag->arity = 1;
-            flag->handler = [handler](std::vector<std::string> ss) { handler(std::move(ss[0])); };
-            return *this;
-        }
-        FlagMaker & category(const std::string & s) { flag->category = s; return *this; }
-
-        template<class T>
-        FlagMaker & dest(T * dest)
-        {
-            flag->arity = 1;
-            flag->handler = [=](std::vector<std::string> ss) { *dest = ss[0]; };
-            return *this;
-        }
-
-        template<class T>
-        FlagMaker & set(T * dest, const T & val)
-        {
-            flag->arity = 0;
-            flag->handler = [=](std::vector<std::string> ss) { *dest = val; };
-            return *this;
-        }
-
-        FlagMaker & mkHashTypeFlag(HashType * ht);
-    };
-
-    FlagMaker mkFlag();
+    void addFlag(Flag && flag);
 
     /* Helper functions for constructing flags / positional
        arguments. */
@@ -116,13 +120,13 @@ public:
         const std::string & label, const std::string & description,
         std::function<void(std::string)> fun)
     {
-        mkFlag()
-            .shortName(shortName)
-            .longName(longName)
-            .labels({label})
-            .description(description)
-            .arity(1)
-            .handler([=](std::vector<std::string> ss) { fun(ss[0]); });
+        addFlag({
+            .longName = longName,
+            .shortName = shortName,
+            .description = description,
+            .labels = {label},
+            .handler = {[=](std::string s) { fun(s); }}
+        });
     }
 
     void mkFlag(char shortName, const std::string & name,
@@ -135,11 +139,12 @@ public:
     void mkFlag(char shortName, const std::string & longName, const std::string & description,
         T * dest, const T & value)
     {
-        mkFlag()
-            .shortName(shortName)
-            .longName(longName)
-            .description(description)
-            .handler([=](std::vector<std::string> ss) { *dest = value; });
+        addFlag({
+            .longName = longName,
+            .shortName = shortName,
+            .description = description,
+            .handler = {[=]() { *dest = value; }}
+        });
     }
 
     template<class I>
@@ -155,18 +160,18 @@ public:
     void mkFlag(char shortName, const std::string & longName,
         const std::string & description, std::function<void(I)> fun)
     {
-        mkFlag()
-            .shortName(shortName)
-            .longName(longName)
-            .labels({"N"})
-            .description(description)
-            .arity(1)
-            .handler([=](std::vector<std::string> ss) {
+        addFlag({
+            .longName = longName,
+            .shortName = shortName,
+            .description = description,
+            .labels = {"N"},
+            .handler = {[=](std::string s) {
                 I n;
-                if (!string2Int(ss[0], n))
+                if (!string2Int(s, n))
                     throw UsageError("flag '--%s' requires a integer argument", longName);
                 fun(n);
-            });
+            }}
+        });
     }
 
     /* Expect a string argument. */
diff --git a/src/libutil/config.cc b/src/libutil/config.cc
index 7551d97d1..f03e444ec 100644
--- a/src/libutil/config.cc
+++ b/src/libutil/config.cc
@@ -177,12 +177,13 @@ void BaseSetting<T>::toJSON(JSONPlaceholder & out)
 template<typename T>
 void BaseSetting<T>::convertToArg(Args & args, const std::string & category)
 {
-    args.mkFlag()
-        .longName(name)
-        .description(description)
-        .arity(1)
-        .handler([=](std::vector<std::string> ss) { overriden = true; set(ss[0]); })
-        .category(category);
+    args.addFlag({
+        .longName = name,
+        .description = description,
+        .category = category,
+        .labels = {"value"},
+        .handler = {[=](std::string s) { overriden = true; set(s); }},
+    });
 }
 
 template<> void BaseSetting<std::string>::set(const std::string & str)
@@ -227,16 +228,18 @@ template<> std::string BaseSetting<bool>::to_string() const
 
 template<> void BaseSetting<bool>::convertToArg(Args & args, const std::string & category)
 {
-    args.mkFlag()
-        .longName(name)
-        .description(description)
-        .handler([=](std::vector<std::string> ss) { override(true); })
-        .category(category);
-    args.mkFlag()
-        .longName("no-" + name)
-        .description(description)
-        .handler([=](std::vector<std::string> ss) { override(false); })
-        .category(category);
+    args.addFlag({
+        .longName = name,
+        .description = description,
+        .category = category,
+        .handler = {[=]() { override(true); }}
+    });
+    args.addFlag({
+        .longName = "no-" + name,
+        .description = description,
+        .category = category,
+        .handler = {[=]() { override(false); }}
+    });
 }
 
 template<> void BaseSetting<Strings>::set(const std::string & str)
diff --git a/src/nix/add-to-store.cc b/src/nix/add-to-store.cc
index ed02227db..00da01f7e 100644
--- a/src/nix/add-to-store.cc
+++ b/src/nix/add-to-store.cc
@@ -14,12 +14,13 @@ struct CmdAddToStore : MixDryRun, StoreCommand
     {
         expectArg("path", &path);
 
-        mkFlag()
-            .longName("name")
-            .shortName('n')
-            .description("name component of the store path")
-            .labels({"name"})
-            .dest(&namePart);
+        addFlag({
+            .longName = "name",
+            .shortName = 'n',
+            .description = "name component of the store path",
+            .labels = {"name"},
+            .handler = {&namePart},
+        });
     }
 
     std::string description() override
diff --git a/src/nix/build.cc b/src/nix/build.cc
index 0b0762836..850e09ce8 100644
--- a/src/nix/build.cc
+++ b/src/nix/build.cc
@@ -11,17 +11,19 @@ struct CmdBuild : InstallablesCommand, MixDryRun, MixProfile
 
     CmdBuild()
     {
-        mkFlag()
-            .longName("out-link")
-            .shortName('o')
-            .description("path of the symlink to the build result")
-            .labels({"path"})
-            .dest(&outLink);
+        addFlag({
+            .longName = "out-link",
+            .shortName = 'o',
+            .description = "path of the symlink to the build result",
+            .labels = {"path"},
+            .handler = {&outLink},
+        });
 
-        mkFlag()
-            .longName("no-link")
-            .description("do not create a symlink to the build result")
-            .set(&outLink, Path(""));
+        addFlag({
+            .longName = "no-link",
+            .description = "do not create a symlink to the build result",
+            .handler = {&outLink, Path("")},
+        });
     }
 
     std::string description() override
diff --git a/src/nix/command.cc b/src/nix/command.cc
index 99b24d2a2..71b027719 100644
--- a/src/nix/command.cc
+++ b/src/nix/command.cc
@@ -35,16 +35,18 @@ StorePathsCommand::StorePathsCommand(bool recursive)
     : recursive(recursive)
 {
     if (recursive)
-        mkFlag()
-            .longName("no-recursive")
-            .description("apply operation to specified paths only")
-            .set(&this->recursive, false);
+        addFlag({
+            .longName = "no-recursive",
+            .description = "apply operation to specified paths only",
+            .handler = {&this->recursive, false},
+        });
     else
-        mkFlag()
-            .longName("recursive")
-            .shortName('r')
-            .description("apply operation to closure of the specified paths")
-            .set(&this->recursive, true);
+        addFlag({
+            .longName = "recursive",
+            .shortName = 'r',
+            .description = "apply operation to closure of the specified paths",
+            .handler = {&this->recursive, true},
+        });
 
     mkFlag(0, "all", "apply operation to the entire store", &all);
 }
@@ -101,11 +103,12 @@ Strings editorFor(const Pos & pos)
 
 MixProfile::MixProfile()
 {
-    mkFlag()
-        .longName("profile")
-        .description("profile to update")
-        .labels({"path"})
-        .dest(&profile);
+    addFlag({
+        .longName = "profile",
+        .description = "profile to update",
+        .labels = {"path"},
+        .handler = {&profile},
+    });
 }
 
 void MixProfile::updateProfile(const StorePath & storePath)
@@ -145,28 +148,30 @@ MixDefaultProfile::MixDefaultProfile()
     profile = getDefaultProfile();
 }
 
-MixEnvironment::MixEnvironment() : ignoreEnvironment(false) {
-    mkFlag()
-        .longName("ignore-environment")
-        .shortName('i')
-        .description("clear the entire environment (except those specified with --keep)")
-        .set(&ignoreEnvironment, true);
+MixEnvironment::MixEnvironment() : ignoreEnvironment(false)
+{
+    addFlag({
+        .longName = "ignore-environment",
+        .shortName = 'i',
+        .description = "clear the entire environment (except those specified with --keep)",
+        .handler = {&ignoreEnvironment, true},
+    });
 
-    mkFlag()
-        .longName("keep")
-        .shortName('k')
-        .description("keep specified environment variable")
-        .arity(1)
-        .labels({"name"})
-        .handler([&](std::vector<std::string> ss) { keep.insert(ss.front()); });
+    addFlag({
+        .longName = "keep",
+        .shortName = 'k',
+        .description = "keep specified environment variable",
+        .labels = {"name"},
+        .handler = {[&](std::string s) { keep.insert(s); }},
+    });
 
-    mkFlag()
-        .longName("unset")
-        .shortName('u')
-        .description("unset specified environment variable")
-        .arity(1)
-        .labels({"name"})
-        .handler([&](std::vector<std::string> ss) { unset.insert(ss.front()); });
+    addFlag({
+        .longName = "unset",
+        .shortName = 'u',
+        .description = "unset specified environment variable",
+        .labels = {"name"},
+        .handler = {[&](std::string s) { unset.insert(s); }},
+    });
 }
 
 void MixEnvironment::setEnviron() {
diff --git a/src/nix/copy.cc b/src/nix/copy.cc
index 85c777d38..77673a1c2 100644
--- a/src/nix/copy.cc
+++ b/src/nix/copy.cc
@@ -19,27 +19,32 @@ struct CmdCopy : StorePathsCommand
     CmdCopy()
         : StorePathsCommand(true)
     {
-        mkFlag()
-            .longName("from")
-            .labels({"store-uri"})
-            .description("URI of the source Nix store")
-            .dest(&srcUri);
-        mkFlag()
-            .longName("to")
-            .labels({"store-uri"})
-            .description("URI of the destination Nix store")
-            .dest(&dstUri);
+        addFlag({
+            .longName = "from",
+            .description = "URI of the source Nix store",
+            .labels = {"store-uri"},
+            .handler = {&srcUri},
+        });
 
-        mkFlag()
-            .longName("no-check-sigs")
-            .description("do not require that paths are signed by trusted keys")
-            .set(&checkSigs, NoCheckSigs);
+        addFlag({
+            .longName = "to",
+            .description = "URI of the destination Nix store",
+            .labels = {"store-uri"},
+            .handler = {&dstUri},
+        });
 
-        mkFlag()
-            .longName("substitute-on-destination")
-            .shortName('s')
-            .description("whether to try substitutes on the destination store (only supported by SSH)")
-            .set(&substitute, Substitute);
+        addFlag({
+            .longName = "no-check-sigs",
+            .description = "do not require that paths are signed by trusted keys",
+            .handler = {&checkSigs, NoCheckSigs},
+        });
+
+        addFlag({
+            .longName = "substitute-on-destination",
+            .shortName = 's',
+            .description = "whether to try substitutes on the destination store (only supported by SSH)",
+            .handler = {&substitute, Substitute},
+        });
     }
 
     std::string description() override
diff --git a/src/nix/dev-shell.cc b/src/nix/dev-shell.cc
index 9c45a935d..2bdf59839 100644
--- a/src/nix/dev-shell.cc
+++ b/src/nix/dev-shell.cc
@@ -237,16 +237,16 @@ struct CmdDevShell : Common, MixEnvironment
 
     CmdDevShell()
     {
-        mkFlag()
-            .longName("command")
-            .shortName('c')
-            .description("command and arguments to be executed insted of an interactive shell")
-            .labels({"command", "args"})
-            .arity(ArityAny)
-            .handler([&](std::vector<std::string> ss) {
+        addFlag({
+            .longName = "command",
+            .shortName = 'c',
+            .description = "command and arguments to be executed insted of an interactive shell",
+            .labels = {"command", "args"},
+            .handler = {[&](std::vector<std::string> ss) {
                 if (ss.empty()) throw UsageError("--command requires at least one argument");
                 command = ss;
-            });
+            }}
+        });
     }
 
     std::string description() override
diff --git a/src/nix/hash.cc b/src/nix/hash.cc
index 01628cf6c..9b9509d3a 100644
--- a/src/nix/hash.cc
+++ b/src/nix/hash.cc
@@ -23,9 +23,7 @@ struct CmdHash : Command
         mkFlag(0, "base64", "print hash in base-64", &base, Base64);
         mkFlag(0, "base32", "print hash in base-32 (Nix-specific)", &base, Base32);
         mkFlag(0, "base16", "print hash in base-16", &base, Base16);
-        mkFlag()
-            .longName("type")
-            .mkHashTypeFlag(&ht);
+        addFlag(Flag::mkHashTypeFlag("type", &ht));
         #if 0
         mkFlag()
             .longName("modulo")
@@ -76,9 +74,7 @@ struct CmdToBase : Command
 
     CmdToBase(Base base) : base(base)
     {
-        mkFlag()
-            .longName("type")
-            .mkHashTypeFlag(&ht);
+        addFlag(Flag::mkHashTypeFlag("type", &ht));
         expectArgs("strings", &args);
     }
 
diff --git a/src/nix/installables.cc b/src/nix/installables.cc
index 1d70ad3d5..937d69206 100644
--- a/src/nix/installables.cc
+++ b/src/nix/installables.cc
@@ -15,12 +15,13 @@ namespace nix {
 
 SourceExprCommand::SourceExprCommand()
 {
-    mkFlag()
-        .shortName('f')
-        .longName("file")
-        .label("file")
-        .description("evaluate FILE rather than the default")
-        .dest(&file);
+    addFlag({
+        .longName = "file",
+        .shortName = 'f',
+        .description = "evaluate FILE rather than the default",
+        .labels = {"file"},
+        .handler = {&file}
+    });
 }
 
 Value * SourceExprCommand::getSourceExpr(EvalState & state)
diff --git a/src/nix/main.cc b/src/nix/main.cc
index 2c64c7476..57b8bed9f 100644
--- a/src/nix/main.cc
+++ b/src/nix/main.cc
@@ -59,15 +59,16 @@ struct NixArgs : virtual MultiCommand, virtual MixCommonArgs
 
     NixArgs() : MultiCommand(*RegisterCommand::commands), MixCommonArgs("nix")
     {
-        mkFlag()
-            .longName("help")
-            .description("show usage information")
-            .handler([&]() { showHelpAndExit(); });
+        addFlag({
+            .longName = "help",
+            .description = "show usage information",
+            .handler = {[&]() { showHelpAndExit(); }},
+        });
 
-        mkFlag()
-            .longName("help-config")
-            .description("show configuration options")
-            .handler([&]() {
+        addFlag({
+            .longName = "help-config",
+            .description = "show configuration options",
+            .handler = {[&]() {
                 std::cout << "The following configuration options are available:\n\n";
                 Table2 tbl;
                 std::map<std::string, Config::SettingInfo> settings;
@@ -76,28 +77,33 @@ struct NixArgs : virtual MultiCommand, virtual MixCommonArgs
                     tbl.emplace_back(s.first, s.second.description);
                 printTable(std::cout, tbl);
                 throw Exit();
-            });
+            }},
+        });
 
-        mkFlag()
-            .longName("print-build-logs")
-            .shortName('L')
-            .description("print full build logs on stderr")
-            .set(&printBuildLogs, true);
+        addFlag({
+            .longName = "print-build-logs",
+            .shortName = 'L',
+            .description = "print full build logs on stderr",
+            .handler = {&printBuildLogs, true},
+        });
 
-        mkFlag()
-            .longName("version")
-            .description("show version information")
-            .handler([&]() { printVersion(programName); });
+        addFlag({
+            .longName = "version",
+            .description = "show version information",
+            .handler = {[&]() { printVersion(programName); }},
+        });
 
-        mkFlag()
-            .longName("no-net")
-            .description("disable substituters and consider all previously downloaded files up-to-date")
-            .handler([&]() { useNet = false; });
+        addFlag({
+            .longName = "no-net",
+            .description = "disable substituters and consider all previously downloaded files up-to-date",
+            .handler = {[&]() { useNet = false; }},
+        });
 
-        mkFlag()
-            .longName("refresh")
-            .description("consider all previously downloaded files out-of-date")
-            .handler([&]() { refresh = true; });
+        addFlag({
+            .longName = "refresh",
+            .description = "consider all previously downloaded files out-of-date",
+            .handler = {[&]() { refresh = true; }},
+        });
     }
 
     void printFlags(std::ostream & out) override
diff --git a/src/nix/run.cc b/src/nix/run.cc
index ebfec36d9..b888281a5 100644
--- a/src/nix/run.cc
+++ b/src/nix/run.cc
@@ -63,16 +63,16 @@ struct CmdShell : InstallablesCommand, RunCommon, MixEnvironment
 
     CmdShell()
     {
-        mkFlag()
-            .longName("command")
-            .shortName('c')
-            .description("command and arguments to be executed; defaults to '$SHELL'")
-            .labels({"command", "args"})
-            .arity(ArityAny)
-            .handler([&](std::vector<std::string> ss) {
+        addFlag({
+            .longName = "command",
+            .shortName = 'c',
+            .description = "command and arguments to be executed; defaults to '$SHELL'",
+            .labels = {"command", "args"},
+            .handler = {[&](std::vector<std::string> ss) {
                 if (ss.empty()) throw UsageError("--command requires at least one argument");
                 command = ss;
-            });
+            }}
+        });
     }
 
     std::string description() override
diff --git a/src/nix/search.cc b/src/nix/search.cc
index 769274543..fcad6be84 100644
--- a/src/nix/search.cc
+++ b/src/nix/search.cc
@@ -40,16 +40,18 @@ struct CmdSearch : SourceExprCommand, MixJSON
     {
         expectArgs("regex", &res);
 
-        mkFlag()
-            .longName("update-cache")
-            .shortName('u')
-            .description("update the package search cache")
-            .handler([&]() { writeCache = true; useCache = false; });
+        addFlag({
+            .longName = "update-cache",
+            .shortName = 'u',
+            .description = "update the package search cache",
+            .handler = {[&]() { writeCache = true; useCache = false; }}
+        });
 
-        mkFlag()
-            .longName("no-cache")
-            .description("do not use or update the package search cache")
-            .handler([&]() { writeCache = false; useCache = false; });
+        addFlag({
+            .longName = "no-cache",
+            .description = "do not use or update the package search cache",
+            .handler = {[&]() { writeCache = false; useCache = false; }}
+        });
     }
 
     std::string description() override
diff --git a/src/nix/show-derivation.cc b/src/nix/show-derivation.cc
index 0ede7b468..b6f24599f 100644
--- a/src/nix/show-derivation.cc
+++ b/src/nix/show-derivation.cc
@@ -15,11 +15,12 @@ struct CmdShowDerivation : InstallablesCommand
 
     CmdShowDerivation()
     {
-        mkFlag()
-            .longName("recursive")
-            .shortName('r')
-            .description("include the dependencies of the specified derivations")
-            .set(&recursive, true);
+        addFlag({
+            .longName = "recursive",
+            .shortName = 'r',
+            .description = "include the dependencies of the specified derivations",
+            .handler = {&recursive, true}
+        });
     }
 
     std::string description() override
diff --git a/src/nix/sigs.cc b/src/nix/sigs.cc
index 5f07448e0..a91465c2a 100644
--- a/src/nix/sigs.cc
+++ b/src/nix/sigs.cc
@@ -13,13 +13,13 @@ struct CmdCopySigs : StorePathsCommand
 
     CmdCopySigs()
     {
-        mkFlag()
-            .longName("substituter")
-            .shortName('s')
-            .labels({"store-uri"})
-            .description("use signatures from specified store")
-            .arity(1)
-            .handler([&](std::vector<std::string> ss) { substituterUris.push_back(ss[0]); });
+        addFlag({
+            .longName = "substituter",
+            .shortName = 's',
+            .description = "use signatures from specified store",
+            .labels = {"store-uri"},
+            .handler = {[&](std::string s) { substituterUris.push_back(s); }},
+        });
     }
 
     std::string description() override
@@ -98,12 +98,13 @@ struct CmdSignPaths : StorePathsCommand
 
     CmdSignPaths()
     {
-        mkFlag()
-            .shortName('k')
-            .longName("key-file")
-            .label("file")
-            .description("file containing the secret signing key")
-            .dest(&secretKeyFile);
+        addFlag({
+            .longName = "key-file",
+            .shortName = 'k',
+            .description = "file containing the secret signing key",
+            .labels = {"file"},
+            .handler = {&secretKeyFile}
+        });
     }
 
     std::string description() override
diff --git a/src/nix/upgrade-nix.cc b/src/nix/upgrade-nix.cc
index 4fcc6a721..32efcc3a7 100644
--- a/src/nix/upgrade-nix.cc
+++ b/src/nix/upgrade-nix.cc
@@ -16,18 +16,20 @@ struct CmdUpgradeNix : MixDryRun, StoreCommand
 
     CmdUpgradeNix()
     {
-        mkFlag()
-            .longName("profile")
-            .shortName('p')
-            .labels({"profile-dir"})
-            .description("the Nix profile to upgrade")
-            .dest(&profileDir);
+        addFlag({
+            .longName = "profile",
+            .shortName = 'p',
+            .description = "the Nix profile to upgrade",
+            .labels = {"profile-dir"},
+            .handler = {&profileDir}
+        });
 
-        mkFlag()
-            .longName("nix-store-paths-url")
-            .labels({"url"})
-            .description("URL of the file that contains the store paths of the latest Nix release")
-            .dest(&storePathsUrl);
+        addFlag({
+            .longName = "nix-store-paths-url",
+            .description = "URL of the file that contains the store paths of the latest Nix release",
+            .labels = {"url"},
+            .handler = {&storePathsUrl}
+        });
     }
 
     std::string description() override
diff --git a/src/nix/verify.cc b/src/nix/verify.cc
index 9b0658803..08a36ac50 100644
--- a/src/nix/verify.cc
+++ b/src/nix/verify.cc
@@ -20,13 +20,13 @@ struct CmdVerify : StorePathsCommand
     {
         mkFlag(0, "no-contents", "do not verify the contents of each store path", &noContents);
         mkFlag(0, "no-trust", "do not verify whether each store path is trusted", &noTrust);
-        mkFlag()
-            .longName("substituter")
-            .shortName('s')
-            .labels({"store-uri"})
-            .description("use signatures from specified store")
-            .arity(1)
-            .handler([&](std::vector<std::string> ss) { substituterUris.push_back(ss[0]); });
+        addFlag({
+            .longName = "substituter",
+            .shortName = 's',
+            .description = "use signatures from specified store",
+            .labels = {"store-uri"},
+            .handler = {[&](std::string s) { substituterUris.push_back(s); }}
+        });
         mkIntFlag('n', "sigs-needed", "require that each path has at least N valid signatures", &sigsNeeded);
     }
 
diff --git a/src/nix/why-depends.cc b/src/nix/why-depends.cc
index f9acc7f13..36a3ee863 100644
--- a/src/nix/why-depends.cc
+++ b/src/nix/why-depends.cc
@@ -37,11 +37,12 @@ struct CmdWhyDepends : SourceExprCommand
         expectArg("package", &_package);
         expectArg("dependency", &_dependency);
 
-        mkFlag()
-            .longName("all")
-            .shortName('a')
-            .description("show all edges in the dependency graph leading from 'package' to 'dependency', rather than just a shortest path")
-            .set(&all, true);
+        addFlag({
+            .longName = "all",
+            .shortName = 'a',
+            .description = "show all edges in the dependency graph leading from 'package' to 'dependency', rather than just a shortest path",
+            .handler = {&all, true},
+        });
     }
 
     std::string description() override