nix: Show help when no arguments are given

Fixes #1464.

src/libutil/args.cc

@@ -146,7 +146,7 @@ bool Args::processArgs(const Strings & args, bool finish)
         res = true;
     }
 
-    if (finish && !expectedArgs.empty())
+    if (finish && !expectedArgs.empty() && !expectedArgs.front().optional)
         throw UsageError("more arguments are required");
 
     return res;

src/libutil/args.hh

@@ -51,6 +51,7 @@ protected:
     {
         std::string label;
         size_t arity; // 0 = any
+        bool optional;
         std::function<void(Strings)> handler;
     };
 
@@ -165,7 +166,7 @@ public:
     /* Expect a string argument. */
     void expectArg(const std::string & label, string * dest)
     {
-        expectedArgs.push_back(ExpectedArg{label, 1, [=](Strings ss) {
+        expectedArgs.push_back(ExpectedArg{label, 1, false, [=](Strings ss) {
             *dest = ss.front();
         }});
     }
@@ -173,7 +174,7 @@ public:
     /* Expect 0 or more arguments. */
     void expectArgs(const std::string & label, Strings * dest)
     {
-        expectedArgs.push_back(ExpectedArg{label, 0, [=](Strings ss) {
+        expectedArgs.push_back(ExpectedArg{label, 0, false, [=](Strings ss) {
             *dest = ss;
         }});
     }

src/nix/command.cc

@@ -24,7 +24,7 @@ void Command::printHelp(const string & programName, std::ostream & out)
 MultiCommand::MultiCommand(const Commands & _commands)
     : commands(_commands)
 {
-    expectedArgs.push_back(ExpectedArg{"command", 1, [=](Strings ss) {
+    expectedArgs.push_back(ExpectedArg{"command", 1, true, [=](Strings ss) {
         assert(!command);
         auto i = commands.find(ss.front());
         if (i == commands.end())

src/nix/main.cc

@@ -15,11 +15,7 @@ struct NixArgs : virtual MultiCommand, virtual MixCommonArgs
 {
     NixArgs() : MultiCommand(*RegisterCommand::commands), MixCommonArgs("nix")
     {
-        mkFlag('h', "help", "show usage information", [=]() {
-            printHelp(programName, std::cout);
-            std::cout << "\nNote: this program is EXPERIMENTAL and subject to change.\n";
-            throw Exit();
-        });
+        mkFlag('h', "help", "show usage information", [&]() { showHelpAndExit(); });
 
         mkFlag(0, "help-config", "show configuration options", [=]() {
             std::cout << "The following configuration options are available:\n\n";
@@ -47,6 +43,13 @@ struct NixArgs : virtual MultiCommand, virtual MixCommonArgs
             "Boolean settings can be overriden using ‘--<name>’ or ‘--no-<name>’. See ‘nix\n"
             "--help-config’ for a list of configuration settings.\n";
     }
+
+    void showHelpAndExit()
+    {
+        printHelp(programName, std::cout);
+        std::cout << "\nNote: this program is EXPERIMENTAL and subject to change.\n";
+        throw Exit();
+    }
 };
 
 void mainWrapped(int argc, char * * argv)
@@ -68,7 +71,7 @@ void mainWrapped(int argc, char * * argv)
 
     args.parseCmdline(argvToStrings(argc, argv));
 
-    assert(args.command);
+    if (!args.command) args.showHelpAndExit();
 
     StartProgressBar bar;