Make it possible to add default subcommand

pkgs/args/CHANGELOG.md

@@ -1,3 +1,12 @@
+## 2.8.0
+
+* Allow designating a top-level command or a subcommand as a default one by
+  passing `isDefault: true` to `addCommand` or `addSubcommand`.
+  Default command will be selected by argument parser if no sibling command
+  matches. This allows creating command line interfaces where both
+  `program command` and `program command subcommand` are runnable
+  (Fixes #103).
+
 ## 2.7.0
 
 * Remove sorting of the `allowedHelp` argument in usage output. Ordering will

pkgs/args/lib/command_runner.dart

@@ -31,9 +31,19 @@ class CommandRunner<T> {
 
   /// A single-line template for how to invoke this executable.
   ///
-  /// Defaults to `"$executableName <command> arguments`". Subclasses can
-  /// override this for a more specific template.
-  String get invocation => '$executableName <command> [arguments]';
+  /// Defaults to `"$executableName <command> arguments"` (if there is no
+  /// default command) or `"$executableName [<command>] arguments"` (otherwise).
+  ///
+  /// Subclasses can override this for a more specific template.
+  String get invocation {
+    var command = '<command>';
+
+    if (argParser.defaultCommand != null) {
+      command = '[$command]';
+    }
+
+    return '$executableName $command [arguments]';
+  }
 
   /// Generates a string displaying usage information for the executable.
   ///
@@ -56,9 +66,10 @@ class CommandRunner<T> {
     );
     buffer.writeln(_wrap('Global options:'));
     buffer.writeln('${argParser.usage}\n');
-    buffer.writeln(
-      '${_getCommandUsage(_commands, lineLength: argParser.usageLineLength)}\n',
-    );
+    buffer.writeln(_getCommandUsage(_commands,
+        lineLength: argParser.usageLineLength,
+        defaultCommand: argParser.defaultCommand));
+    buffer.writeln();
     buffer.write(_wrap(
         'Run "$executableName help <command>" for more information about a '
         'command.'));
@@ -105,12 +116,25 @@ class CommandRunner<T> {
       throw UsageException(message, _usageWithoutDescription);
 
   /// Adds [Command] as a top-level command to this runner.
-  void addCommand(Command<T> command) {
+  ///
+  /// If [isDefault] is `true` then added command will be designated as a
+  /// default one. Default command is selected if no other sibling command
+  /// matches. Only a single leaf-command can be designated as a default.
+  void addCommand(Command<T> command, {bool isDefault = false}) {
+    if (isDefault && command.subcommands.isNotEmpty) {
+      throw ArgumentError('default command must be a leaf command');
+    }
+    if (isDefault && argParser.defaultCommand != null) {
+      throw StateError('default command already defined');
+    }
     var names = [command.name, ...command.aliases];
     for (var name in names) {
       _commands[name] = command;
       argParser.addCommand(name, command.argParser);
     }
+    if (isDefault) {
+      argParser.defaultCommand = command.name;
+    }
     command._runner = this;
   }
 
@@ -288,9 +312,13 @@ abstract class Command<T> {
     parents.add(runner!.executableName);
 
     var invocation = parents.reversed.join(' ');
-    return _subcommands.isNotEmpty
-        ? '$invocation <subcommand> [arguments]'
-        : '$invocation [arguments]';
+    if (argParser.defaultCommand != null) {
+      return '$invocation [<subcommand>] [arguments]';
+    } else if (_subcommands.isNotEmpty) {
+      return '$invocation <subcommand> [arguments]';
+    } else {
+      return '$invocation [arguments]';
+    }
   }
 
   /// The command's parent command, if this is a subcommand.
@@ -363,11 +391,10 @@ abstract class Command<T> {
 
     if (_subcommands.isNotEmpty) {
       buffer.writeln();
-      buffer.writeln(_getCommandUsage(
-        _subcommands,
-        isSubcommand: true,
-        lineLength: length,
-      ));
+      buffer.writeln(_getCommandUsage(_subcommands,
+          isSubcommand: true,
+          lineLength: length,
+          defaultCommand: argParser.defaultCommand));
     }
 
     buffer.writeln();
@@ -446,12 +473,26 @@ abstract class Command<T> {
   }
 
   /// Adds [Command] as a subcommand of this.
-  void addSubcommand(Command<T> command) {
+  ///
+  /// If [isDefault] is `true` then added command will be designated as a
+  /// default one. Default subcommand is selected if no other sibling subcommand
+  /// matches. Only a single leaf-command can be designated as a default.
+  void addSubcommand(Command<T> command, {bool isDefault = false}) {
+    if (isDefault && command.subcommands.isNotEmpty) {
+      throw ArgumentError('default command must be a leaf command');
+    }
+    if (isDefault && argParser.defaultCommand != null) {
+      throw StateError('default command already defined');
+    }
+
     var names = [command.name, ...command.aliases];
     for (var name in names) {
       _subcommands[name] = command;
       argParser.addCommand(name, command.argParser);
     }
+    if (isDefault) {
+      argParser.defaultCommand = command.name;
+    }
     command._parent = this;
   }
 
@@ -470,8 +511,10 @@ abstract class Command<T> {
 ///
 /// [isSubcommand] indicates whether the commands should be called "commands" or
 /// "subcommands".
+///
+/// [defaultCommand] indicate which command (if any) is designated as default.
 String _getCommandUsage(Map<String, Command> commands,
-    {bool isSubcommand = false, int? lineLength}) {
+    {bool isSubcommand = false, int? lineLength, String? defaultCommand}) {
   // Don't include aliases.
   var names =
       commands.keys.where((name) => !commands[name]!.aliases.contains(name));
@@ -502,7 +545,8 @@ String _getCommandUsage(Map<String, Command> commands,
       buffer.write(category);
     }
     for (var command in commandsByCategory[category]!) {
-      var lines = wrapTextAsLines(command.summary,
+      var defaultMarker = defaultCommand == command.name ? '(default) ' : '';
+      var lines = wrapTextAsLines(defaultMarker + command.summary,
           start: columnStart, length: lineLength);
       buffer.writeln();
       buffer.write('  ${padRight(command.name, length)}   ${lines.first}');
@@ -515,6 +559,15 @@ String _getCommandUsage(Map<String, Command> commands,
     }
   }
 
+  if (defaultCommand != null) {
+    buffer.writeln();
+    buffer.writeln();
+    buffer.write(wrapText(
+        'Default command ($defaultCommand) will be selected if no command'
+        ' is explicitly specified.',
+        length: lineLength));
+  }
+
   return buffer.toString();
 }
 

pkgs/args/lib/src/allow_anything_parser.dart

@@ -104,4 +104,11 @@ class AllowAnythingParser implements ArgParser {
 
   @override
   Option? findByNameOrAlias(String name) => null;
+
+  @override
+  String? get defaultCommand => null;
+
+  @override
+  set defaultCommand(String? value) => throw UnsupportedError(
+      "ArgParser.allowAnything().defaultCommand= isn't supported.");
 }

pkgs/args/lib/src/arg_parser.dart

@@ -25,6 +25,11 @@ class ArgParser {
   /// The commands that have been defined for this parser.
   final Map<String, ArgParser> commands;
 
+  /// Command which will be executed by default if no command is specified.
+  ///
+  /// When `null` it is a usage error to omit the command name.
+  String? defaultCommand;
+
   /// A list of the [Option]s in [options] intermingled with [String]
   /// separators.
   final _optionsAndSeparators = <Object>[];

pkgs/args/lib/src/parser.dart

@@ -49,7 +49,7 @@ class Parser {
           _grammar, const {}, _commandName, null, arguments, arguments);
     }
 
-    ArgResults? commandResults;
+    ({String name, ArgParser parser})? command;
 
     // Parse the args.
     while (_args.isNotEmpty) {
@@ -61,26 +61,19 @@ class Parser {
 
       // Try to parse the current argument as a command. This happens before
       // options so that commands can have option-like names.
-      var command = _grammar.commands[_current];
-      if (command != null) {
-        _validate(_rest.isEmpty, 'Cannot specify arguments before a command.',
-            _current);
-        var commandName = _args.removeFirst();
-        var commandParser = Parser(commandName, command, _args, this, _rest);
-
-        try {
-          commandResults = commandParser.parse();
-        } on ArgParserException catch (error) {
-          throw ArgParserException(
-              error.message,
-              [commandName, ...error.commands],
-              error.argumentName,
-              error.source,
-              error.offset);
-        }
-
-        // All remaining arguments were passed to command so clear them here.
-        _rest.clear();
+      //
+      // Otherwise, if there is a default command then select it before parsing
+      // any arguments. We make exception for situations when help flag is
+      // passed because we want `program command -h` to display help for
+      // `command` rather than display help for the default subcommand of the
+      // `command`.
+      if (_grammar.commands[_current] case final parser?) {
+        command = (name: _args.removeFirst(), parser: parser);
+        break;
+      } else if (_grammar.defaultCommand case final defaultCommand?
+          when !(_current == '-h' || _current == '--help')) {
+        command =
+            (name: defaultCommand, parser: _grammar.commands[defaultCommand]!);
         break;
       }
 
@@ -96,6 +89,38 @@ class Parser {
       _rest.add(_args.removeFirst());
     }
 
+    // If there is a default command and we did not select any other commands
+    // and we don't have any trailing arguments then select the default
+    // command unless user requested help.
+    if (command == null && _rest.isEmpty && !_results.containsKey('help')) {
+      if (_grammar.defaultCommand case final defaultCommand?) {
+        command =
+            (name: defaultCommand, parser: _grammar.commands[defaultCommand]!);
+      }
+    }
+
+    ArgResults? commandResults;
+    if (command != null) {
+      _validate(_rest.isEmpty, 'Cannot specify arguments before a command.',
+          command.name);
+      var commandParser =
+          Parser(command.name, command.parser, _args, this, _rest);
+
+      try {
+        commandResults = commandParser.parse();
+      } on ArgParserException catch (error) {
+        throw ArgParserException(
+            error.message,
+            [command.name, ...error.commands],
+            error.argumentName,
+            error.source,
+            error.offset);
+      }
+
+      // All remaining arguments were passed to command so clear them here.
+      _rest.clear();
+    }
+
     // Check if mandatory and invoke existing callbacks.
     _grammar.options.forEach((name, option) {
       var parsedOption = _results[name];

pkgs/args/pubspec.yaml

@@ -1,5 +1,5 @@
 name: args
-version: 2.7.0
+version: 2.8.0
 description: >-
   Library for defining parsers for parsing raw command-line arguments into a set
   of options and values using GNU and POSIX style options.