Add support for default help command.

Author: lorentey

OptionParser/Command.swift

@@ -91,15 +91,17 @@ extension OptionParser {
         }
 
         override func parse(from context: inout ParseContext, _ parentRecord: Record) throws {
+            let toolName = context.toolName
+            let printer = context.printer
             try syntax.parse(from: &context, parentRecord: parentRecord) { record in
                 if let commandName = record.command {
                     guard let command = self.parentSyntax!.commandsByName[commandName] else {
                         throw OptionError("Unknown command '\(commandName)'")
                     }
-                    try command.printHelp(toolName: context.toolName, printer: context.printer)
+                    try command.printHelp(toolName: toolName, printer: printer)
                 }
                 else {
-                    try self.parentSyntax!.printHelp(toolName: context.toolName, printer: context.printer)
+                    try self.parentSyntax!.printHelp(toolName: toolName, printer: printer)
                 }
             }
         }

OptionParser/OptionParser.swift

@@ -12,7 +12,7 @@ public struct OptionError: Error {
 
 public struct OptionParser<Record> {
     let syntax: Syntax<Void, Record>
-    let action: (Record) throws -> Void
+    let action: ((Record) throws -> Void)?
 
     public init(docs: String,
                 initial: Record,
@@ -26,6 +26,17 @@ public struct OptionParser<Record> {
                   action: defaultAction)
     }
 
+    public init(docs: String,
+                initial: Record,
+                options: [Option] = [],
+                commands: [AnyCommand]) {
+        self.init(_docs: docs,
+                  initial: initial,
+                  options: options,
+                  commands: commands,
+                  action: nil)
+    }
+
     public init(docs: String,
                 initial: Record,
                 options: [Option] = [],
@@ -43,7 +54,7 @@ public struct OptionParser<Record> {
          options: [Option] = [],
          parameters: [Parameter] = [],
          commands: [AnyCommand] = [],
-         action: @escaping (Record) throws -> Void) {
+         action: ((Record) throws -> Void)?) {
 
         self.syntax = Syntax<Void, Record>(docs: docs,
                                            initial: initial,

OptionParser/Syntax.swift

@@ -56,7 +56,7 @@ final class Syntax<Record, Subrecord> {
         self.init(name: name, docs: docs, initial: initial, options: options, parameters: parameters, commands: [])
     }
 
-    func parse(from context: inout ParseContext, parentRecord: Record, action: (Subrecord) throws -> Void) throws {
+    func parse(from context: inout ParseContext, parentRecord: Record, action: ((Subrecord) throws -> Void)?) throws {
         var record = try initial(parentRecord)
         var arguments: [String] = []
         var parametersOnly = false
@@ -92,7 +92,12 @@ final class Syntax<Record, Subrecord> {
             }
         }
         try parsePositionalArguments(arguments, record: &record)
-        try action(record)
+        if let action = action {
+            try action(record)
+        }
+        else {
+            try self.printHelp(toolName: context.toolName, printer: context.printer)
+        }
     }
 
     private func parsePositionalArguments(_ arguments: [String], record: inout Subrecord) throws {