Add UsageOnMissing option to Kong

Adds an UsageOnMissing option which configures Kong to omit errors and
display usage when a command is invalid due to a missing argument. This
can be useful if a user wishes for the top-level command to print usage
when no args are supplied, or if they would prefer child commands which
have their own subcommands to print usage specific to that command.

Signed-off-by: hasheddan <georgedanielmangum@gmail.com>

context.go

@@ -187,6 +187,9 @@ func (c *Context) Validate() error { // nolint: gocyclo
 	}
 
 	if err := checkMissingChildren(node); err != nil {
+		if c.Kong.usageOnMissing {
+			return c.PrintUsage(false)
+		}
 		return err
 	}
 	if err := checkMissingPositionals(positionals, node.Positional); err != nil {
@@ -624,6 +627,9 @@ func (c *Context) Run(binds ...interface{}) (err error) {
 	defer catch(&err)
 	node := c.Selected()
 	if node == nil {
+		if c.Kong.usageOnMissing {
+			return nil
+		}
 		return fmt.Errorf("no command selected")
 	}
 	return c.RunNode(node, binds...)

kong.go

@@ -49,6 +49,7 @@ type Kong struct {
 
 	noDefaultHelp  bool
 	usageOnError   bool
+	usageOnMissing bool
 	help           HelpPrinter
 	helpFormatter  HelpValueFormatter
 	helpOptions    HelpOptions

options.go

@@ -216,6 +216,14 @@ func UsageOnError() Option {
 	})
 }
 
+// UsageOnMissing configures Kong to display usage and exit successfully if command is missing a child argument.
+func UsageOnMissing() Option {
+	return OptionFunc(func(k *Kong) error {
+		k.usageOnMissing = true
+		return nil
+	})
+}
+
 // ClearResolvers clears all existing resolvers.
 func ClearResolvers() Option {
 	return OptionFunc(func(k *Kong) error {