Support default commands.

Fixes #41.

README.md

@@ -293,6 +293,8 @@ type CLI struct {
 }
 ```
 
+If a sub-command is tagged with `default:"1"` it will be selected if there are no further arguments.
+
 <a id="markdown-branching-positional-arguments" name="branching-positional-arguments"></a>
 ## Branching positional arguments
 
@@ -419,6 +421,7 @@ Tag                    | Description
 `type:"X"`             | Specify [named types](#custom-named-types) to use.
 `placeholder:"X"`      | Placeholder text.
 `default:"X"`          | Default value.
+`default:"1"`          | On a command, make it the default.
 `short:"X"`            | Short name, if flag.
 `required`             | If present, flag/arg is required.
 `optional`             | If present, flag/arg is optional.

context.go

@@ -397,6 +397,24 @@ func (c *Context) trace(node *Node) (err error) { // nolint: gocyclo
 			return fmt.Errorf("unexpected token %s", token)
 		}
 	}
+
+	// End of the line, check for a default command.
+	var defaultNode *Path
+	for _, child := range node.Children {
+		if child.Type == CommandNode && child.Tag.Default != "" {
+			if defaultNode != nil {
+				return fmt.Errorf("can't have more than one default command under %s", node.Summary())
+			}
+			defaultNode = &Path{
+				Parent:  child,
+				Command: child,
+				Flags:   child.Flags,
+			}
+		}
+	}
+	if defaultNode != nil {
+		c.Path = append(c.Path, defaultNode)
+	}
 	return nil
 }
 
@@ -617,6 +635,7 @@ func checkMissingChildren(node *Node) error {
 		missing = append(missing, strconv.Quote(strings.Join(missingArgs, " ")))
 	}
 
+	haveDefault := 0
 	for _, child := range node.Children {
 		if child.Hidden {
 			continue
@@ -627,10 +646,19 @@ func checkMissingChildren(node *Node) error {
 			}
 			missing = append(missing, strconv.Quote(child.Summary()))
 		} else {
+			if child.Tag.Default != "" {
+				if len(child.Children) > 0 {
+					return fmt.Errorf("default command %s must not have subcommands or arguments", child.Summary())
+				}
+				haveDefault++
+			}
 			missing = append(missing, strconv.Quote(child.Name))
 		}
 	}
-	if len(missing) == 0 {
+	if haveDefault > 1 {
+		return fmt.Errorf("more than one default command found under %s", node.Summary())
+	}
+	if len(missing) == 0 || haveDefault > 0 {
 		return nil
 	}
 

kong_test.go

@@ -815,3 +815,38 @@ func TestIssue40EnumAcrossCommands(t *testing.T) {
 	_, err = p.Parse([]string{"three", "c"})
 	require.NoError(t, err)
 }
+
+func TestEnumArg(t *testing.T) {
+	var cli struct {
+		Nested struct {
+			One string `arg:"" enum:"a,b,c"`
+			Two string `arg:""`
+		} `cmd:""`
+	}
+	p := mustNew(t, &cli)
+	_, err := p.Parse([]string{"nested", "a", "b"})
+	require.NoError(t, err)
+	require.Equal(t, "a", cli.Nested.One)
+	require.Equal(t, "b", cli.Nested.Two)
+}
+
+func TestDefaultCommand(t *testing.T) {
+	var cli struct {
+		One struct{} `cmd:"" default:"1"`
+		Two struct{} `cmd:""`
+	}
+	p := mustNew(t, &cli)
+	ctx, err := p.Parse([]string{})
+	require.NoError(t, err)
+	require.Equal(t, "one", ctx.Command())
+}
+
+func TestMultipleDefaultCommands(t *testing.T) {
+	var cli struct {
+		One struct{} `cmd:"" default:"1"`
+		Two struct{} `cmd:"" default:"1"`
+	}
+	p := mustNew(t, &cli)
+	_, err := p.Parse([]string{})
+	require.EqualError(t, err, "can't have more than one default command under  <command>")
+}