args: add new `UnknownCommandError` type

This commit makes unknown commands a proper error type so that
the caller can check for this specific error type without having
to do string comparisons.

This is useful in the context of https://github.com/spf13/cobra/issues/823

Signed-off-by: Michael Vogt <mvogt@redhat.com>
Signed-off-by: Michael Vogt <michael.vogt@gmail.com>

args.go

@@ -21,6 +21,17 @@ import (
 
 type PositionalArgs func(cmd *Command, args []string) error
 
+// UnknownCommandError is returned for unknown command
+type UnknownCommandError struct {
+	unknownCmd  string
+	cmdPath     string
+	suggestions string
+}
+
+func (e UnknownCommandError) Error() string {
+	return fmt.Sprintf("unknown command %q for %q%s", e.unknownCmd, e.cmdPath, e.suggestions)
+}
+
 // legacyArgs validation has the following behaviour:
 // - root commands with no subcommands can take arbitrary arguments
 // - root commands with subcommands will do subcommand validity checking
@@ -33,7 +44,7 @@ func legacyArgs(cmd *Command, args []string) error {
 
 	// root command with subcommands, do subcommand checking.
 	if !cmd.HasParent() && len(args) > 0 {
-		return fmt.Errorf("unknown command %q for %q%s", args[0], cmd.CommandPath(), cmd.findSuggestions(args[0]))
+		return UnknownCommandError{args[0], cmd.CommandPath(), cmd.findSuggestions(args[0])}
 	}
 	return nil
 }
@@ -41,7 +52,7 @@ func legacyArgs(cmd *Command, args []string) error {
 // NoArgs returns an error if any args are included.
 func NoArgs(cmd *Command, args []string) error {
 	if len(args) > 0 {
-		return fmt.Errorf("unknown command %q for %q", args[0], cmd.CommandPath())
+		return UnknownCommandError{args[0], cmd.CommandPath(), ""}
 	}
 	return nil
 }

command_test.go

@@ -147,7 +147,10 @@ func TestRootExecuteUnknownCommand(t *testing.T) {
 	rootCmd := &Command{Use: "root", Run: emptyRun}
 	rootCmd.AddCommand(&Command{Use: "child", Run: emptyRun})
 
-	output, _ := executeCommand(rootCmd, "unknown")
+	output, err := executeCommand(rootCmd, "unknown")
+	if _, ok := err.(UnknownCommandError); !ok {
+		t.Errorf("Expected:\n %T\nGot:\n %T\n", err, UnknownCommandError{})
+	}
 
 	expected := "Error: unknown command \"unknown\" for \"root\"\nRun 'root --help' for usage.\n"
 
@@ -156,6 +159,16 @@ func TestRootExecuteUnknownCommand(t *testing.T) {
 	}
 }
 
+func TestRootFindUnknownCommandErrorType(t *testing.T) {
+	rootCmd := &Command{Use: "root", Run: emptyRun}
+	rootCmd.AddCommand(&Command{Use: "child", Run: emptyRun})
+
+	_, _, err := rootCmd.Find([]string{"unknown"})
+	if _, ok := err.(UnknownCommandError); !ok {
+		t.Errorf("Expected:\n %T\nGot:\n %T\n", err, UnknownCommandError{})
+	}
+}
+
 func TestSubcommandExecuteC(t *testing.T) {
 	rootCmd := &Command{Use: "root", Run: emptyRun}
 	childCmd := &Command{Use: "child", Run: emptyRun}