Better usage display, rework flags

cmd/jargon/main.go

@@ -4,6 +4,7 @@ import (
 	"bufio"
 	"flag"
 	"fmt"
+	"io/ioutil"
 	"os"
 	"strconv"
 	"strings"
@@ -19,30 +20,6 @@ import (
 var version, commit, date string
 
 func main() {
-	//
-	// Flags. Prefer local instead of global, allowing other funcs to be stateless.
-	//
-	filein := flag.String("file", "", "input file path (if none, stdin is used as input)")
-	fileout := flag.String("out", "", "output file path (if none, stdout is used as input)")
-	html := flag.Bool("html", false, "parse input as html (keep tags whole)")
-	lang := flag.String("lang", "english", "language of input, relevant when used with -stem. options:\n"+strings.Join(langs, ", "))
-
-	// These flags aren't actually consumed, see setFilters below
-	// Need to declare them anyway, or flags package will consider them errors
-	flag.Bool("ascii", false, "a filter to replace diacritics with ascii equivalents, e.g. café → cafe")
-	flag.Bool("contractions", false, "a filter to expand contractions, e.g. Would've → Would have")
-	flag.Bool("stack", false, "a filter to recognize tech terms as Stack Overflow tags, e.g. Ruby on Rails → ruby-on-rails")
-	flag.Bool("stem", false, "a filter to stem words using snowball stemmer, e.g. management|manager → manag")
-	flag.Bool("lemmas", false, "only return tokens that have been changed by a filter (lemmatized)")
-	flag.Bool("distinct", false, "only return unique tokens")
-
-	count := flag.Bool("count", false, "count the tokens")
-	lines := flag.Bool("lines", false, "add a line break between all tokens")
-
-	v := flag.Bool("version", false, "display the version")
-
-	flag.Parse()
-
 	// Local to prevent mistaken use in other funcs
 	check := func(err error) {
 		if err != nil {
@@ -52,6 +29,48 @@ func main() {
 		}
 	}
 
+	//
+	// Flags. We're doing multiple flag sets to organize the Usage output a bit
+	//
+	args := os.Args[1:]
+
+	flags := flag.NewFlagSet("Flags", flag.ContinueOnError)
+	flags.SetOutput(ioutil.Discard) // mute errors by default
+
+	filein := flags.String("file", "", "input file path (if none, stdin is used as input)")
+	fileout := flags.String("out", "", "output file path (if none, stdout is used as input)")
+	html := flags.Bool("html", false, "parse input as html (keep tags whole)")
+	count := flags.Bool("count", false, "count the tokens")
+	lines := flags.Bool("lines", false, "add a line break between all tokens")
+	flags.Bool("lemmas", false, "only return tokens that have been changed by a filter (lemmatized)")
+	flags.Bool("distinct", false, "only return unique tokens")
+	v := flags.Bool("version", false, "display the version")
+
+	filters := flag.NewFlagSet("Filters", flag.ContinueOnError)
+	filters.SetOutput(ioutil.Discard) // mute errors by default
+
+	// We don't actually use these flags, see setFilters below; included here for Usage and errors
+	filters.Bool("ascii", false, "a filter to replace diacritics with ascii equivalents, e.g. café → cafe")
+	filters.Bool("contractions", false, "a filter to expand contractions, e.g. Would've → Would have")
+	filters.Bool("stack", false, "a filter to recognize tech terms as Stack Overflow tags, e.g. Ruby on Rails → ruby-on-rails")
+	filters.Bool("stem", false, "a filter to stem words using snowball stemmer, e.g. management|manager → manag")
+	lang := filters.String("lang", "english", "language of input, relevant when used with -stem. options:\n"+strings.Join(langs, ", "))
+
+	// Parse both flag sets
+	flagsErr := flags.Parse(args)
+	filtersErr := filters.Parse(args)
+
+	// Working around the default error behavior, a bit of acrobatics here
+	// If one flag set returns a "not defined" error, see if it's defined in the other flag set
+	err := checkDefined(flagsErr, filters)
+	check(err)
+	err = checkDefined(filtersErr, flags)
+	check(err)
+
+	// Unmute output
+	flags.SetOutput(os.Stderr)
+	filters.SetOutput(os.Stderr)
+
 	if *v {
 		fmt.Println("Version: " + version)
 		fmt.Println("Commit: " + commit)
@@ -77,8 +96,12 @@ func main() {
 	if err == errNoInput {
 		// Display usage
 		os.Stderr.WriteString(flag.CommandLine.Name() + " takes text from std input and processes it with one or more filters\n\n")
+
+		os.Stderr.WriteString("Filters:\n")
+		filters.PrintDefaults()
+
 		os.Stderr.WriteString("Flags:\n")
-		flag.PrintDefaults()
+		flags.PrintDefaults()
 		return
 	}
 	check(err)
@@ -120,6 +143,23 @@ func main() {
 	check(err)
 }
 
+func checkDefined(err error, other *flag.FlagSet) error {
+	if err != nil && strings.Contains(err.Error(), "not defined") {
+		// Get the name of the undefined arg
+		missing := strings.Split(err.Error(), ": -")
+		if len(missing) == 2 {
+			// See if it's defined elsewhere
+			arg := missing[1]
+			f := other.Lookup(arg)
+			if f == nil {
+				return fmt.Errorf("flag provided but not defined: -%s", arg)
+			}
+		}
+	}
+
+	return nil
+}
+
 type config struct {
 	Fs afero.Fs