Custom parsers implementing encoding.TextMarshaler() can have default values printed via --help

README.md

@@ -265,15 +265,33 @@ func (n *NameDotName) UnmarshalText(b []byte) error {
 	return nil
 }
 
+// optional, implement in case you want to use defaults
+func (n *NameDotName) MarshalText() (text []byte, err error) {
+	text = []byte(fmt.Sprintf("%s.%s", n.Head, n.Tail))
+	return
+}
+
 func main() {
 	var args struct {
 		Name *NameDotName
 	}
+	// set default
+	args.Name = &NameDotName{"file", "txt"}
 	arg.MustParse(&args)
 	fmt.Printf("%#v\n", args.Name)
 }
 ```
 ```shell
+$ ./example --help
+Usage: test [--name NAME]
+
+Options:
+  --name NAME [default: file.txt]
+  --help, -h             display this help and exit
+
+$ ./example
+&main.NameDotName{Head:"file", Tail:"txt"}
+
 $ ./example --name=foo.bar
 &main.NameDotName{Head:"foo", Tail:"bar"}
 

usage.go

@@ -6,6 +6,7 @@ import (
 	"os"
 	"reflect"
 	"strings"
+	"encoding"
 )
 
 // the width of the left column
@@ -134,9 +135,17 @@ func printOption(w io.Writer, spec *spec) {
 	if v.IsValid() {
 		z := reflect.Zero(v.Type())
 		if (v.Type().Comparable() && z.Type().Comparable() && v.Interface() != z.Interface()) || v.Kind() == reflect.Slice && !v.IsNil() {
+			if scalar, ok := v.Interface().(encoding.TextMarshaler); ok {
+				if value, err := scalar.MarshalText(); err != nil {
+					fmt.Fprintf(w, " [default: error: %v]", err)
+				} else {
+					fmt.Fprintf(w, " [default: %v]", string(value))
+				}
+			} else {
 				fmt.Fprintf(w, " [default: %v]", v)
 			}
 		}
+	}
 	fmt.Fprint(w, "\n")
 }
 

usage_test.go

@@ -7,12 +7,34 @@ import (
 
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
+	"strings"
+	"fmt"
 )
 
-func TestWriteUsage(t *testing.T) {
-	expectedUsage := "Usage: example [--name NAME] [--value VALUE] [--verbose] [--dataset DATASET] [--optimize OPTIMIZE] [--ids IDS] [--values VALUES] [--workers WORKERS] INPUT [OUTPUT [OUTPUT ...]]\n"
+type NameDotName struct {
+	Head, Tail string
+}
 
-	expectedHelp := `Usage: example [--name NAME] [--value VALUE] [--verbose] [--dataset DATASET] [--optimize OPTIMIZE] [--ids IDS] [--values VALUES] [--workers WORKERS] INPUT [OUTPUT [OUTPUT ...]]
+func (n *NameDotName) UnmarshalText(b []byte) error {
+	s := string(b)
+	pos := strings.Index(s, ".")
+	if pos == -1 {
+		return fmt.Errorf("missing period in %s", s)
+	}
+	n.Head = s[:pos]
+	n.Tail = s[pos+1:]
+	return nil
+}
+
+func (n *NameDotName) MarshalText() (text []byte, err error) {
+	text = []byte(fmt.Sprintf("%s.%s", n.Head, n.Tail))
+	return
+}
+
+func TestWriteUsage(t *testing.T) {
+	expectedUsage := "Usage: example [--name NAME] [--value VALUE] [--verbose] [--dataset DATASET] [--optimize OPTIMIZE] [--ids IDS] [--values VALUES] [--workers WORKERS] [--file FILE] INPUT [OUTPUT [OUTPUT ...]]\n"
+
+	expectedHelp := `Usage: example [--name NAME] [--value VALUE] [--verbose] [--dataset DATASET] [--optimize OPTIMIZE] [--ids IDS] [--values VALUES] [--workers WORKERS] [--file FILE] INPUT [OUTPUT [OUTPUT ...]]
 
 Positional arguments:
   INPUT
@@ -29,6 +51,7 @@ Options:
   --values VALUES        Values [default: [3.14 42 256]]
   --workers WORKERS, -w WORKERS
                          number of workers to start
+  --file FILE, -f FILE   File with mandatory extension [default: scratch.txt]
   --help, -h             display this help and exit
 `
 	var args struct {
@@ -42,11 +65,13 @@ Options:
 		Ids      []int64   `help:"Ids"`
 		Values   []float64 `help:"Values"`
 		Workers  int       `arg:"-w,env:WORKERS" help:"number of workers to start"`
+		File     *NameDotName `arg:"-f" help:"File with mandatory extension"`
 	}
 	args.Name = "Foo Bar"
 	args.Value = 42
 	args.Values = []float64{3.14, 42, 256}
-	p, err := NewParser(Config{}, &args)
+	args.File = &NameDotName{"scratch", "txt"}
+	p, err := NewParser(Config{"example"}, &args)
 	require.NoError(t, err)
 
 	os.Args[0] = "example"