Merge pull request #62 from mwlazlo-tls/master

Custom parsers implementing encoding.TextMarshaler() can have default…

README.md

@@ -265,15 +265,33 @@ func (n *NameDotName) UnmarshalText(b []byte) error {
 	return nil
 }
 
+// optional: implement in case you want to display a default value in the usage string
+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
@@ -134,7 +135,15 @@ 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() {
-			fmt.Fprintf(w, " [default: %v]", v)
+			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 +7,35 @@ import (
 
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
+	"strings"
+	"fmt"
+	"errors"
 )
 
+type NameDotName struct {
+	Head, Tail string
+}
+
+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] INPUT [OUTPUT [OUTPUT ...]]\n"
+	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] INPUT [OUTPUT [OUTPUT ...]]
+	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 +52,7 @@ 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 +66,13 @@ 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"
@@ -60,6 +86,42 @@ Options:
 	assert.Equal(t, expectedHelp, help.String())
 }
 
+type MyEnum int
+
+func (n *MyEnum) UnmarshalText(b []byte) error {
+	b = []byte("Hello")
+	return nil
+}
+
+func (n *MyEnum) MarshalText() (text []byte, err error) {
+	s := "There was a problem"
+	text = []byte(s)
+	err = errors.New(s)
+	return
+}
+
+func TestUsageError(t *testing.T) {
+	expectedHelp := `Usage: example [--name NAME]
+
+Options:
+  --name NAME [default: error: There was a problem]
+  --help, -h             display this help and exit
+`
+	var args struct {
+		Name *MyEnum
+	}
+	v := MyEnum(42)
+	args.Name = &v
+	p, err := NewParser(Config{"example"}, &args)
+
+	// NB: some might might expect there to be an error here
+	require.NoError(t, err)
+
+	var help bytes.Buffer
+	p.WriteHelp(&help)
+	assert.Equal(t, expectedHelp, help.String())
+}
+
 func TestUsageLongPositionalWithHelp_legacyForm(t *testing.T) {
 	expectedHelp := `Usage: example VERYLONGPOSITIONALWITHHELP