Add default values to usage

Check if the value isn't it's zero value and if not add a default value
to the usage text.

usage.go

@@ -76,7 +76,6 @@ func (p *Parser) WriteHelp(w io.Writer) {
 		}
 	}
 
-	// write the list of options
 	fmt.Fprint(w, "\noptions:\n")
 	for _, spec := range options {
 		printOption(w, spec)
@@ -101,6 +100,13 @@ func printOption(w io.Writer, spec *spec) {
 		}
 		fmt.Fprint(w, spec.help)
 	}
+	// Check if spec.dest is zero value or not
+	// If it isn't a default value have been added
+	v := spec.dest
+	z := reflect.Zero(v.Type())
+	if v.Interface() != z.Interface() {
+		fmt.Fprintf(w, " [default: %v]", v)
+	}
 	fmt.Fprint(w, "\n")
 }
 

usage_test.go

@@ -10,15 +10,17 @@ import (
 )
 
 func TestWriteUsage(t *testing.T) {
-	expectedUsage := "usage: example [--verbose] [--dataset DATASET] [--optimize OPTIMIZE] INPUT [OUTPUT [OUTPUT ...]]\n"
+	expectedUsage := "usage: example [--name NAME] [--value VALUE] [--verbose] [--dataset DATASET] [--optimize OPTIMIZE] INPUT [OUTPUT [OUTPUT ...]]\n"
 
-	expectedHelp := `usage: example [--verbose] [--dataset DATASET] [--optimize OPTIMIZE] INPUT [OUTPUT [OUTPUT ...]]
+	expectedHelp := `usage: example [--name NAME] [--value VALUE] [--verbose] [--dataset DATASET] [--optimize OPTIMIZE] INPUT [OUTPUT [OUTPUT ...]]
 
 positional arguments:
   input
   output
 
 options:
+  --name NAME            name to use [default: Foo Bar]
+  --value VALUE          secret value [default: 42]
   --verbose, -v          verbosity level
   --dataset DATASET      dataset to use
   --optimize OPTIMIZE, -O OPTIMIZE
@@ -28,10 +30,14 @@ options:
 	var args struct {
 		Input    string   `arg:"positional"`
 		Output   []string `arg:"positional"`
+		Name     string   `arg:"help:name to use"`
+		Value    int      `arg:"help:secret value"`
 		Verbose  bool     `arg:"-v,help:verbosity level"`
 		Dataset  string   `arg:"help:dataset to use"`
 		Optimize int      `arg:"-O,help:optimization level"`
 	}
+	args.Name = "Foo Bar"
+	args.Value = 42
 	p, err := NewParser(&args)
 	require.NoError(t, err)