Print defaults for multiples

Check if the default value supplied is a slice and not nil, if so
print the list of values supplied.
Test case for slice argument with and without default values.
Default values for slices was not printed because slice is not
comparable, but the zero value for slices is nil.

usage.go

@@ -117,7 +117,7 @@ func printOption(w io.Writer, spec *spec) {
 	v := spec.dest
 	if v.IsValid() {
 		z := reflect.Zero(v.Type())
-		if v.Type().Comparable() && z.Type().Comparable() && v.Interface() != z.Interface() {
+		if (v.Type().Comparable() && z.Type().Comparable() && v.Interface() != z.Interface()) || v.Kind() == reflect.Slice && !v.IsNil() {
 			fmt.Fprintf(w, " [default: %v]", v)
 		}
 	}

usage_test.go

@@ -10,9 +10,9 @@ import (
 )
 
 func TestWriteUsage(t *testing.T) {
-	expectedUsage := "usage: example [--name NAME] [--value VALUE] [--verbose] [--dataset DATASET] [--optimize OPTIMIZE] [--ids IDS] [--workers WORKERS] INPUT [OUTPUT [OUTPUT ...]]\n"
+	expectedUsage := "usage: example [--name NAME] [--value VALUE] [--verbose] [--dataset DATASET] [--optimize OPTIMIZE] [--ids IDS] [--values VALUES] [--workers WORKERS] INPUT [OUTPUT [OUTPUT ...]]\n"
 
-	expectedHelp := `usage: example [--name NAME] [--value VALUE] [--verbose] [--dataset DATASET] [--optimize OPTIMIZE] [--ids IDS] [--workers WORKERS] INPUT [OUTPUT [OUTPUT ...]]
+	expectedHelp := `usage: example [--name NAME] [--value VALUE] [--verbose] [--dataset DATASET] [--optimize OPTIMIZE] [--ids IDS] [--values VALUES] [--workers WORKERS] INPUT [OUTPUT [OUTPUT ...]]
 
 positional arguments:
   input
@@ -26,23 +26,26 @@ options:
   --optimize OPTIMIZE, -O OPTIMIZE
                          optimization level
   --ids IDS              Ids
+  --values VALUES        Values [default: [3.14 42 256]]
   --workers WORKERS, -w WORKERS
                          number of workers to start
   --help, -h             display this help and exit
 `
 	var args struct {
-		Input    string   `arg:"positional"`
-		Output   []string `arg:"positional,help:list of outputs"`
-		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"`
-		Ids      []int64  `arg:"help:Ids"`
-		Workers  int      `arg:"-w,env:WORKERS,help:number of workers to start"`
+		Input    string    `arg:"positional"`
+		Output   []string  `arg:"positional,help:list of outputs"`
+		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"`
+		Ids      []int64   `arg:"help:Ids"`
+		Values   []float64 `arg:"help:Values"`
+		Workers  int       `arg:"-w,env:WORKERS,help:number of workers to start"`
 	}
 	args.Name = "Foo Bar"
 	args.Value = 42
+	args.Values = []float64{3.14, 42, 256}
 	p, err := NewParser(Config{}, &args)
 	require.NoError(t, err)