add support for description string

README.md

@@ -263,6 +263,33 @@ usage: example [--name NAME]
 error: error processing --name: missing period in "oops"
 ```
 
+### Description strings
+
+```go
+type args struct {
+	Foo string
+}
+
+func (args) Description() string {
+	return "this program does this and that"
+}
+
+func main() {
+	var args args
+	arg.MustParse(&args)
+}
+```
+
+```shell
+$ ./example -h
+this program does this and that
+usage: example [--foo FOO]
+
+options:
+  --foo FOO
+  --help, -h             display this help and exit
+```
+
 ### Documentation
 
 https://godoc.org/github.com/alexflint/go-arg

parse.go

@@ -70,6 +70,7 @@ type Parser struct {
 	spec        []*spec
 	config      Config
 	version     string
+	description string
 }
 
 // Versioned is the interface that the destination struct should implement to
@@ -80,6 +81,14 @@ type Versioned interface {
 	Version() string
 }
 
+// Described is the interface that the destination struct should implement to
+// make a description string appear at the top of the help message.
+type Described interface {
+	// Description returns the string that will be printed on a line by itself
+	// at the top of the help message.
+	Description() string
+}
+
 // walkFields calls a function for each field of a struct, recursively expanding struct fields.
 func walkFields(v reflect.Value, visit func(field reflect.StructField, val reflect.Value, owner reflect.Type) bool) {
 	t := v.Type()
@@ -102,6 +111,9 @@ func NewParser(config Config, dests ...interface{}) (*Parser, error) {
 		if dest, ok := dest.(Versioned); ok {
 			p.version = dest.Version()
 		}
+		if dest, ok := dest.(Described); ok {
+			p.description = dest.Description()
+		}
 		v := reflect.ValueOf(dest)
 		if v.Kind() != reflect.Ptr {
 			panic(fmt.Sprintf("%s is not a pointer (did you forget an ampersand?)", v.Type()))

usage.go

@@ -29,6 +29,9 @@ func (p *Parser) WriteUsage(w io.Writer) {
 		}
 	}
 
+	if p.description != "" {
+		fmt.Fprintln(w, p.description)
+	}
 	if p.version != "" {
 		fmt.Fprintln(w, p.version)
 	}

usage_test.go

@@ -129,3 +129,31 @@ options:
 		t.Fail()
 	}
 }
+
+type described struct{}
+
+// Described returns the description for this program
+func (described) Description() string {
+	return "this program does this and that"
+}
+
+func TestUsageWithDescription(t *testing.T) {
+	expectedHelp := `this program does this and that
+usage: example
+
+options:
+  --help, -h             display this help and exit
+`
+	os.Args[0] = "example"
+	p, err := NewParser(Config{}, &described{})
+	require.NoError(t, err)
+
+	var help bytes.Buffer
+	p.WriteHelp(&help)
+	actual := help.String()
+	t.Logf("Expected:\n%s", expectedHelp)
+	t.Logf("Actual:\n%s", actual)
+	if expectedHelp != actual {
+		t.Fail()
+	}
+}