Merge pull request #39 from alexflint/embedded

add support for embedded structs

README.md

@@ -188,6 +188,34 @@ $ ./example --version
 someprogram 4.3.0
 ```
 
+### Embedded structs
+
+The fields of embedded structs are treated just like regular fields:
+
+```go
+
+type DatabaseOptions struct {
+	Host     string
+	Username string
+	Password string
+}
+
+type LogOptions struct {
+	LogFile string
+	Verbose bool
+}
+
+func main() {
+	var args struct {
+		DatabaseOptions
+		LogOptions
+	}
+	arg.MustParse(&args)
+}
+```
+
+As usual, any field tagged with `arg:"-"` is ignored.
+
 ### Custom parsing
 
 You can implement your own argument parser by implementing `encoding.TextUnmarshaler`:

parse.go

@@ -21,7 +21,6 @@ type spec struct {
 	env        string
 	wasPresent bool
 	boolean    bool
-	fieldName  string // for generating helpful errors
 }
 
 // ErrHelp indicates that -h or --help were provided
@@ -81,6 +80,19 @@ type Versioned interface {
 	Version() 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()
+	for i := 0; i < t.NumField(); i++ {
+		field := t.Field(i)
+		val := v.Field(i)
+		expand := visit(field, val, t)
+		if expand && field.Type.Kind() == reflect.Struct {
+			walkFields(val, visit)
+		}
+	}
+}
+
 // NewParser constructs a parser from a list of destination structs
 func NewParser(config Config, dests ...interface{}) (*Parser, error) {
 	p := Parser{
@@ -99,19 +111,22 @@ func NewParser(config Config, dests ...interface{}) (*Parser, error) {
 			panic(fmt.Sprintf("%T is not a struct pointer", dest))
 		}
 
-		t := v.Type()
-		for i := 0; i < t.NumField(); i++ {
+		var errs []string
+		walkFields(v, func(field reflect.StructField, val reflect.Value, t reflect.Type) bool {
 			// Check for the ignore switch in the tag
-			field := t.Field(i)
 			tag := field.Tag.Get("arg")
 			if tag == "-" {
-				continue
+				return false
+			}
+
+			// If this is an embedded struct then recurse into its fields
+			if field.Anonymous && field.Type.Kind() == reflect.Struct {
+				return true
 			}
 
 			spec := spec{
-				long:      strings.ToLower(field.Name),
-				dest:      v.Field(i),
-				fieldName: t.Name() + "." + field.Name,
+				long: strings.ToLower(field.Name),
+				dest: val,
 			}
 
 			// Check whether this field is supported. It's good to do this here rather than
@@ -121,7 +136,9 @@ func NewParser(config Config, dests ...interface{}) (*Parser, error) {
 			var parseable bool
 			parseable, spec.boolean, spec.multiple = canParse(field.Type)
 			if !parseable {
-				return nil, fmt.Errorf("%s.%s: %s fields are not supported", t.Name(), field.Name, field.Type.String())
+				errs = append(errs, fmt.Sprintf("%s.%s: %s fields are not supported",
+					t.Name(), field.Name, field.Type.String()))
+				return false
 			}
 
 			// Look at the tag
@@ -138,7 +155,9 @@ func NewParser(config Config, dests ...interface{}) (*Parser, error) {
 						spec.long = key[2:]
 					case strings.HasPrefix(key, "-"):
 						if len(key) != 2 {
-							return nil, fmt.Errorf("%s.%s: short arguments must be one character only", t.Name(), field.Name)
+							errs = append(errs, fmt.Sprintf("%s.%s: short arguments must be one character only",
+								t.Name(), field.Name))
+							return false
 						}
 						spec.short = key[1:]
 					case key == "required":
@@ -155,11 +174,19 @@ func NewParser(config Config, dests ...interface{}) (*Parser, error) {
 							spec.env = strings.ToUpper(field.Name)
 						}
 					default:
-						return nil, fmt.Errorf("unrecognized tag '%s' on field %s", key, tag)
+						errs = append(errs, fmt.Sprintf("unrecognized tag '%s' on field %s", key, tag))
+						return false
 					}
 				}
 			}
 			p.spec = append(p.spec, &spec)
+
+			// if this was an embedded field then we already returned true up above
+			return false
+		})
+
+		if len(errs) > 0 {
+			return nil, errors.New(strings.Join(errs, "\n"))
 		}
 	}
 	if p.config.Program == "" {

parse_test.go

@@ -633,3 +633,24 @@ func TestInvalidMailAddr(t *testing.T) {
 	err := parse("--recipient xxx", &args)
 	assert.Error(t, err)
 }
+
+type A struct {
+	X string
+}
+
+type B struct {
+	Y int
+}
+
+func TestEmbedded(t *testing.T) {
+	var args struct {
+		A
+		B
+		Z bool
+	}
+	err := parse("--x=hello --y=321 --z", &args)
+	require.NoError(t, err)
+	assert.Equal(t, "hello", args.X)
+	assert.Equal(t, 321, args.Y)
+	assert.Equal(t, true, args.Z)
+}