feat!: narrow unions with UnionAs and name member lists Types

README.md

@@ -51,7 +51,7 @@ A `Union` holds one ID of any of several types and picks the type by prefix:
 ```go
 type principalUnion struct{}
 
-func (principalUnion) Members() []trogonobjectid.MemberType {
+func (principalUnion) Types() []trogonobjectid.MemberType {
 	return []trogonobjectid.MemberType{
 		trogonobjectid.MemberOf[userType](),
 		trogonobjectid.MemberOf[systemType](),
@@ -61,9 +61,8 @@ func (principalUnion) Members() []trogonobjectid.MemberType {
 type PrincipalID = trogonobjectid.Union[principalUnion]
 
 p, err := trogonobjectid.ParseUnion[principalUnion]("system_cron")
-switch m := p.Member().(type) {
-case SystemID:
-	// m.Bare() == "cron"
+if sys, ok := trogonobjectid.UnionAs[systemType](p); ok {
+	// sys.Bare() == "cron"
 }
 ```
 

union.go

@@ -43,7 +43,7 @@ func MemberOf[T Definer]() MemberType {
 //
 //	type principalUnion struct{}
 //
-//	func (principalUnion) Members() []trogonobjectid.MemberType {
+//	func (principalUnion) Types() []trogonobjectid.MemberType {
 //		return []trogonobjectid.MemberType{
 //			trogonobjectid.MemberOf[userType](),
 //			trogonobjectid.MemberOf[systemType](),
@@ -52,11 +52,11 @@ func MemberOf[T Definer]() MemberType {
 //
 //	type PrincipalID = trogonobjectid.Union[principalUnion]
 type UnionDefiner interface {
-	Members() []MemberType
+	Types() []MemberType
 }
 
 // Union holds one ID of any of its member types, chosen by prefix when parsing.
-// Use a type switch on Member to get the typed ID back.
+// Use UnionAs, or a type switch on Member, to get the typed ID back.
 //
 // Every encoding uses the member's full form, because the prefix is the only thing
 // that tells the member types apart. The zero value is an absent ID.
@@ -77,7 +77,7 @@ func unionSpecFor[U UnionDefiner]() (*unionSpec, error) {
 
 func newUnionSpec[U UnionDefiner]() (unionSpec, error) {
 	var marker U
-	members := marker.Members()
+	members := marker.Types()
 	if len(members) == 0 {
 		return unionSpec{}, fmt.Errorf("%w: %T has no members", ErrInvalidDefinition, marker)
 	}
@@ -169,6 +169,13 @@ func MustParseUnion[U UnionDefiner](str string) Union[U] {
 // Member returns the wrapped ID, or nil for the zero union.
 func (u Union[U]) Member() Member { return u.member }
 
+// UnionAs returns u's member as an ID[T], and false when u holds another type or
+// is zero. It narrows a union without a type switch.
+func UnionAs[T Definer, U UnionDefiner](u Union[U]) (ID[T], bool) {
+	id, ok := u.member.(ID[T])
+	return id, ok
+}
+
 // IsZero reports whether u is the zero (absent) union.
 func (u Union[U]) IsZero() bool { return u.member == nil }
 

union_test.go

@@ -24,7 +24,7 @@ type SystemID = trogonobjectid.ID[systemType]
 
 type principalUnion struct{}
 
-func (principalUnion) Members() []trogonobjectid.MemberType {
+func (principalUnion) Types() []trogonobjectid.MemberType {
 	return []trogonobjectid.MemberType{
 		trogonobjectid.MemberOf[userType](),
 		trogonobjectid.MemberOf[systemType](),
@@ -48,7 +48,7 @@ func (acmeAdminType) Definition() trogonobjectid.Definition {
 
 type overlapUnion struct{}
 
-func (overlapUnion) Members() []trogonobjectid.MemberType {
+func (overlapUnion) Types() []trogonobjectid.MemberType {
 	return []trogonobjectid.MemberType{
 		trogonobjectid.MemberOf[acmeType](),
 		trogonobjectid.MemberOf[acmeAdminType](),
@@ -57,11 +57,11 @@ func (overlapUnion) Members() []trogonobjectid.MemberType {
 
 type emptyUnion struct{}
 
-func (emptyUnion) Members() []trogonobjectid.MemberType { return nil }
+func (emptyUnion) Types() []trogonobjectid.MemberType { return nil }
 
 type duplicateUnion struct{}
 
-func (duplicateUnion) Members() []trogonobjectid.MemberType {
+func (duplicateUnion) Types() []trogonobjectid.MemberType {
 	return []trogonobjectid.MemberType{
 		trogonobjectid.MemberOf[userType](),
 		trogonobjectid.MemberOf[userType](),
@@ -70,7 +70,7 @@ func (duplicateUnion) Members() []trogonobjectid.MemberType {
 
 type badMemberUnion struct{}
 
-func (badMemberUnion) Members() []trogonobjectid.MemberType {
+func (badMemberUnion) Types() []trogonobjectid.MemberType {
 	return []trogonobjectid.MemberType{trogonobjectid.MemberOf[badType]()}
 }
 
@@ -249,6 +249,21 @@ func TestUnionZero(t *testing.T) {
 	assert.Empty(t, p.String())
 }
 
+func TestUnionAs(t *testing.T) {
+	user := trogonobjectid.MustNew[userType]("abc-123")
+	u := trogonobjectid.MustNewUnion[principalUnion](user)
+
+	got, ok := trogonobjectid.UnionAs[userType](u)
+	require.True(t, ok)
+	assert.Equal(t, user, got)
+
+	_, ok = trogonobjectid.UnionAs[systemType](u)
+	assert.False(t, ok)
+
+	_, ok = trogonobjectid.UnionAs[userType](trogonobjectid.Union[principalUnion]{})
+	assert.False(t, ok)
+}
+
 func TestUnionSpanner(t *testing.T) {
 	want := trogonobjectid.MustNewUnion[principalUnion](trogonobjectid.MustNew[userType]("abc-123"))