Allow configoptional to wrap scalar values

Author: evan-bradley

config/configoptional/optional.go

@@ -48,20 +48,6 @@ func deref(t reflect.Type) reflect.Type {
 	return t
 }
 
-// assertStructKind checks if T can be dereferenced into a type with struct kind.
-//
-// We assert this because our unmarshaling logic currently only supports structs.
-// This can be removed if we ever support scalar values.
-func assertStructKind[T any]() error {
-	var instance T
-	t := deref(reflect.TypeOf(instance))
-	if t.Kind() != reflect.Struct {
-		return fmt.Errorf("configoptional: %q does not have a struct kind", t)
-	}
-
-	return nil
-}
-
 // assertNoEnabledField checks that a struct type
 // does not have a field with a mapstructure tag "enabled".
 //
@@ -101,11 +87,9 @@ func Some[T any](value T) Optional[T] {
 
 // Default creates an Optional with a default value for unmarshaling.
 //
-// It panics if
-// - T is not a struct OR
-// - T has a field with the mapstructure tag "enabled".
+// It panics if T has a field with the mapstructure tag "enabled".
 func Default[T any](value T) Optional[T] {
-	err := errors.Join(assertStructKind[T](), assertNoEnabledField[T]())
+	err := errors.Join(assertNoEnabledField[T]())
 	if err != nil {
 		panic(err)
 	}
@@ -149,7 +133,7 @@ func (o *Optional[T]) Get() *T {
 // - T is not a struct OR
 // - T has a field with the mapstructure tag "enabled".
 func (o *Optional[T]) GetOrInsertDefault() *T {
-	err := errors.Join(assertStructKind[T](), assertNoEnabledField[T]())
+	err := errors.Join(assertNoEnabledField[T]())
 	if err != nil {
 		panic(err)
 	}
@@ -167,7 +151,10 @@ func (o *Optional[T]) GetOrInsertDefault() *T {
 	return o.Get()
 }
 
-var _ confmap.Unmarshaler = (*Optional[any])(nil)
+var (
+	_ confmap.Unmarshaler        = (*Optional[any])(nil)
+	_ xconfmap.ScalarUnmarshaler = (*Optional[any])(nil)
+)
 
 // Unmarshal the configuration into the Optional value.
 //
@@ -205,7 +192,7 @@ func (o *Optional[T]) Unmarshal(conf *confmap.Conf) error {
 		}
 	}
 
-	if err := conf.Unmarshal(&o.value, xconfmap.WithForceUnmarshaler()); err != nil {
+	if err := conf.Unmarshal(&o.value, xconfmap.WithForceUnmarshaler(), xconfmap.WithScalarMarshaler()); err != nil {
 		return err
 	}
 
@@ -221,7 +208,36 @@ func (o *Optional[T]) Unmarshal(conf *confmap.Conf) error {
 	return nil
 }
 
-var _ confmap.Marshaler = (*Optional[any])(nil)
+// UnmarshalScalar unmarshals a scalar value into the Optional.
+//
+// A `nil` value will set the Optional to None, disabling it as setting
+// `enabled: false` for a struct-type Optional or `null` for a pointer field
+// would.
+func (o *Optional[T]) UnmarshalScalar(val any) error {
+	if val == nil {
+		var zero T
+		o.value = zero
+		o.flavor = noneFlavor
+		return nil
+	}
+
+	v, ok := val.(T)
+	if !ok {
+		return fmt.Errorf("val is %T, not %T", val, v)
+	}
+	o.value = v
+	o.flavor = someFlavor
+	return nil
+}
+
+func (o *Optional[T]) ScalarType() any {
+	return o.value
+}
+
+var (
+	_ confmap.Marshaler        = (*Optional[any])(nil)
+	_ xconfmap.ScalarMarshaler = (*Optional[any])(nil)
+)
 
 // Marshal the Optional value into the configuration.
 // If the Optional is None or Default, it does not marshal anything.
@@ -230,22 +246,26 @@ var _ confmap.Marshaler = (*Optional[any])(nil)
 // T must be derefenceable to a type with struct kind.
 // Scalar values are not supported.
 func (o Optional[T]) Marshal(conf *confmap.Conf) error {
-	if err := assertStructKind[T](); err != nil {
-		return err
-	}
-
 	if o.flavor == noneFlavor || o.flavor == defaultFlavor {
 		// Optional is None or Default, do not marshal anything.
 		return conf.Marshal(map[string]any(nil))
 	}
 
-	if err := conf.Marshal(o.value); err != nil {
+	if err := conf.Marshal(o.value, xconfmap.WithScalarMarshaler()); err != nil {
 		return fmt.Errorf("configoptional: failed to marshal Optional value: %w", err)
 	}
 
 	return nil
 }
 
+func (o Optional[T]) GetScalarValue() (any, error) {
+	if o.flavor == noneFlavor || o.flavor == defaultFlavor {
+		return nil, nil
+	}
+
+	return o.value, nil
+}
+
 var _ xconfmap.Validator = (*Optional[any])(nil)
 
 // Validate implements [xconfmap.Validator]. This is required because the