An Objective-C pattern for making sum types - Example.h

Example.h

#import <Foundation/Foundation.h>

@class ExampleFoo;
@class ExampleBar;
  
@interface Example : NSObject
// notice there is no public designated initializer, so consumers can't instantiate Example directly.
// They must instantiate a subclass.
+ (instancetype _Nonnull)new NS_UNAVAILABLE;
- (instancetype _Nonnull)init NS_UNAVAILABLE;

// Consumers will use this method to unpack the sum type
- (void)switchFoo:(void (^ _Nonnull)(ExampleFoo * _Nonnull foo_))fooBlock
              bar:(void (^ _Nonnull)(ExampleBar * _Nonnull bar_))barBlock NS_REQUIRES_SUPER;
@end