An Objective-C pattern for making sum types - ExampleSwitcher.m

ExampleSwitcher.m

#import "ExampleSwitcher.h" // https://gist.github.com/hborders/f378b4e0d0f1d74ff7c8149cb04c6268

@implementation ExampleSwitcher
+ (NSObject * _Nonnull)nonnullValueFrom:(Example * _Nonnull)example
                              switchFoo:(NSObject * _Nonnull (^ _Nonnull)(ExampleFoo * _Nonnull foo_))fooBlock
                                    bar:(NSObject * _Nonnull (^ _Nonnull)(ExampleBar * _Nonnull bar_))barBlock {
    NSParameterAssert(example);
    NSParameterAssert(fooBlock);
    NSParameterAssert(barBlock);
    
    // Unfortunately, we still have this local block assignment,
    // but at least it's only in one spot. All other code will
    // be verified correct by the compiler.
    NSObject * _Nullable __block maybeValue;
    [example switchFoo:^(ExampleFoo * _Nonnull foo_) {
        maybeValue = fooBlock(foo_);
    }
                   bar:^(ExampleBar * _Nonnull bar_) {
                       maybeValue = barBlock(bar_);
                   }];
    assert(maybeValue);
    return (NSObject * _Nonnull) maybeValue;
}

+ (NSObject * _Nullable)nullableValueFrom:(Example * _Nonnull)example
                                switchFoo:(NSObject * _Nullable (^ _Nonnull)(ExampleFoo * _Nonnull foo_))fooBlock
                                      bar:(NSObject * _Nullable (^ _Nonnull)(ExampleBar * _Nonnull bar_))barBlock {
    NSParameterAssert(example);
    NSParameterAssert(fooBlock);
    NSParameterAssert(barBlock);
    
    // Unfortunately, we still have this local block assignment,
    // but at least it's only in one spot. All other code will
    // be verified correct by the compiler.
    NSObject * _Nullable __block maybeValue;
    [example switchFoo:^(ExampleFoo * _Nonnull foo_) {
        maybeValue = fooBlock(foo_);
    }
                   bar:^(ExampleBar * _Nonnull bar_) {
                       maybeValue = barBlock(bar_);
                   }];
    return maybeValue;
}
@end