Leak-Free Recursive Blocks. See http://jeremywsherman.com/blog/2013/02/27/leak-free-recursive-blocks/

gistfile1.m

#import <Foundation/Foundation.h>

// clang -framework Foundation main.m -o main && ./main
int main(int argc, char *argv[]) {
    @autoreleasepool {
        NSUInteger (^__block factorial)(NSUInteger n);
        NSUInteger (^__block __weak weakFactorial)(NSUInteger n);
        weakFactorial = factorial = ^ NSUInteger (NSUInteger n) {
            return n==0 ? 1 : n * weakFactorial(n-1);
        };
        NSLog(@"%ld",factorial(3));
    }
}