Y combinator in ObjC

gistfile1.c

/* This is an exercise, basically. See http://www.ece.uc.edu/~franco/C511/html/Scheme/ycomb.html 
   for why you might care, or https://gist.github.com/kongtomorrow/e95bea13162ca0e29d4b for the 
   same in Swift.
 */


#define Y(Domain,Range) ({ \
        typedef Range(^DtoR)(Domain); \
\
        ^(DtoR (^almost)(DtoR)){ \
            typedef id (^IdToId)(id); \
            return (DtoR) (^(IdToId next){ /* the real type of next is DtoR(^)(typeof(next)). It's a recursive definition, so we just make it IdToId to keep the compiler happy.*/ \
                return (id)^(Domain args){ \
                    return (Range) almost(next(next))(args); \
                }; \
            }(^(IdToId next){ \
                return (id)^(Domain args){ \
                    return (Range) almost(next(next))(args); \
                }; \
            })); \
        }; \
    })

    typedef int (^IntToInt)(int);
    IntToInt factorial = Y(int,int)(^(IntToInt recur){
        return ^(int n){
            switch (n) {
                case  0: return 1;
                default: return n*recur(n-1);
            }
        };
    });
    printf("%d\n", factorial(5));

    typedef int (^CharsToInt)(char *str);
    CharsToInt maxIntInString = Y(char*,int)(^(CharsToInt recur){
        return ^(char *str) {
            if (*str == '\0') {
                return 0;
            } else {
                return MAX(*str - '0', recur(str+1));
            }
        };
    });
    printf("%d\n", maxIntInString("0523562"));