draft of a dictionary comprehension idea for obj-c: generating dictionaries over collections using simple-ish block syntax.

comprehension.m

- (NSDictionary *)dictionaryOfPeople:(NSArray *)people {
    return [people dictionaryViaEnumeration:^(Person *person, id *key, id *value) {
        *key = person.identifier;
        *value = person.name;
    }];
}
////////////////////

typedef void (^TransformBlock)(id obj, id *key, id *value);

@implementation NSObject (DictionaryGenerator)

- (NSDictionary *)dictionaryViaEnumeration:(TransformBlock)block {
    NSAssert([self conformsToProtocol:@protocol(NSFastEnumeration)], @"dictionaryViaEnumeration only works on objects conforming to NSFastEnumeration");

    NSInteger count = 0;
    if ([self respondsToSelector:@selector(count)]) {
        count = [(id)self count];
    } else if ([self respondsToSelector:@selector(length)]) {
        count = [(id)self length];
    }
    NSMutableDictionary *result = [NSMutableDictionary dictionaryWithCapacity:count];
    for (id object in (id<NSFastEnumeration>)self) {
        id key = nil;
        id value = nil;
        block(object, &key, &value);
        if (!value) {
            value = [NSNull null];
        }
        if (key) {
            result[key] = value;
        }
    }
    return result;
}

@end

How about modifying this to use dictionary syntax in the block:

- (NSDictionary *)dictionaryOfPeople:(NSArray *)people {
    return [people dictionaryViaEnumeration:^NSDictionary *(Person *person) {
        return @{ person.identifier : person.name };
    }];
}