How do I specify the block object / predicate required by NSDictionary's keysOfEntriesPassingTest ?

Question

For learning (not practical -- yet) purposes, I'd like to use the following method on an NSDictionary to give me back a set of keys that have values using a test I've defined. Unfortunately have no idea how to specify the predicate.

NSDictionary keysOfEntriesPassingTest:
- (NSSet *)keysOfEntriesPassingTest:(BOOL (^)(id key, id obj, BOOL *stop))predicate

Let's say for example all my values are NSURLs, and I'd like to get back all the URLs that are on port 8080. Here's my stab at coding that -- though it doesn't really make sense to me that it'd be correct:

NSSet * mySet = [myDict keysOfEntriesPassingTest:^(id key, id obj, BOOL *stop) {
                 if( [[obj port] isEqual: [NSNumber numberWithInt: 8080]]) {
                     return key;
                 }]

And that's because I get back the following compiler error:

incompatible block pointer types initializing 'void (^)(struct objc_object *, struct objc_object *, BOOL *)', expected 'BOOL (^)(struct objc_object *, struct objc_object *, BOOL *)'

What am I missing? I'd appreciate a pointer at some docs that go into more detail about the "Block object" that the predicate is supposed to be.
Thanks!

And this is the code that works:

NSSet * mySet = [myDict keysOfEntriesPassingTest:^(id key, id obj, BOOL *stop) {
                 if( [[obj port] isEqual: [NSNumber numberWithInt: 8080]]) {
                     return YES;
                 else
                     return NO;
                 }]

Answer 1

The error indicates that your block must return a BOOL, rather than id. Check the docs. I suspect you are expected to return YES if the key/obj pair passes the desired test.

Answer 2

A little expansion on Barry's answer....

A block is just like a function pointer with one key difference. A block will infer the return type from the return statements contained within or it will give an error message like the one you said (the LLVM compiler's error should be a bit more reasonable).

When you wrote:

NSSet * mySet = [myDict keysOfEntriesPassingTest:^(id key, id obj, BOOL *stop) {
                 if( [[obj port] isEqual: [NSNumber numberWithInt: 8080]]) {
                     return key;
                 }]

Consider that as a function:

BOOL bob(id key, id obj) {
    if( [[obj port] isEqual: [NSNumber numberWithInt: 8080]]) {
        return key;
    }
}

See the problem? There is a code path that doesn't return a value and, thus, the compiler would complain.