Is it possible to compare two Objective-C blocks by content?

Question

float pi = 3.14;
float (^piSquare)(void) = ^(void){ return pi * pi; };
float (^piSquare2)(void) = ^(void){ return pi * pi; };

[piSquare isEqualTo: piSquare2]; // -> want it to behave like -isEqualToString...

Answer 1

I don't think this is possible. Blocks can be roughly seen as advanced functions (with access to global or local variables). The same way you cannot compare functions' content, you cannot compare blocks' content.

All you can do is to compare their low-level implementation, but I doubt that the compiler will guarantee that two blocks with the same content share their implementation.

Answer 2

To expand on Laurent's answer.

A Block is a combination of implementation and data. For two blocks to be equal, they would need to have both the exact same implementation and have captured the exact same data. Comparison, thus, requires comparing both the implementation and the data.

One might think comparing the implementation would be easy. It actually isn't because of the way the compiler's optimizer works.

While comparing simple data is fairly straightforward, blocks can capture objects-- including C++ objects (which might actually work someday)-- and comparison may or may not need to take that into account. A naive implementation would simply do a byte level comparison of the captured contents. However, one might also desire to test equality of objects using the object level comparators.

Then there is the issue of __block variables. A block, itself, doesn't actually have any metadata related to __block captured variables as it doesn't need it to fulfill the requirements of said variables. Thus, comparison couldn't compare __block values without significantly changing compiler codegen.

All of this is to say that, no, it isn't currently possible to compare blocks and to outline some of the reasons why. If you feel that this would be useful, file a bug via http://bugreport.apple.com/ and provide a use case.

Answer 3

Putting aside issues of compiler implementation and language design, what you're asking for is provably undecidable (unless you only care about detecting 100% identical programs). Deciding if two programs compute the same function is equivalent to solving the halting problem. This is a classic consequence of Rice's Theorem: Any "interesting" property of Turing machines is undecidable, where "interesting" just means that it's true for some machines and false for others.

Just for fun, here's the proof. Assume we can create a function to decide if two blocks are equivalent, called EQ(b1, b2). Now we'll use that function to solve the halting problem. We create a new function HALT(M, I) that tells us if Turing machine M will halt on input I like so:

BOOL HALT(M,I) {
  return EQ(
    ^(int) {return 0;},
    ^(int) {M(I); return 0;}
  );
}

If M(I) halts then the blocks are equivalent, so HALT(M,I) returns YES. If M(I) doesn't halt then the blocks are not equivalent, so HALT(M,I) returns NO. Note that we don't have to execute the blocks -- our hypothetical EQ function can compute their equivalence just by looking at them.

We have now solved the halting problem, which we know is not possible. Therefore, EQ cannot exist.