Methods vs Messages

JavaScript classes allow calling methods

class Counter {
  add(amount) {
    // Code I write
  }
}

const counter = new Counter();
// Call add method.
counter.add(1);

Objective-C classes allow sending messages

// Send add message.
[counter add:@1];

// Here is the identifier for this message.
SEL messageIdentifier = @selector(add:);

// Send add message.
[counter performSelector:@selector(add:) withObject:@1];

// Create message ready to send.
// Yes, it is a fair amount of code…
NSMethodSignature *signature = [Counter
  instanceMethodSignatureForSelector:@selector(add:)];
NSInvocation *sender = [NSInvocation
  invocationWithMethodSignature:signature];
NSNumber *amount = @1;
[sender setArgument:&amount atIndex:2];
[sender setTarget:counter];

// Now, let’s send the add message twice.
[sender invoke];
[sender invoke];

Adding sent numbers

// This is our message generator:
function* GenerateNumbers() {
  yield 1; // These
  yield 2; // are
  yield 3; // our messages
}

// This is our message processor:
function adder(messageGenerator) {
  let total = 0;
  for (const message of messageGenerator()) {
    total += message;
  }
  return total;
}

adder(GenerateNumbers);
// 6