What is the purpose of tokenRecipient contract in token example?

Question

The example of how to create a token (https://www.ethereum.org/token) includes contract tokenRecipient. Why is it there?

Answer 1

I'ts just an example contract where you send tokens to. It represents a contract which you could sent tokens to.

You can see it where it says:

/* Allow another contract to spend some tokens in your behalf */
function approveAndCall(address _spender, uint256 _value, bytes _extraData) 
    returns (bool success) {
    allowance[msg.sender][_spender] = _value;     
    tokenRecipient spender = tokenRecipient(_spender);
    spender.receiveApproval(msg.sender, _value, this, _extraData); 
    return true; 
}

Answer 2

Good question. I know I'm late, but still posting the answer as I was also wondering about it. I agree with @arodriguezdonaire. But ethereum explains it clearly in the documentation.

You'll notice that there some more functions in your basic token contract, like approve, sendFrom and others. These functions are there for your token to interact with other contracts: if you want, say, sell tokens to a decentralized exchange, just sending them to an address will not be enough as the exchange will not be aware of the new tokens or who sent them, because contracts aren't able to subscribe to Events only to function calls. So for contracts, you should first approve an amount of tokens they can move from your account and then ping them to let them know they should do their thing - or do the two actions in one, with approveAndCall.

If you want to see the sample implementation of toeknRecipient, have a look at "shareholdersassociation".