Write an Erlang function named calculator that takes no arguments. The function, when run in a process, should wait to receive a message. If the message is in the form of a tuple of four items: { Pid, Operand, X, Y } , do this:

• If Operand is add, send a message to Pid with the result of adding X and Y.
• If Operand is subtract, send a message to Pid with the result of subtracting Y from X.
• If Operand is multiply, send a message to Pid with the result of multiplying X and Y.
• If Operand is divide, send a message to Pid with the result of dividing X by Y.

Then, rerun the function.

• If the message is in the form of a tuple with two items: { Pid, terminate }, then send a message to Pid of done. Do not rerun the function.
• If the message is of any other form, ignore it and rerun the function.

My code:

calculate() ->
  receive
    {Pid, Operand, X, Y} 
      if 
         Operand == "add" -> Pid ! X+Y
         Operand == "substract" -> Pid ! Y - X
         Operand == "multiply" -> Pid ! X*Y
         Operand == "divide" -> Pid ! X/Y
    {Pid, terminate} -> Pid ! "done"
    _ -> calculate();
  end.

Can someone help me with this problem?