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\section{Functions}  When you find yourself doing the same task several times, it may be a good time to create a reusable function. A function, as you learned in the Enclosures section, is something enclosed within curly braces. David Touretzky introduces the idea of function in this way: ``think of a function as a box through which data flows. The function operates on the data in some way, and the result is what flows out.''\footnote{Touretzky, David. COMMON LISP: A Gentle Introduction to Symbolic Computation. The Benjamin/Cummings Publishing Company, Inc, 1990, p. 1. That's the book that inspired the title of this tutorial.}    \begin{lstlisting}[style=SuperCollider-IDE, basicstyle=\scttfamily\footnotesize]  f = { 2 + 2 }; // define the function  f.value; // put the function to work  \end{lstlisting}    The function above is not terribly useful, as it only knows how to do one thing (add 2 and 2). Normally you will want to define functions that can give you different results depending depending on varying input arguments.    \begin{lstlisting}[style=SuperCollider-IDE, basicstyle=\scttfamily\footnotesize]  f = {arg a, b; ["a plus b", a+b, "a times b", a*b].postln}; // define function  f.value(3, 7); // now you can give any two numbers as arguments to the function  f.value(10, 14);  // Compare:  ~sillyRand = rrand(0, 10); // not a function  ~sillyRand.value; // evaluate several times  ~sillyRand2 = {rrand(0, 10)}; // a function  ~sillyRand2.value; // evaluate several times  \end{lstlisting}    As a last example, here's one very useful function.    \begin{lstlisting}[style=SuperCollider-IDE, basicstyle=\scttfamily\footnotesize]  // Use this function to decide how to spend your Summer days  (  ~whatToDo = {   var today, dayName, actions;  today = Date.getDate.dayOfWeek;  dayName =   case  {today==0} {"Sunday"}  {today==1} {"Monday"}  {today==2} {"Tuesday"}  {today==3} {"Wednesday"}  {today==4} {"Thursday"}  {today==5} {"Friday"}  {today==6} {"Saturday"};  actions = ["boomerang throwing", "arm wrestling", "stair climbing", "rope climbing", "underwater hockey", "pea shooting", "a nap marathon"];  "Ah, " ++ dayName ++ "...! " ++ "What a good day for " ++ actions.choose;  };  )  // Run it in the morning  ~whatToDo.value;  \end{lstlisting}  \bigskip  \todo[inline, color=green!40]{ TIP: Another common notation to declare arguments at the beginning of a Function is: \texttt{f = \{|a, b| a + b\}}. This is equivalent to \texttt{f = \{arg a, b; a + b\}}}