(list) vs list

One problem that I’ve seen a lot is confusion between using list and (list). The two are different and knowing the difference between them is important.

Consider the following Java code:

class Example {
  Example Example;

  Example Example() { return Example; }
}

This is perfectly valid, albeit confusing Java code. We have a class named Example and it contains an attribute and a method also named Example. Now, in this code Java always knows how to handle each different version of Example, and it so it allows us to assign this name multiple times because each Example exists in a different namespace. Our class Example lives in the class’ namespace, the attribute Example lives in the class’s attribute namespace and finally the Example method exists in the class’s method namespace. If we have the following code, Java knows exactly how to handle it:

Example Example = new Example();
System.out.println(Example.Example);
System.out.println(Example.Example());

When we say Example.Example, the lack of parentheses indicate to Java to get this value out of the attribute namespace. When we say Example.Example(), it indicated to Java to call this method from the method namespace.

This is the same thing as we have going on Lisp. In Lisp we have 2 namespace: a function and a variable namespace. When we say:

(let ((list '(1 2 3 4 5)))
  (print list)
  (print (list)))

it knows exactly how to handle both uses of list. In the first print, the output will be the list that we defined in the let block and in the second print the output will be a call to the list function with no parameters, which is an empty list.

Be mindful when using list as a variable whether you’ve surrounded it with parentheses or not, because the two mean different things and will give unexpected results if you miss your problem.