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Complications

Now, for the first complication. In addition to lists, the Lisp interpreter can evaluate a symbol that is not quoted and does not have parentheses around it. The Lisp interpreter will attempt to determine the symbol's value as a variable. This situation is described in the section on variables. (See Variables.)

The second complication occurs because some functions are unusual and do not work in the usual manner. Those that don't are called special forms. They are used for special jobs, like defining a function, and there are not many of them. In the next few chapters, you will be introduced to several of the more important special forms.

As well as special forms, there are also macros. A macro is a construct defined in Lisp, which differs from a function in that it translates a Lisp expression into another expression that is to be evaluated in place of the original expression. (See Lisp macro.)

For the purposes of this introduction, you do not need to worry too much about whether something is a special form, macro, or ordinary function. For example, if is a special form (see if), but when is a macro (see Lisp macro). In earlier versions of Emacs, defun was a special form, but now it is a macro (see defun). It still behaves in the same way.

The final complication is this: if the function that the Lisp interpreter is looking at is not a special form, and if it is part of a list, the Lisp interpreter looks to see whether the list has a list inside of it. If there is an inner list, the Lisp interpreter first figures out what it should do with the inside list, and then it works on the outside list. If there is yet another list embedded inside the inner list, it works on that one first, and so on. It always works on the innermost list first. The interpreter works on the innermost list first, to evaluate the result of that list. The result may be used by the enclosing expression.

Otherwise, the interpreter works left to right, from one expression to the next.