Function Calls

The evaluation rule for function call forms is simple: evaluate the remaining elements of the list as Lisp forms and pass the resulting values to the named function. This rule obviously places some additional syntactic constraints on a function call form: all the elements of the list after the first must themselves be well-formed Lisp forms. In other words, the basic syntax of a function call form is as follows, where each of the arguments is itself a Lisp form:

  1. (function-name argument*)

Thus, the following expression is evaluated by first evaluating 1, then evaluating 2, and then passing the resulting values to the **+** function, which returns 3:

  1. (+ 1 2)

A more complex expression such as the following is evaluated in similar fashion except that evaluating the arguments (+ 1 2) and (- 3 4) entails first evaluating their arguments and applying the appropriate functions to them:

  1. (* (+ 1 2) (- 3 4))

Eventually, the values 3 and -1 are passed to the ***** function, which returns -3.

As these examples show, functions are used for many of the things that require special syntax in other languages. This helps keep Lisp’s syntax regular.