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Programming in Emacs Lisp

1.8.2 An Argument as the Value of a Variable or List

An argument can be a symbol that returns a value when it is evaluated. For example, when the symbol fill-column by itself is evaluated, it returns a number. This number can be used in an addition.

Position the cursor after the following expression and type C-x C-e:

(+ 2 fill-column)

The value will be a number two more than what you get by evaluating fill-column alone. For me, this is 74, because the value of fill-column is 72.

As we have just seen, an argument can be a symbol that returns a value when evaluated. In addition, an argument can be a list that returns a value when it is evaluated. For example, in the following expression, the arguments to the function concat are the strings "The " and " red foxes." and the list (number-to-string (+ 2 fill-column)).

(concat "The " (number-to-string (+ 2 fill-column)) " red foxes.")

If you evaluate this expression--and if, as with my Emacs, fill-column evaluates to 72---"The 74 red foxes." will appear in the echo area. (Note that you must put spaces after the word `The' and before the word `red' so they will appear in the final string. The function number-to-string converts the integer that the addition function returns to a string. number-to-string is also known as int-to-string.)