15.1 Sequencing

Evaluating forms in the order they appear is the most common way control passes from one form to another. In some contexts, such as in a function body, this happens automatically. Elsewhere you must use a control structure construct to do this: progn, the simplest control construct of Lisp.

A progn special form looks like this:

(progn a b c …)

and it says to execute the forms a, b, c and so on, in that order. These forms are called the body of the progn form. The value of the last form in the body becomes the value of the entire progn.

In the early days of Lisp, progn was the only way to execute two or more forms in succession and use the value of the last of them. But programmers found they often needed to use a progn in the body of a function, where (at that time) only one form was allowed. So the body of a function was made into an “implicit progn”: several forms are allowed just as in the body of an actual progn. Many other control structures likewise contain an implicit progn. As a result, progn is not used as often as it used to be. It is needed now most often inside an unwind-protect, and, or, or in the then-part of an if.

Special Form: progn forms…

This special form evaluates all of the forms, in textual order, returning the result of the final form.

(progn (print "The first form")
       (print "The second form")
       (print "The third form"))
     -| "The first form"
     -| "The second form"
     -| "The third form"
⇒ "The third form"

Two other control constructs likewise evaluate a series of forms but return a different value:

Special Form: prog1 form1 forms…

This special form evaluates form1 and all of the forms, in textual order, returning the result of form1.

(prog1 (print "The first form")
       (print "The second form")
       (print "The third form"))
     -| "The first form"
     -| "The second form"
     -| "The third form"
⇒ "The first form"

Here is a way to remove the first element from a list in the variable x, then return the value of that former element:

(prog1 (car x) (setq x (cdr x)))

Special Form: prog2 form1 form2 forms…

This special form evaluates form1, form2, and all of the following forms, in textual order, returning the result of form2.

(prog2 (print "The first form")
       (print "The second form")
       (print "The third form"))
     -| "The first form"
     -| "The second form"
     -| "The third form"
⇒ "The second form"