Each of the constructs in this section simply evaluates all the argument forms in order. They differ only in what results are returned.
The progn construct takes a number of forms and evaluates them sequentially, in order, from left to right. The values of all the forms but the last are discarded; whatever the last form returns is returned by the progn form. One says that all the forms but the last are evaluated for effect, because their execution is useful only for the side effects caused, but the last form is executed for value.
progn is the primitive control structure construct for “compound statements,” such as begin-end blocks in Algol-like languages. Many Lisp constructs are “implicit progn” forms: as part of their syntax each allows many forms to be written that are to be evaluated sequentially, discarding the results of all forms but the last and returning the results of the last form.
If the last form of the progn returns multiple values, then those multiple values are returned by the progn form. If there are no forms for the progn, then the result is nil. These rules generally hold for implicit progn forms as well.
prog1 is similar to progn, but it returns the value of its first form. All the argument forms are executed sequentially; the value of the first form is saved while all the others are executed and is then returned.
prog1 is most commonly used to evaluate an expression with side effects and to return a value that must be computed before the side effects happen. For example:
alters the car of x to be foo and returns the old car of x.
prog1 always returns a single value, even if the first form tries to return multiple values. As a consequence, (prog1 x) and (progn x) may behave differently if x can produce multiple values. See multiple-value-prog1. A point of style: although prog1 can be used to force exactly a single value to be returned, it is conventional to use the function values for this purpose.
prog2 is similar to prog1, but it returns the value of its second form. All the argument forms are executed sequentially; the value of the second form is saved while all the other forms are executed and is then returned. prog2 is provided mostly for historical compatibility.
Occasionally it is desirable to perform one side effect, then a value-producing operation, then another side effect. In such a peculiar case, prog2 is fairly perspicuous. For example:
prog2, like prog1, always returns a single value, even if the second form tries to return multiple values. As a consequence of this, (prog2 x y) and (progn x y) may behave differently if y can produce multiple values.