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2.8 Sequencing

The begin special form is used to evaluate expressions in a particular order.

standard special form: begin expression expression … ¶

The expressions are evaluated sequentially from left to right, and the value of the last expression is returned. This expression type is used to sequence side effects such as input and output.

(define x 0)
(begin (set! x 5)
       (+ x 1))                 ⇒  6

(begin (display "4 plus 1 equals ")
       (display (+ 4 1)))
                                -|  4 plus 1 equals 5
                                ⇒  unspecified

Often the use of begin is unnecessary, because many special forms already support sequences of expressions (that is, they have an implicit begin). Some of these special forms are:

case
cond
define          ;“procedure define” only
do
lambda
let
let*
letrec
letrec*
let-values
let*-values
named-lambda
parameterize