4.4.7 Sequencing

Syntax: begin ?form …

Syntax: begin ?expression ?expression …

The ?begin keyword has two different roles, depending on its context:

• It may appear as a form in a ?body, ?library-body, or ?top-level-body, or directly nested in a begin form that appears in a body. In this case, the begin form must have the shape specified in the first header line. This use of begin acts as a splicing form: the forms inside the ?body are spliced into the surrounding body, as if the begin wrapper were not actually present.

  A begin form in a ?body or ?library-body must be non–empty if it appears after the first ?expression within the body.

• It may appear as an ordinary expression and must have the shape specified in the second header line. In this case, the ?expressions are evaluated sequentially from left to right, and the values of the last ?expression are returned. This expression type is used to sequence side effects such as assignments or input and output.

(define x 0)

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

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