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5.2. Sequencing
(begin exp1 exp2 ...) | syntax |
returns: the result of the last expression | |
The expressions exp1 exp2 … are evaluated in sequence from left to right. begin is used to sequence assignments, input/output, or other operations that cause side effects.
(define x 3)
(begin
(set! x (+ x 1))
(+ x x)) ⇒ 8
A begin form may contain zero or more definitions in place of the expressions exp1 exp2 …, in which case it is considered to be a definition and may appear only where definitions are valid.
(let ()
(begin (define x 3) (define y 4))
(+ x y)) ⇒ 7
This form of begin is primarily used by syntactic extensions that must expand into multiple definitions. (See page 91.)The bodies of many syntactic forms, including lambda, let, let*, and letrec, as well as the result clauses of cond, case, and do, are treated as if they were inside an implicit begin; that is, the expressions making up the body or result clause are executed in sequence.
(define swap-pair!
(lambda (x)
(let ((temp (car x)))
(set-car! x (cdr x))
(set-cdr! x temp)
x)))
(swap-pair! (cons 'a 'b)) ⇒ (b . a)
