4.7 Grouping

Special Form: begin form … ¶

[R7RS base] Evaluates forms sequentially, and returns the last result(s).

Begin doesn’t introduce new scope like let, that is, you can’t place "internal define" at the beginning of forms generally. Semantically begin behaves as if forms are spliced into the surrounding context. For example, toplevel expression like the following is the same as two toplevel definitions:

(begin (define x 1) (define y 2))

Here’s a trickier example:

(let ()
  (begin
    (define x 2)
    (begin
      (define y 3)
    ))
  (+ x y))

  ≡

(let ()
  (define x 2)
  (define y 3)
  (+ x y))

Macro: begin0 exp0 exp1 … ¶

Evaluates exp0, exp1, …, then returns the result(s) of exp0. The name is taken from MzScheme. This is called prog1 in CommonLisp.

Unlike begin, this does creates a new scope, for the begin0 form is expanded as follows.

(receive tmp exp0
  exp1 …
  (apply values tmp))

Macro: independently exp … ¶

[SRFI-236] Evaluates exp … in an unspecified order, and returns an undefined result. This form is defined in SRFI-236.

You can use begin to group multiple expressions, but begin also specifies the order of evaluation even when it’s unnecessary. Using independently, you can express the intention to the readers that the order does not matter, and also gives the compiler more opportunities for optimization.

As the order doesn’t matter, SRFI-236 does not specify the return value of this form. Gauche returns #<undef>, but the caller shoudn’t rely on it.