2.3.3 Rationale

In case of an ordinary and formed of proper boolean expressions:

(and e1 e2 ...)

expression e2, if it gets to be evaluated, knows that e1 has returned non–#f. Moreover, e2 knows exactly what the result of e1 was (true) which e2 can use to its advantage. If e1 however is an extended boolean expression, e2 can no longer tell which particular non–#f value e1 has returned. Chances are it took a lot of work to evaluate e1, and the produced result (a number, a vector, a string, etc) may be of value to e2. Alas, the and form merely checks that the result is not an #f, and throws it away. If e2 needs it, it has to compute that value anew.

This proposed and-let* special form lets constituent expressions get hold of the results of already evaluated expressions, without re–doing their work.

and-let* can be thought of as a combination of let* and and, or a generalization of cond’s send operator, =>. An and-let* form can also be considered a sequence of guarded expressions.

In a regular program, forms may produce results, bind them to variables and let other forms use these results. and-let* differs in that it checks to make sure that every produced result “makes sense” (that is, not an #f). The first “failure” triggers the guard and aborts the rest of the sequence (which presumably would not make any sense to execute anyway). Examples:

(and-let* ([my-list     (compute-list)]
           [            (not (null? my-list))])
  (do-something my-list))

(define (look-up key alist)
  (and-let* ([x (assq key alist)])
    (cdr x)))

(or
  (and-let* ([c (read-char)]
             [  (not (eof-object? c))])
    (string-set! some-str i c)
    (set! i (+ 1 i)))
  (begin
    (do-process-eof)))

;; A more realistic example
;; Parse the 'timestamp' ::= 'token1' 'token2'
;;   token1 ::= 'YY' 'MM' 'J'
;;   token2 ::= 'GG' 'gg' "/"
(define (parse-full-timestamp token1 token2)
  (and-let* ([  (= 5 (string-length token1))]
             [  (= 5 (string-length token2))]
             [timestamp
                (OS:string->time "%m/%d/%y %H:%M"
                  (string
                    (string-ref token1 2) (string-ref token1 3) #\/
                    (string-ref token1 0) (string-ref token1 1) #\/
                    (case (string-ref token1 4)
                      ((#\8 #\9) #\9) (else #\0))
                    (string-ref token1 4) #\space
                    (string-ref token2 0) (string-ref token2 1) #\:
                    (string-ref token2 2) (string-ref token2 3)))]
             [  (positive? timestamp)])
           timestamp))

and-let* is also similar to an “anaphoric and” Lisp macro². and-let* allows however more than one intermediate result, each of which continues to be bound through the rest of the form.