Like an ordinary AND, an AND-LET* special form evaluates its arguments -- expressions -- one after another in order, till the first one that yields #f. Unlike AND, however, a non-#f result of one expression can be bound to a fresh variable and used in the subsequent expressions. AND-LET* is a cross-breed between LET* and AND.
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 -- #t --
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 macro [Rob Warnock, comp.lang.scheme, 26 Feb 1998 09:06:43 GMT, Message-ID: [email protected]]. AND-LET* allows however more than one intermediate result, each of which continues to be bound through the rest of the form.
AND-LET* (CLAWS) BODY
CLAWS ::= '() | (cons CLAW CLAWS)
CLAW ::= (VARIABLE EXPRESSION) | (EXPRESSION) |
BOUND-VARIABLE
eval[ (AND-LET* (CLAW1 ...) BODY), env] =
eval_claw[ CLAW1, env ] andalso
eval[ (AND-LET* ( ...) BODY), ext_claw_env[CLAW1, env]]
eval[ (AND-LET* (CLAW) ), env] = eval_claw[ CLAW, env ]
eval[ (AND-LET* () FORM1 ...), env] = eval[ (BEGIN FORM1 ...), env ]
eval[ (AND-LET* () ), env] = #t
eval_claw[ BOUND-VARIABLE, env ] =
eval[ BOUND-VARIABLE, env ]
eval_claw[ (EXPRESSION), env ] =
eval[ EXPRESSION, env ]
eval_claw[ (VARIABLE EXPRESSION), env ] =
eval[ EXPRESSION, env ]
ext_claw_env[ BOUND-VARIABLE, env ] = env
ext_claw_env[ (EXPRESSION), env ] =
env-after-eval[ EXPRESSION, env ]
ext_claw_env[ (VARIABLE EXPRESSION), env ] =
extend-env[ env-after-eval[ EXPRESSION, env ],
VARIABLE boundto eval[ EXPRESSION, env ]]
The full implementation plus the validation code are available here (which is a copy of http://pobox.com/~oleg/ftp/Scheme/vland.scm).
This is an implementation of AND-LET* as a (Gambit) low-level macro that re-writes AND-LET* as a "tree" of AND and LET forms. A validation code is also presented, which verifies not only that everything works as expected, but also that AND-LET* finds syntax errors where expected.
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Last modified: Wed Feb 6 17:21:57 MET 2002