by Oleg Kiselyov
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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).
(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: firstname.lastname@example.org]. 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[ (LET* () 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 ]]
A full sample implementation is available at http://okmij.org/ftp/Scheme/lib/myenv-chez.scm. The test suite is at http://okmij.org/ftp/Scheme/tests/vland.scm.
Copyright (C) Oleg Kiselyov (1998). All Rights Reserved.
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Last modified: Sun Jan 28 13:40:28 MET 2007