and-let*
Location: lib, 26 Lines
; Scheme 9 from Empty Space, Function Library ; By Nils M Holm, 2010,2012 ; Placed in the Public Domain ; ; (and-let* <binding> ... <body>) ==> object ; ; (load-from-library "and-letstar.scm") ; ; Each <binding> has the form (<variable> <expression>) and binds ; the given <variable> to the normal form of <expression>. ; ; Like LET*, AND-LET* evaluates its <binding>s in sequence, so each ; <expression> is evaluated in an environment that includes all previous ; <binding>s of the same AND-LET*. Unlike LET*, though, AND-LET* returns ; #F immediately as soon as one of its <expression>s evaluates to #F. ; Only when all <expression>s evaluate to non-#F values, it evaluates ; <body> and returns its value. AND-LET* expands as follows: ; ; (and-let* ((<var1> <expr1>) ---> (let ((<var1> <expr1>)) ; ... (and <var1> ; (<varN> <exprN>)) ... ; <body>) (let ((<varN> <exprN>)) ; (and <varN> ; <body>)))) ; ; This is only a subset of SRFI-2 AND-LET*. ; ; Example: (and-let* ((a '((x . 1))) ; (a (assq 'x a))) ; (cdr a)) ==> 1 ; ; (and-let* ((a '((x . 1))) ; (a (assq 'z a))) ; (cdr a)) ==> #f (define-syntax (and-let* clauses expr . exprs) (letrec ((nest-let (lambda (c) (cond ((null? c) (cons expr exprs)) ((null? (cdr c)) `(let ((,(caar c) ,(cadar c))) (and ,(caar c) ,@(cons expr exprs)))) (else `(let ((,(caar c) ,(cadar c))) (and ,(caar c) ,(nest-let (cdr c))))))))) (let loop ((c clauses)) (cond ((null? c)) ((or (not (pair? (car c))) (null? (cdar c)) (not (null? (cddar c))) (not (symbol? (caar c)))) (error "and-let*: syntax error" c)) (else (loop (cdr c))))) (if (null? clauses) `(let () ,expr ,@exprs) (nest-let clauses))))