John Cowan writes: > Per Bothner scripsit: > > > Assuming guard is definable in terms of with-exception-handler, > > there should be a definition in section 7.3 "Derived expression > > types". > > There should, indeed, but no one has written one. Contributions are > earnestly solicited.
What about the one in <http://srfi.schemers.org/srfi-34/srfi-34.html>? Cut-and-pasted from there: (define-syntax guard (syntax-rules () ((guard (var clause ...) e1 e2 ...) ((call-with-current-continuation (lambda (guard-k) (with-exception-handler (lambda (condition) ((call-with-current-continuation (lambda (handler-k) (guard-k (lambda () (let ((var condition)) ; clauses may SET! var (guard-aux (handler-k (lambda () (raise condition))) clause ...)))))))) (lambda () (call-with-values (lambda () e1 e2 ...) (lambda args (guard-k (lambda () (apply values args))))))))))))) (define-syntax guard-aux (syntax-rules (else =>) ((guard-aux reraise (else result1 result2 ...)) (begin result1 result2 ...)) ((guard-aux reraise (test => result)) (let ((temp test)) (if temp (result temp) reraise))) ((guard-aux reraise (test => result) clause1 clause2 ...) (let ((temp test)) (if temp (result temp) (guard-aux reraise clause1 clause2 ...)))) ((guard-aux reraise (test)) test) ((guard-aux reraise (test) clause1 clause2 ...) (let ((temp test)) (if temp temp (guard-aux reraise clause1 clause2 ...)))) ((guard-aux reraise (test result1 result2 ...)) (if test (begin result1 result2 ...) reraise)) ((guard-aux reraise (test result1 result2 ...) clause1 clause2 ...) (if test (begin result1 result2 ...) (guard-aux reraise clause1 clause2 ...))))) _______________________________________________ Scheme-reports mailing list [email protected] http://lists.scheme-reports.org/cgi-bin/mailman/listinfo/scheme-reports
