79 lines
2.5 KiB
Scheme
79 lines
2.5 KiB
Scheme
(define *current-exception-handlers*
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(list (lambda (condition)
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(error "*current-exception-handler*" "unhandled exception" condition))))
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(define (with-exception-handlers new-handlers thunk)
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(let ((previous-handlers *current-exception-handlers*))
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(dynamic-wind
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(lambda ()
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(set! *current-exception-handlers* new-handlers))
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thunk
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(lambda ()
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(set! *current-exception-handlers* previous-handlers)))))
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(define (with-exception-handler handler thunk)
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(with-exception-handlers (cons handler *current-exception-handlers*)
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thunk))
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(define (raise obj)
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(let ((handlers *current-exception-handlers*))
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(with-exception-handlers (cdr handlers)
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(lambda ()
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((car handlers) obj)
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(error "handler returned"
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(car handlers)
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obj)))))
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(define-syntax guard
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(syntax-rules ()
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((guard (var clause ...) e1 e2 ...)
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((call-with-current-continuation
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(lambda (guard-k)
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(with-exception-handler
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(lambda (condition)
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((call-with-current-continuation
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(lambda (handler-k)
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(guard-k
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(lambda ()
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(let ((var condition)) ; clauses may SET! var
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(guard-aux (handler-k (lambda ()
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(raise condition)))
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clause ...))))))))
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(lambda ()
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(call-with-values
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(lambda () e1 e2 ...)
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(lambda args
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(guard-k (lambda ()
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(apply values args)))))))))))))
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(define-syntax guard-aux
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(syntax-rules (else =>)
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((guard-aux reraise (else result1 result2 ...))
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(begin result1 result2 ...))
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((guard-aux reraise (test => result))
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(let ((temp test))
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(if temp
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(result temp)
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reraise)))
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((guard-aux reraise (test => result) clause1 clause2 ...)
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(let ((temp test))
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(if temp
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(result temp)
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(guard-aux reraise clause1 clause2 ...))))
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((guard-aux reraise (test))
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test)
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((guard-aux reraise (test) clause1 clause2 ...)
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(let ((temp test))
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(if temp
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temp
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(guard-aux reraise clause1 clause2 ...))))
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((guard-aux reraise (test result1 result2 ...))
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(if test
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(begin result1 result2 ...)
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reraise))
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((guard-aux reraise (test result1 result2 ...) clause1 clause2 ...)
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(if test
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(begin result1 result2 ...)
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(guard-aux reraise clause1 clause2 ...)))))
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