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  1. ; replace exceptions with defaults
  2. ; response to chaw
  3.  
  4. (import (rnrs exceptions (6)))
  5.  
  6. (define-syntax try
  7.   (syntax-rules (trying)
  8.     ((try trying expr default)
  9.       (call-with-current-continuation
  10.         (lambda (return)
  11.           (with-exception-handler
  12.             (lambda (x) (return default))
  13.             (lambda () expr)))))
  14.     ((try) #f)
  15.     ((try expr) (try trying expr #f))
  16.     ((try expr0 expr1 ...)
  17.       (let ((t (try trying expr0 #f)))
  18.         (if t t (try expr1 ...))))))
  19.  
  20. (display (try (car '(2)) (begin (display "hello") (newline) 3))) (newline) (newline)
  21.  
  22. (define (make-try/default proc dflt)
  23.   (lambda args
  24.     (guard (exc (else dflt))
  25.       (apply proc args))))
  26.  
  27. (define-syntax try/default
  28.   (syntax-rules ()
  29.     ((_  dflt body body1 ...)
  30.      ((make-try/default (lambda () body body1 ...) dflt)))))
  31.  
  32. (display (try/default (begin (display "hello") (newline) 3) (car '(2)))) (newline)
Success #stdin #stdout 0.08s 9704KB
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2

hello
2
https://ideone.com/9ajhyO
language:
Scheme (guile 2.2.4)
created:
6 years ago
visibility:
public

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