/usr/lib/s9fes/catch.scm is in scheme9 2010.11.13-2.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 | ; Scheme 9 from Empty Space, Function Library
; By Nils M Holm, 2010
; See the LICENSE file of the S9fES package for terms of use
;
; (catch <tag> <statement> ...) ==> object
; (throw <tag> <expression>) ==> undefined
;
; (load-from-library "catch.scm")
;
; Implement Common LISP-style CATCH and THROW. <Tag> must evaluate to
; an object that can be checked for identity using EQ? (typically a
; symbol). CATCH establishes a catch named <tag> and then evaluates
; the given <statement>s in an implicit BEGIN. Unless one of the
; statements executes THROW, the value of the last <statement> will
; be returned. When a <statement> executes THROW, controll will be
; passed to the innermost catch with the same <tag> as the throw.
; In this case the catch will exit immediately, returning the value
; of <expression> as its result.
;
; Example: (let ((v #f))
; (let ((r (catch 'foo
; (set! v 0)
; (throw 'foo 1)
; (set! v 2)
; 3)))
; (list v r))) ==> (0 1)
(load-from-library "setters.scm")
(define *catch-stack* '())
(define-syntax (catch tag . body)
`(call/cc
(lambda (c)
(push! (cons ,tag c) *catch-stack*)
(let ((result (begin ,@body)))
(pop! *catch-stack*)
result))))
(define (throw tag value)
(do ((cs *catch-stack* (cdr cs)))
((null? cs)
(set! *catch-stack* '())
(error "throw: no catcher for" tag))
(if (eq? tag (caar cs))
(begin (set! *catch-stack* (cdr cs))
((cdar cs) value)))))
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