On 10/22/10 5:12 PM, scouic wrote:
Hi,
i learn with-handlers mechanism, and i don't understand what's wrong in
this piece of code ?
#lang racket
(require xml)
(define a-good-string "<doc><bold>hi</bold> there!</doc>")
(define a-bad-string "<doc><boooooold>hi</bold> there!</doc>")
(define (panoramix s)
(with-handlers ((exn:invalid-xexpr? (lambda (e) 'UFO)))
(string->xexpr s)))
(panoramix a-good-string)
; this is ok and returns > '(doc () (bold () "hi") " there!")
(panoramix a-bad-string)
; <unsaved editor>::6: read-xml: parse-error: start tag `boooooold' at
[1.5/6 1.16/17] doesn't match end tag `bold' at [1.18/19 1.25/26]
Why it returns an error and not UFO ?
Because the exception raised is not a value that answers true to
exn:invalid-xexpr?. If you change the program to this, you can see what
is raised:
(define (panoramix s)
(with-handlers (((lambda (x) true) (lambda (e) e)))
(string->xexpr s)))
which is:
(exn:xml
"read-xml: parse-error: start tag `boooooold' at [1.5/6 1.16/17]
doesn't match end tag `bold' at [1.18/19 1.25/26]"
#<continuation-mark-set>
(list (srcloc 'string #f #f 6 11) (srcloc 'string #f #f 19 7))
...)
The problem is not in converting XML to an Xexpr, but rather in reading
XML from a string. If you change the predicate to exn:xml? it will work
as you expected.
(define (panoramix s)
(with-handlers ((exn:xml? (lambda (e) 'UFO)))
(string->xexpr s)))
David
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