I can't seem to get throw/catch to work

I can't seem to get throw/catch to work

[Tim Brown]

I'm just starting to get up and running with guile (and scheme).

I'm trying to write a C extension which (eventually) will throw an
exception/error back to guile. I cannot catch an error generated with
scm_throw(). More importantly I can't seem to throw and error within
guile and catch it myself.

guile> (catch #t (throw 'foo) (lambda (key . args) (display key) (newline)))
<unnamed port>:1:11: In procedure gsubr-apply in expression (throw 
(quote foo)):
<unnamed port>:1:11: unhandled-exception: foo
ABORT: (misc-error)

Type "(backtrace)" to get more information or "(debug)" to enter the 
debugger.
guile>

Ok, so that's me throwing my own error to myself. But I can't even catch
an exception generated by guile.

Neither generally, using the (catch #t ...) nor specifically, using
(catch 'numerical-overflow ...):

guile> (catch #t (/ 1 0) (lambda (key . args) (display key) (newline)))
<unnamed port>:2:11: In procedure / in expression (/ 1 0):
<unnamed port>:2:11: Numerical overflow
ABORT: (numerical-overflow)
guile> (catch 'numerical-overflow (/ 1 0) (lambda (key . args) (display 
key) (newline)))
<unnamed port>:3:28: In procedure / in expression (/ 1 0):
<unnamed port>:3:28: Numerical overflow
ABORT: (numerical-overflow)
guile> (version)
"1.6.4"

This happens on linux, solaris and hpux builds, so I assume there's a
problem with my use of the language rather than anything else.

What am I missing here?

Tim

-- 
Tim Brown <tim.brown@cityc.co.uk> |            City Computing Limited |
T: +44 20 8770 2110               |      City House, Sutton Park Road |
F: +44 20 8770 2130               |       Sutton, Surrey, SM1 2AE, GB |
BEAUTY: What's in your eye when you have a bee in your hand.__________/





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Re: I can't seem to get throw/catch to work

[Peter S. Christopher]

Hi there,

	The second argument to catch must be a thunk (zero
argument procedure). So your first example should be

(catch #t 
	(lambda () (throw 'foo))
	(lambda (key . args) (display key) (newline)))

The way you had it written, the (throw 'foo) was being evaluated before
the catch was fully established.

cheers, 
Pete

On Wed, 30 Jul 2003, Tim Brown wrote:

> I'm just starting to get up and running with guile (and scheme).
> 
> I'm trying to write a C extension which (eventually) will throw an
> exception/error back to guile. I cannot catch an error generated with
> scm_throw(). More importantly I can't seem to throw and error within
> guile and catch it myself.
> 
> guile> (catch #t (throw 'foo) (lambda (key . args) (display key) (newline)))
> <unnamed port>:1:11: In procedure gsubr-apply in expression (throw 
> (quote foo)):
> <unnamed port>:1:11: unhandled-exception: foo
> ABORT: (misc-error)
> 
> Type "(backtrace)" to get more information or "(debug)" to enter the 
> debugger.
> guile>
> 
> Ok, so that's me throwing my own error to myself. But I can't even catch
> an exception generated by guile.
> 
> Neither generally, using the (catch #t ...) nor specifically, using
> (catch 'numerical-overflow ...):
> 
> guile> (catch #t (/ 1 0) (lambda (key . args) (display key) (newline)))
> <unnamed port>:2:11: In procedure / in expression (/ 1 0):
> <unnamed port>:2:11: Numerical overflow
> ABORT: (numerical-overflow)
> guile> (catch 'numerical-overflow (/ 1 0) (lambda (key . args) (display 
> key) (newline)))
> <unnamed port>:3:28: In procedure / in expression (/ 1 0):
> <unnamed port>:3:28: Numerical overflow
> ABORT: (numerical-overflow)
> guile> (version)
> "1.6.4"
> 
> This happens on linux, solaris and hpux builds, so I assume there's a
> problem with my use of the language rather than anything else.
> 
> What am I missing here?
> 
> Tim
> 
> -- 
> Tim Brown <tim.brown@cityc.co.uk> |            City Computing Limited |
> T: +44 20 8770 2110               |      City House, Sutton Park Road |
> F: +44 20 8770 2130               |       Sutton, Surrey, SM1 2AE, GB |
> BEAUTY: What's in your eye when you have a bee in your hand.__________/
> 
> 
> 
> 
> 
> _______________________________________________
> Guile-user mailing list
> Guile-user@gnu.org
> http://mail.gnu.org/mailman/listinfo/guile-user
> 



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Re: I can't seem to get throw/catch to work

[Dirk Eßer]

Tim Brown <tim.brown@cityc.co.uk> schrieb am 30.07.03 13:25:12:

[ ... ]
> guile> (catch #t (throw 'foo) (lambda (key . args) (display key) (newline)))
> guile> (catch #t (/ 1 0) (lambda (key . args) (display key) (newline)))

AFAIK, catch is not a special form (as it is in CL), but a procedure, which takes
procedures as arguments:

  (catch THE-KEY-TO-CATCH BODY-THUNK HANDLER-THUNK)

where

  BODY-THUNK is a procedure (lambda () ...), and
  HANDLER-THUNK is something like (lambda (key . rest) ...)

So,

  (catch #t (lambda () (throw ´foo)) (lambda (key . rest) ...))
  (catch #t (lambda () (/ 1 0)) (lambda (key . rest) ...))

should actually do, what you expect.

The mistake in the original examples was, that due to catch being a procedure,
(throw ´foo) and (/ 1 0) are actually evaluated as part of the standard argument
evaluation process, which happens before the actual call; the catch-handler is
installed during execution of BODY-THUNK inside catch. But since evaluating arguments
fails, catch itself is never actually called.

-- dirk

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