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

[Dirk Eßer]

Tim Brown <address@hidden> 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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