Re: ‘with-exception-handler’ rational e

[Neil Jerram]

On 2014-03-07 13:34, Nikita Karetnikov wrote:
> I’ve expected ‘with-exception-handler’ to behave like ‘catch’, but it
> doesn’t.
>
> scheme@(guile-user)> ,use (srfi srfi-34)
> scheme@(guile-user)> (with-exception-handler (const "got it") (lambda
> () (raise "boom!")))
> ice-9/boot-9.scm:106:20: In procedure #<procedure 94b0e40 at
> ice-9/boot-9.scm:97:6 (thrown-k . args)>:
> ice-9/boot-9.scm:106:20: Throw to key `srfi-34' with args `("boom!")'.
>
> Entering a new prompt.  Type `,bt' for a backtrace or `,q' to continue.
> scheme@(guile-user) [1]>
> scheme@(guile-user)> (catch 'misc-error (lambda () (error "boom!"))
> (const "got it"))
> $1 = "got it"
>
> Is there an explanation that doesn’t involve diving into the land of
> continuations and dynamic wind?

Yes: the HANDLER that you specify in '(catch TAG THUNK HANDLER)' is 
implicitly extended, by Guile, by a non-local jump to the continuation 
of the '(catch ...)'.  The HANDLER that you specify in 
'(with-exception-handler HANDLER THUNK)' isn't.

So, to get catch-like behaviour you'd need to write something like this:

(call/cc (lambda (k)
           (with-exception-handler (lambda (e)
                                     (k "got it"))
             (lambda () (raise "boom!")))))

i.e. explicitly call the escape continuation in your handler code.

Why?  I think, simply because:
- the catch behaviour is traditional for catch/throw in Lisp and Scheme
- the with-exception-handler is what SRFI-34 specifies.

Alternatively you can use SRFI-34's 'guard' form, which encapsulates a 
pattern like that.

Hope that helps,

     Neil