Re: [Scheme-reports] possible error in specification of guard

[Alex Shinn <alexshinn@x>]

On Sun, Dec 23, 2012 at 6:55 AM, Per Bothner <per@x> wrote:

Is the following a correct translation of this guard clause:

(guard (var cond-clause ....) body)

First assume there is an else clause:

(call/cc (k)
   (with-exception-handler
     (lambda (var)
       (k (cond cond-clause ...)))
     (lambda () body)))

If there is no else clause:

(call/cc (k)
   (with-exception-handler
     (lambda (var)
       (k (cond cond-clause ... (else (raise-continuable var)))))
     (lambda () body)))

It's actually more complicated than that, because

in the final (else (raise-continuable var)) case, the

continuation has to be raised from the original

exception handler.

The trick used in the R6RS reference implementation[1]

(which Chibi uses) is to return a thunk and apply it outside

the with-exception-handler.

[1] http://www.r6rs.org/refimpl/r6rs-exceptions.tar.gz

-- 

Alex

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