[Larceny-users] A useful chunk of code...

[Ray Racine]

On Mon, 2008-01-14 at 18:25 +0000, David Rush wrote:
> Hi y'all -
...
> It may also be useful as a model for writing other sorts of error/exit
> handlers. Code is below the fold...
> 
> david rush

Here is a parsimonious pearl for non-batch, REPL work, which I suspect
is full of unintended consequences and land mines but is working so
far ...

(library
 (rl3 env debug)

 (export debug-enable)

 (import (rnrs base)
         (only (rnrs io simple)
               display)
         (only (rnrs control)
               unless)                     
         (primitives call-with-output-string decode-error error-handler
reset))
 
 (define simple-error-handler
   (lambda e
     (display (call-with-output-string
               (lambda (sp) (decode-error e sp))))
     (reset)))
 
 (define default-error-handler #f)
 
 (define debug-enable
   (lambda (state)
     (if state
         (error-handler default-error-handler)
         (error-handler simple-error-handler))))
 
 (unless default-error-handler
   (set! default-error-handler (error-handler))))

=====================================================

Larceny v0.961 "Fluoridation" (Jan 13 2008 08:39:06,
precise:Linux:unified)
larceny.heap, built on Sat Dec 29 17:39:47 EST 2007
ERR5RS mode (no libraries have been imported)

> (import (rnrs base))

> (import (rl3 env debug))
Autoloading (rl3 env debug)
Autoloading (rnrs control)

> (define x 3)

> (x x)

Error: Attempt to apply 3, which is not a procedure.
Entering debugger; type "?" for help.
debug> #<EOF>

> (debug-enable #f)
#f

> (x x)

Error: Attempt to apply 3, which is not a procedure.

> (string-ref "ray" 10)

Error: string-ref: 10 is not a valid index into string

> (debug-enable #t)
#t

> (string-ref "ray" 10)

Error: string-ref: 10 is not a valid index into string
Entering debugger; type "?" for help.
debug> #<EOF>

For the full stack trace as well as the error message without entering
the debugger, use David's posted code with (reset) in lieu of
(exit ...).