Re: [Chicken-users] Problem with amb - or nice bug?

[Tobia Conforto]

> Hello,
> I am learning scheme and am trying to implement the "amb" operator.

Hi. That's great!

Let me start with a piece of advice: consider learning syntax-rules,  
or hygienic, new-style macros instead of the old-style define-macro.   
Besides being Scheme's standard macro system, syntax-rules is much  
more friendly, readable and safe to use.  The next version of Chicken  
(Chicken 4) will support syntax-rules by default and will remove  
define-macro.  In Chicken 3 you can load one of a number of eggs that  
provide it, for example alexpander.

Your amb macro looks like this in syntax-rules:

(define-syntax amb
  (syntax-rules (m-fail)
    ((amb alt ...)
     (let ((mf m-fail))
       (call/cc
         (lambda (sk)
           (call/cc
             (lambda (ck)
               (set! m-fail
                 (lambda ()
                   (set! m-fail mf)
                   (ck 'fail)))
               (sk alt)))
           ...
           (mf)))))))

Anyways, back to the problem you are having.

I traced it down to the use of (amb) in the top-level and I have a  
half idea I've stumbled into a bug in the Chicken interpreter, so I'm  
asking for confirmation from the more experienced Chicken hackers here.

In the meantime, as a workaround, you can convert your *street* global  
variable into a local variable:

(let ((street (list (make-house ;etc.
                    (make-house ;etc.
  (if (eq? (house-nationality (car street))
           (house-nationality (cadr street)))
      (amb))
  (map show-house street))

Written like this, it works.


                                --//--


Here are my findings.

I got the same results with all these (amb) implementations:
 - the one in the amb egg;
 - the define-macro in the OP;
 - my hygienic one above, both in Chicken 3 with alexpander and in  
Chicken svn head;
so it's not a problem in the specific macro.

This is the smallest example that fails: it prints the unreacheable  
message instead of raising the appropriate "Program failed!" or  
"expression tree exhausted" error.

(amb 1 2)
(amb)
(print "if you read this, there's a bug somewhere")

To see it fail, you need to:
 - put these three calls at the top level, NOT inside any lambda or  
begin;
 - load the file with csi -s or with (load), NOT compiling it;
 - remember to reset the top amb exception (m-fail or amb-failure- 
continuation) between tests, better yet if you start a new csi each  
time.


Tobia