[plt-scheme] how to test for optional requirements in nondeterministic programming

Hi Thomas,

thanks a lot! I really should have thought of this - just wrap the soft
preconditions in an additional amb and pass this amb the existing
solution as a last option.
This way I can even rank the soft conditions between themselves.

Thanks again,
Sigrid



Thomas Chust schrieb:
> Sigrid Keydana wrote:
>   
>> [...]
>> like in the SICP "puzzles", I am using amb to choose people from a list
>> and then verify that requirements are fulfilled with an assert method.
>> Now, in addition to the requirements, I also want to make the result
>> satisfy some "optional" requirements or "nice-to-have"s if possible, but
>> in contrast to the "hard requirements" the program may not fail if this
>> doesn't work out.
>> [...]
>>     
>
> Hello,
>
> this sounds as if you could solve your problem simply by nesting the
> dynamic scope of amb backtracking contexts. Here is a stupid example
> that wraps the results satisfying all hard preconditions in pairs whose
> car is #t iff the soft preconditions are met and whose cdr contains the
> result itself:
>
>   #lang scheme
>   (require
>    (planet murphy/amb:1:1/amb))
>
>   (define (solutions)
>     (amb-collect
>      (let ([i (let ([i (for/amb ([i (in-range 0 10)])
>                          i)])
>                 ;; Check hard preconditions
>                 (amb-assert (odd? i))
>                 (amb-assert (> i 1))
>                 i)])
>        (cons (amb
>               (begin
>                 ;; Check soft preconditions
>                 (amb-assert (> i 5))
>                 (amb-assert (< i 9))
>                 #t)
>               #f)
>              i))))
>
> [The for/amb syntax is just a shorthand from my implementation of the
> amb operator. In this particular example it could be replaced with (amb
> 0 1 ... 9)]
>
> cu,
> Thomas
>
>