Re: how to make this fail driven loop succesfull ?

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> Date: Sun, 9 Mar 2014 09:46:23 +0100
> From: [email protected]
> To: [email protected]; [email protected]
> Subject: Re: [SWIPL] how to make this fail driven loop succesfull ?
>
> On 03/08/2014 10:43 PM, Bob Minors wrote:
>> Roelof,
>>
>> You can make your failure-drive loop finish as 'true' like this:
>>
>> siblings(Father,Mother) :- is_father(X, Father), is_mother(X, Mother), write(X), nl, fail ; true.
>>
>> Reading from left-to-right, when it has found all the possible values
>> for X and failed to find any more with backtracking, it will try the
>> right-hand branch of the ';' and finally succeed with the 'true'.
>
> Over time, as proposed here before, I generally prefer recursion or
> forall/2 and avoid side effects and lists of objects satisfying some
> criterion as much as possible. So, you just write
>
> sibling(Father, Mother, X) :-
> is_father(X, Father),
> is_mother(X, Mother).
>
> You can easily combine this with other nice logical relations. If you want
> all of them, you use findall/3, but as late as possible because you cannot
> easily combine siblings(Father, Mother, Siblings) without using member,
> set operations, etc, which all make your code less readable. If you merely
> want to write them, use
>
> forall(sibling(Father, Mother, X), writeln(X)).
>
> These are a few lessons I learned while helping non-experienced programmers
> using Prolog to explore their data.
>
> Cheers --- Jan
>


Thanks, 


As I stated Im following the adventure in Prolog tutorial.

Im now at chapter 5 Rules where Recursion is chapter 8.


So I cannot use recurrsion yet and have to use the fail-driven approach.


Roelof