Re: Just crashed gprolog

[Lindsey Spratt]

On Sunday, May 4, 2003, at 02:26  AM, Emmanuel Mogenet wrote:

> ancestor(X,Y) :- parent(X,Y).
> ancestor(X,Y) :- ancestor(X,Z), ancestor(Z,Y)
[snip]
> | ?- ancestor(X,leo).
>
> X = manu ? ;
>
> X = jolene ? ;
>
> X = francis ? ;
> Segmentation fault (core dumped)
>
>
> Am I doing something wrong ?
yes, you have an infinite recursion.

> Is the tree of ancestors that huge that I'm
> causing the unification to fail ?
no.

You have encountered gprolog's graceful method of telling you that you 
have an infinite recursion: it recurses until it overwrites some memory 
and then faults.

Your second ancestor/2 clause: "ancestor(X,Y) :- ancestor(X,Z), 
ancestor(Z,Y)." is "left-recursive" (the first goal is a recursive 
call). This is not supported by Prolog. Once ancestor/2 exhausts the 
solutions for 'ancestor(X,leo)', it gets into an infinite recursive 
loop through this second clause with 'ancestor(X,Z)' trying first to 
find a satisfactory parent/2 solution (which is no longer possible), 
then falling info the second ancestor/2 clause (recursively), which 
tries the parent/2 clauses again then recursively falls into the 2nd 
ancestor/2 clause, etcetera.

This can be simply fixed by:
> ancestor(X,Y) :- parent(X,Y).
> ancestor(X,Y) :- parent(X,Z), ancestor(Z,Y)


Lindsey Spratt
http://homepage.mac.com/lspratt