Re: metaprogramming

On 30/08/2013, at 5:50 AM, Josef Frydl wrote:

> Hi, does somebody already know how to take the list of predicates and enclose all the member in parentheses  and members are separated by comma.
> 
> example
> [ a(A),b(B),c(C) ]  -->  (a(A),b(B),c(C) )


If only more people bought The Craft of Prolog.

You *cannot* enclose all the members in parentheses;
parentheses are an artefact of a particular printing style
and context.  They are *never* part of a data structure per se.

[a,b,c] is the same as '.'(a, '.'(b, '.'(c, [])))
 a,b,c  is the same as ','(a, ','(b, c))

From this, it should be pretty obvious both how to do it and
why this is almost always a bad idea.

For example, did x,y arise from [x,y] or from [(x,y)]?

Amongst other things, the empty list is a perfectly good list,
so what do you want _that_ to turn into?   Here I am going to
assume that you are turning a list of goals into a Prolog
conjunction.

goal_list_to_conjunction([], true).
goal_list_to_conjunction([G], C) :- !,
    C = G.
goal_list_to_conjunction([G|Gs], (G,C)) :-
    goal_list_to_conjunction(Gs, C).

Now we start wondering what exactly should happen if some
of the terms in the list are already comma-terms.  Should
[(a,b),(c,d),(e,f)] turn into ((a,b),(c,d),e,f)
or into a,b,c,d,e,f?  What if some of the elements are
true or false?  Should [a,true,b,false,c] => [a,b,false]?

What I am getting at is that
 - as a *data structure* a so-called "round list" is about
   as horrible as it gets in Prolog
 - for at least some uses, simply "change square brackets
   to round parentheses" is not enough to get high-quality
   output.