Re: notational question about :Goal

On 26/08/2014, at 8:05 AM, Ross Boylan wrote:

> The manual defines
> findall(+Template, :Goal, -Bag)
> and the "Notation of predicate descriptions"
> (http://www.swi-prolog.org/pldoc/man?section=3Dpreddesc), say ":"
> indicates a meta-argument.
> =

> What's a meta-argument?

An argument that represents something to do.
The convention always was that : represented
something that WASN'T a goal but needed module
annotation, while 0, 1, 2, .... represented a
a goal that was missing the rightmost n arguments.

SWI Prolog documentation appears to use :Goal
for an argument that is a complete callable term.

> =

> I'm wondering about this because I was wondering if I could construct
> a composite goal, perhaps like =

> (mount(A, B, W1), mount(A, B, W2), W1\=3D W2).

Yes, certainly.

> I realize that could be turned into a helper predicate.

For findall/3, it's just a style issue.

For setof/3 and bagof/3, you have to think
about the r=F4les the variables play.
If a variable
 - will be ground when findall, setof, or bagof is called
   =3D> no worries
 - is captured in the Template
   =3D> no worries
 - is local to your 'composite goal'
   =3D> it must be explicitly existentially quantified
 - is a wildcard
   =3D> it is local to your 'composite goal' and cannot be
   existentially quantified so you mustn't do that.

Let's suppose for argument's sake that in your example,
A will be ground, B is captured by the template, and W1
W2 are local to the composite goal.  Then either you
should existentially quantify them
  W1^W2^(mount(A, B, W1), mount(A, B, W2), W1 \=3D=3D W2)
or you should introduce an auxiliary predicate
  has_dual_mount(A, B)
where
  has_dual_mount(A, B) :-
    mount(A, B, W1), mount(A, B, W2), W1 \=3D=3D W2.

> I also notice the description of setof is
> setof(+Template, +Goal, -Set)
> for which Goal gets a "+" rather than a ":".

Since bagof/3 in the same page uses :, I think you
can take it that this is an incomplete edit of the
documentation, not a subtle difference.