understanding the reif library

[mvolkmann]

I'm learning about the reif library and trying to understand when it is preferable to use if_ instead of ->.

It seems the reif library requires having rules that can set their last argument to the atoms true and false. Is it correct that Prolog does not officially have a Boolean type, but the atoms true and false are commonly used for this purpose?

In the simple code below, is it necessary to create the is_dog rule in order to use if_ in the report_reif rule?

Are there reasons why using if_ instead of -> is preferable in this case?

:- use_module(library(reif)).

dog(comet).

writeln(X) :- write(X), nl.

report(Name) :-
  dog(Name) ->
    writeln('dog');
    writeln('not a dog').

% Can this rule be simplified further?
is_dog(X, B) :- dog(X) -> B = true; B = false.

report_reif(Name) :-
  % The first argument must be a predicate that accepts
  % an extra variable argument to receive true or false.
  if_(
    is_dog(Name),
    writeln('dog'),
    writeln('not a dog')
  ).

:- initialization((
  report(comet), % dog
  report(mark), % not a dog
  report_reif(comet), % dog
  report_reif(mark), % not a dog
  halt
)).

[triska]

Almost invariably, (->)/2 yields wrong results. In your case, consider for example:

?- report(X).
dog
   X = comet.

So X = comet is the only solution, right?

Wrong! not_comet is also a solution:

?- report(not_comet).
not a dog
   true.

So, which is it? Is comet the only solution or not?

[mvolkmann]

The way I have written the report and report_reif rules, they always succeed.
Their only purpose is to write something to stdout so I can see which path (then or else) was taken.
So you are correct that logically speaking they consider everything to be a dog.

Despite this I still have these questions which I think are still valid questions:

1. It seems the reif library requires having rules that set their last argument to the atoms true or false. Is it correct that Prolog does not officially have a Boolean type, but the atoms true and false are commonly used for this purpose?

2. In the simple code below, is it necessary to create the is_dog rule in order to use if_ in the report_reif rule?

[UWN]

In general, read this article which explains if_/3 and the limits of the traditional if-then-else construct. More examples are on Stack Overflow.

[mvolkmann]

I would still like to get feedback on my questions numbered 1 and 2 above.