Is there value in Prolog that is not equal to itself? I write answer to some question about min of tree and this answer also says that if tree is empty min is null. Sounds good idea first but now when I think it sounds like bad idea.
It is kinda OK if null <> null, no problem. But in Prolog I see null is just atom so....
?- null = null.
true.
?- null == null.
true.
?- dif(null, null).
false.
How can I make some term in Prolog that always say:
?- dif(Something, Something).
true.
But if it is any other thing and not this term that is the null thing still say false.?
Or if this is not how I should think in Prolog then how should I think about not true. and also not false. but "neither true nor false because something is missing"?
Just for fun, not really the answer you're looking for, taking the question title quite literally:
?- _ == _ .
false.
But dif/2 is not fouled (hint: each occurrence of the anonymous variable represents a different variable):
?- dif(_, _).
true.
Now, seriously. Starting with your tree minimum predicate example, there's a trivial alternative: the predicate can simply fail when the tree is empty. A better alternative may be to use optional or expected term libraries. The concepts behind these libraries are found in several programming languages, where they provide a better alternative to null. You have both libraries in Logtalk, which you can use with most Prolog systems. See:
and
You use one library or the other depending on your interpretation of "missing" meaning something that is optional (absence of a value is fine) or expected (absence of a value is an error). For example, assume that in your particular application it makes sense to use 0 as the minimum value of an empty tree when doing a specific computation (e.g. the sum of the minimums of a set of trees). If the tree minimum predicate returns an optional term reference, Ref, instead of an integer, you could do e.g.
...,
optional(Ref)::or_else(Minimum, 0),
Sum1 is Sum0 + Minimum,
...
This is a cleaner solution compared with using an if-then-else construct:
...,
( tree_minimum(Tree, Minimum) ->
Sum1 is Sum0 + Minimum
; Sum1 is Sum0
),
...
It also allows you to use different defaults for different computations. For example:
...,
optional(Ref)::or_else(Minimum, 1),
Product1 is Product0 * Minimum,
...
More important, it doesn't mask that you're processing an empty tree in the same way that a default value would do. For example, the following code will only write the minimum values of non-empty trees:
print_tree_minimums(Refs) :-
meta::map(print_tree_minimum, Refs).
print_tree_minimum(Ref) :-
optional(Ref)::if_present(write).
or, using a lambda expression:
print_tree_minimums(Refs) :-
meta::map([Ref]>>(optional(Ref)::if_present(write)), Refs).
This answer is getting long and I don't want to transform it into a general discussion of the pros and cons of optionals and expecteds. But descriptions on both concepts and libraries is easy to find. E.g.