The major difficulty in writing good Prolog code lies in not only understanding but also adequately transmitting the intention or purpose of a program. In contrast to other programming languages there are several quite different kinds of Prolog code often within the same program. By confusing such levels bugs and problems ensue: Pure, monotonic code. … Read more
Let us first get used to a declarative reading of logic programs. Declaratively, a Prolog program states what is true. For example natural_number(0). natural_number(s(X)) :- natural_number(X). The first clause states: 0 is a natural number. The second clause states: If X is a natural number, then s(X) is a natural number. Let us now consider … Read more
Yes, there is such a control construct in ISO Prolog, called ->. You use it like this: ( condition -> then_clause ; else_clause ) Here is an example that uses a chain of else-if-clauses: ( X < 0 -> writeln(‘X is negative. That’s weird! Failing now.’), fail ; X =:= 0 -> writeln(‘X is zero.’) … Read more
For elegance and didactic reasons alone, dif/2 is clearly preferable here and also in the vast majority of other cases, since as you already note “a lot of unnecessary unifications might take place” otherwise, and also because dif/2 is a pure and nicely declarative predicate that can be used in all directions and at any … Read more