?- fred = fred.
true.
  

?- X = fred.
X = fred
  

?- X = likes(mary, pizza).
X = likes(mary, pizza).
  

?- likes(
|        mary,
|        book(
|             title(Title),
|             author(
|                    given('Herman'),
|                    SurnameTerm)))
|  =
|  likes(
|        Who,
|        book(
|             title('Moby Dick'),
|             author(
|                    given('Herman'),
|                    surname('Melville')))).
Title = 'Moby Dick',
SurnameTerm = surname('Melville'),
Who = mary.
  

The goal X = Y succeeds if X = Y would fail; it is the negated form of =.

Unnecessary Use of "=" to Force Unification

This is basically a style issue. Consider the two following alternative pieces of Prolog code for computing the maximum of two numbers.

max1(X, Y, X) :- X > Y.
max1(X, Y, Y) :- X =< Y.

max2(X, Y, Max) :- X > Y, Max = X.
max2(X, Y, Max) :- X =< Y, Max = Y.

The first version is to be preferred, particularly for a novice Prolog programmer, because it reinforces how Prolog works. It also does not involve the unnecessary Max = X or Max = Y.

Whenever you write "=" in a Prolog procedure, review the code to see whether you can get rid of the "=" clause by replacing the item on the left of "=" by the item to the right of it, elsewhere in the procedure. If you do this with max2, you get max1. Sometimes you may have written the "=" with the variable on the right, in which case you need to instead replace the item on the right of the "=" with the item to the left of it.

It should be possible to do this whenever there is a variable (rather than a more complex term) on at least one side of the "=".

?- Term =.. [likes, mary, pizza].
Term = likes(mary, pizza).

?- likes(mary, pizza) =.. List.
List = [likes, mary, pizza].

Here are some other examples:

?- Term =.. [this, predicate, works, with, longer, lists, too].
Term = this(predicate, works, with, longer, lists, too).
?- Term =.. [one].
Term = one.
?- Term =.. [one, two].
Term = one(two).

univ works with infix operators, too, though perhaps in a slightlyunexpected way. Two examples:

?- op(700, xfy, and).
true.
?- Term =.. [and, p, q].
Term = p and q
?- Expression =.. [+, 2, 3], Value is Expression.
Expression = 2+3
Value = 5

is_a_parent(X) :- father_of(X, _).
is_a_parent(X) :- mother_of(X, _).

Variables are used in more than one way:

• to express constraints in parts of a rule:
  likes(A,B) :- dog(B), feeds(A,B).
  Note that both A and B appear on both sides of the neck symbol - the appearance of the same variable in two (or more places) in effect says that when the variable is bound to a value, it must be bound to the same value in all the places that it appears in a given rule.
  Note that if the same variable appears in two or more different rules, it might be bound to different values in the different rules. This is achieved by Prolog renaming the variables internally, when it comes to use the rule - usually the names are things like _1, _2, _3, etc. This is why variables with names like these sometimes turn up in error messages when your Prolog program goes wrong.

   

• to formulate queries, as in the following example:
  ?- studies(X, 9311). Prolog responds by finding all the values for X for which studies(X, 9311) is true, and successively listing them, e.g.

  X = fred ;X = jane ;X = abdul ;X = maria ;

• Put a blank line before the start of any prolog procedure.
• Group collections of facts using blank lines.
• Sometimes it helps to line up comments, but not at the expense of making lines longer than about 75-80 characters.
• Don't put helper procedures in the middle of other procedures: separate them out before or after the procedure they help. If your helperprocedure is used in several other procedures, you might want to put it right at the end of all your code.

Example. In moderate size browser windows, the bad example will have long lines that are folded by the browser, and the good example will not. In practice, you could make the lines in the good example rather longer - up to 75-80 characters.

See also indentation and comments.