2.16.1.9 Singleton variable checking

A singleton variable is a variable that appears only one time in a clause. It can always be replaced by _, the anonymous variable. In some cases, however, people prefer to give the variable a name. As mistyping a variable is a common mistake, Prolog systems generally give a warning (controlled by style_check/1) if a variable is used only once. The system can be informed that a variable is meant to appear once by starting it with an underscore, e.g., _Name. Please note that any variable, except plain _, shares with variables of the same name. The term t(_X, _X) is equivalent to t(X, X), which is different from t(_, _).

As Unicode requires variables to start with an underscore in many languages, this schema needs to be extended.^{29After a proposal by Richard O'Keefe.} First we define the two classes of named variables.

• Named singleton variables
  Named singletons start with a double underscore (__) or a single underscore followed by an uppercase letter, e.g., __var or _Var.

• Normal variables
  All other variables are `normal' variables. Note this makes _var a normal variable.^{30Some Prolog dialects write variables this way.}

Any normal variable appearing exactly once in the clause and any named singleton variables appearing more than once are reported. Below are some examples with warnings in the right column. Singleton messages can be suppressed using the style_check/1 directive.

test(_).
test(_a).	Singleton variables: [_a]
test(_12).	Singleton variables: [_12]
test(A).	Singleton variables: [A]
test(_A).
test(__a).
test(_, _).
test(_a, _a).
test(__a, __a).	Singleton-marked variables appearing more than once: [__a]
test(_A, _A).	Singleton-marked variables appearing more than once: [_A]
test(A, A).

Semantic singletons

Starting with version 6.5.1, SWI-Prolog has syntactic singletons and semantic singletons. The first are checked by read_clause/3 (and read_term/3 using the option singletons(warning)). The latter are generated by the compiler for variables that appear alone in a branch. For example, in the code below the variable X is not a syntactic singleton, but the variable X does not communicate any bindings and replacing X with _ does not change the semantics.

test :-
        (   test_1(X)
        ;   test_2(X)
        ).