- Reference manual
- The SWI-Prolog syntax
- Reference manual
variable is a variable that appears only one time in a clause. It
can always be replaced by
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
_Name. Please note that any variable, except plain
shares with variables of the same name. The term
is equivalent to
t(X, X), which is different from
As Unicode requires variables to start with an underscore in many languages, this schema needs to be extended.28After 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.,
- Normal variables
All other variables are `normal' variables. Note this makes
_vara normal variable.29Some 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(_a).||Singleton variables: [_a]|
|test(_12).||Singleton variables: [_12]|
|test(A).||Singleton variables: [A]|
|test(__a, __a).||Singleton-marked variables appearing more than once: [__a]|
|test(_A, _A).||Singleton-marked variables appearing more than once: [_A]|
Starting with version 6.5.1, SWI-Prolog has syntactic singletons
and semantic singletons. The first are checked by
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) ).