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 | Predicate numbervars/3 |
numbervars(+Term,
+Start, -End)
$VAR(N),
where N is the number of the variable. Counting starts at
Start. End is unified with the number that should
be given to the next variable.bugOnly tagged
integers are supported (see the Prolog flag max_tagged_integer).
This suffices to count all variables that can appear in the largest term
that can be represented, but does not support arbitrary large integer
values for Start. On overflow, a representation_error(tagged_integer)
exception is raised. The example below illustrates this.
Note that the toplevel prints '$VAR'(0) as A due
to the
numbervars(true) option used to print answers.
?- Term = f(X,Y,X),
numbervars(Term, 0, End, [singleton(true)]),
write_canonical(Term), nl.
f('$VAR'(0),'$VAR'('_'),'$VAR'(0))
Term = f(A, _, A),
X = A,
Y = B,
End = 2.
See also the numbervars option to write_term/3
and numbervars/4.
1 
One consequence of "Note that the toplevel prints '$VAR'(0) as A due to the numbervars(true) option used to print answers.":
?- length(A, 1), numbervars(A, 0, _). A = [A].
It looks like a cyclic term, but it is not:
?- length(A, 1), numbervars(A, 0, _), cyclic_term(A). false. ?- A = [A], cyclic_term(A). A = [A].
0 
The output has changed:
?- Term = f(X,Y,X),
numbervars(Term, 0, End),
write_canonical(Term).
f('$VAR'(0),'$VAR'(1),'$VAR'(0))
Term = f($VAR(0),$VAR(1),$VAR(0)),
X = $VAR(0),
Y = $VAR(1),
End = 2.
In effect, the option singletons(false) is given to numbervars/4 underneath.
"Note that the toplevel prints '$VAR'(0) as A due to the numbervars(true) option used to print answers."
is thus no longer the case.
0
numbervars
Rewrites the term.
functor_name(+Atom), attvar(+Action), singletons(+Bool)write_term
Prints a term. The option numbervars(Bool) transforms a $VAR(N) term (would be generated by numbervars/4 with a default/missing functor_name option) into an alphabetic character like A or uses the argument to `$VAR(N) if N` is actually an atom when printing out. This is badly named. Should have been called transmogrify_numbered_vars.
There is also the option variable_names(+List) which performs a ǹumbervars`-style renaming directly in write_term but with the added flexibility to allowing you map variables to names via the List.
write_canonical
Equivalent to processing through numbervars/4 with singletons(false), and then calling write_term/2 using the options ignore_ops(true), quoted(true) and numbervars(false).
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