instantiation_error() if Term is unbound
and Name/Arity is insufficiently instantiated.
SWI-Prolog also supports terms with arity 0, as in a()
(see
section 5). Such terms
must be processed using
functor/4
or compound_name_arity/3.
The predicate functor/3
and
=../2 raise a domain_error
when faced with these terms. Without this precaution a round trip
of a term with arity 0 over functor/3
would create an atom.
a(), see section
5). Type is one of atom,
compound, callable or atomic. Type
must be instantiated if Name is an atom and Arity
is 0 (zero). In other cases Type may be a variable. This
predicate is true if Term (either initially or after haveing
been created from Name and Type) and Type
are related as below
compound or callable.
If
Type is unbound is is unified with compound.
atom
or callable. If Type is unbound is is unified
with
atom.
atomic.
This predicate provides a safe round trip for zero-arity compounds and atoms. It can also be used as a variant of functor/3 that only processes compound or callable terms. See also compound/1, callable/1 and compound_name_arity/3.
domain_error(not_less_than_zero, Arg) if Arg
< 0.?- foo(hello, X) =.. List. List = [foo, hello, X] ?- Term =.. [baz, foo(1)]. Term = baz(foo(1))
SWI-Prolog also supports terms with arity 0, as in a()
(see
section 5). Such terms
must be processed using
compound_name_arguments/3.
This predicate raises a domain error as shown below. See also functor/3.
?- a() =.. L. ERROR: Domain error: `compound_non_zero_arity' expected, found `a()'
name()). See also compound_name_arguments/3.
See also functor/4.$VAR(N),
where N is the number of the variable. Counting starts at
Start. End is unified with the number that should
be given to the next variable.bugOnly tagged
integers are supported (see the Prolog flag max_tagged_integer).
This suffices to count all variables that can appear in the largest term
that can be represented, but does not support arbitrary large integer
values for Start. On overflow, a representation_error(tagged_integer)
exception is raised. The example below illustrates this.
Note that the toplevel prints '$VAR'(0) as A due
to the
numbervars(true) option used to print answers.
?- Term = f(X,Y,X),
numbervars(Term, 0, End),
write_canonical(Term), nl.
f('$VAR'(0),'$VAR'(1),'$VAR'(0))
Term = f(A, B, A),
X = A,
Y = B,
End = 2.
See also the numbervars option to write_term/3
and numbervars/4.
$VAR.skip, which causes numbervars/3
to ignore the attributed variable, bind which causes it to
treat it as a normal variable and assign the next '$VAR'(N)
term to it, or (default)
error which raises a type_error exception.113This
behaviour was decided after a long discussion between David Reitter,
Richard O'Keefe, Bart Demoen and Tom Schrijvers.true (default false), numbervars/4
does singleton detection. Singleton variables are unified with '$VAR'('_'),
causing them to be printed as _ by write_term/2
using the numbervars option. This option is exploited by portray_clause/2
and write_canonical/2.bugCurrently
this option is ignored for cyclic terms.
'$VAR'(X)
and opens the path for replacing this valid Prolog term by an internal
representation that has no textual equivalent.library(backcomp). The variables in List
are ordered in order of appearance traversing Term
depth-first and left-to-right. See also
term_variables/3
and nonground/2.
For example:
?- term_variables(a(X, b(Y, X), Z), L). L = [X, Y, Z].
representation_error
is raised. Note that, if a variable appears in a shared
subterm, it is not considered singleton. Thus,
A is not a singleton in the example below. See also
the singleton option of numbervars/4.
?- S = a(A), term_singletons(t(S,S), L). L = [].
1 is_most_general_term(1)false 2 is_most_general_term(p)true 3 is_most_general_term(p(_))true 4 is_most_general_term(p(_,a))false 5 is_most_general_term(p(X,X))false 6 is_most_general_term([])true 7 is_most_general_term([_|_])false 8 is_most_general_term([_,_])true 9 is_most_general_term([X,X])false
?- copy_term([X], q(X,Y), Vars, Term). Vars = [_A], Term = q(_A, Y).
Note that if VarsIn and In do not share any
variables,
Out is equivalent to In and VarsOut is
a copy (as copy_term/2)
of VarsIn. If In does not contain any variables
not in VarsIn the result is the same as
copy_term(VarsIn-In, VarsOut-Out).
Prolog is not able to modify instantiated parts of a term. Lacking that capability makes the language much safer, but unfortunately there are problems that suffer severely in terms of time and/or memory usage. Always try hard to avoid the use of these primitives, but they can be a good alternative to using dynamic predicates. See also section 4.33, discussing the use of global variables.
This predicate may be used for destructive assignment to terms, using them as an extra-logical storage bin. Always try hard to avoid the use of setarg/3 as it is not supported by many Prolog systems and one has to be very careful about unexpected copying as well as unexpected noncopying of terms. A good practice to improve somewhat on this situation is to make sure that terms whose arguments are subject to setarg/3 have one unused and unshared variable in addition to the used arguments. This variable avoids unwanted sharing in, e.g., copy_term/2, and causes the term to be considered as non-ground. An alternative is to use put_attr/3 to attach information to attributed variables (see section 8.1).
setarg(A,T,V,false), removing
the type restriction on Value. See also nb_linkarg/3.
Below is an example for counting the number of solutions of a goal. Note
that this implementation is thread-safe, reentrant and capable of
handling exceptions. Realising these features with a traditional
implementation based on assert/retract or flag/3
is much more complicated.
:- meta_predicate
succeeds_n_times(0, -).
succeeds_n_times(Goal, Times) :-
Counter = counter(0),
( Goal,
arg(1, Counter, N0),
N is N0 + 1,
nb_setarg(1, Counter, N),
fail
; arg(1, Counter, Times)
).