 See also
  all solution predicates findall/3, bagof/3
and setof/3.
 library(aggregate)
The meta predicates of this library modify the sequence of solutions
of a goal. The modifications and the predicate names are based on the
classical database operations DISTINCT, LIMIT, OFFSET, ORDER BY and
GROUP BY.
These predicates were introduced in the context of the
SWISH Prolog
browserbased shell, which can represent the solutions to a predicate as
a table. Notably wrapping a goal in distinct/1
avoids duplicates in the result table and using
order_by/2
produces a nicely ordered table.
However, the predicates from this library can also be used to stay
longer within the clean paradigm where nondeterministic predicates are
composed from simpler nondeterministic predicates by means of
conjunction and disjunction. While evaluating a conjunction, we might
want to eliminate duplicates of the first part of the conjunction. Below
we give both the classical solution for solving variations of (a(X)
,
b(X)
) and the ones using this library sidebyside.
 Avoid duplicates of earlier steps

setof(X, a(X), Xs), distinct(a(X)),
member(X, Xs), b(X)
b(X).
Note that the distinct/1
based solution returns the first result of distinct(a(X))
immediately after a/1 produces a result,
while the setof/3
based solution will first compute all results of a/1.
 Only try
b(X)
only for the top10 a(X)

setof(X, a(X), Xs), limit(10, order_by([desc(X)], a(X))),
reverse(Xs, Desc), b(X)
first_max_n(10, Desc, Limit),
member(X, Limit),
b(X)
Here we see power of composing primitives from this library and
staying within the paradigm of pure nondeterministic relational
predicates.
 distinct(:Goal)
 distinct(?Witness,
:Goal)
 True if Goal is true and no previous solution of Goal
bound
Witness to the same value. As previous answers need to be
copied, equivalence testing is based on term variance (=@=/2).
The variant distinct/1
is equivalent to
distinct(Goal,Goal)
.
If the answers are ground terms, the predicate behaves as the code
below, but answers are returned as soon as they become available rather
than first computing the complete answer set.
distinct(Goal) :
findall(Goal, Goal, List),
list_to_set(List, Set),
member(Goal, Set).
 reduced(:Goal)
 reduced(?Witness,
:Goal, +Options)
 Similar to distinct/1,
but does not guarantee unique results in return for using a limited
amount of memory. Both distinct/1
and
reduced/1
create a table that block duplicate results. For
distinct/1,
this table may get arbitrary large. In contrast,
reduced/1
discards the table and starts a new one of the table size exceeds a
specified limit. This filter is useful for reducing the number of
answers when processing large or infinite long tail distributions. Options:
 size_limit(+Integer)
 Max number of elements kept in the table. Default is 10,000.
 limit(+Count,
:Goal)
 Limit the number of solutions. True if Goal is true,
returning at most Count solutions. Solutions are returned as
soon as they become available.
 offset(+Count,
:Goal)
 Ignore the first Count solutions. True if Goal is
true and produces more than Count solutions. This predicate
computes and ignores the first Count solutions.
 call_nth(:Goal,
?Nth)
 True when Goal succeeded for the Nth time. If Nth
is bound on entry, the predicate succeeds deterministically if there are
at least Nth solutions for Goal.
 order_by(+Spec,
:Goal)
 Order solutions according to Spec. Spec is a list
of terms, where each element is one of. The ordering of solutions of Goal
that only differ in variables that are not shared with Spec
is not changed.
 asc(Term)
 Order solution according to ascending Term
 desc(Term)
 Order solution according to descending Term
 [nondet]group_by(+By,
+Template, :Goal, Bag)
 Group bindings of Template that have the same value for By.
This predicate is almost the same as bagof/3,
but instead of specifying the existential variables we specify the free
variables. It is provided for consistency and complete coverage of the
common database vocabulary.