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
These predicates were introduced in the context of the
SWISH Prolog browser-based 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 non-deterministic predicates are
composed from simpler non-deterministic 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 (
b(X)) and the ones using this library side-by-side.
- Avoid duplicates of earlier steps
setof(X, a(X), Xs), distinct(a(X)),
member(X, Xs), b(X)
Note that the distinct/1 based solution returns the first result
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 top-10
setof(X, a(X), Xs), limit(10, order_by([desc(X)], a(X))),
reverse(Xs, Desc), b(X)
first_max_n(10, Desc, Limit),
Here we see power of composing primitives from this library and
staying within the paradigm of pure non-deterministic relational
- See also
- - all solution predicates findall/3, bagof/3 and setof/3.
- - library(aggregate)
- 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
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.
findall(Goal, Goal, List),
- 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
- 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
- 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
- Order solution according to ascending Term
- Order solution according to descending Term
- group_by(+By, +Template, :Goal, -Bag) is nondet
- 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.
The following predicates are exported, but not or incorrectly documented.
- reduced(Arg1, Arg2, Arg3)
- distinct(Arg1, Arg2)