This library provides aggregating operators over the solutions of a predicate. The operations are a generalisation of the bagof/3, setof/3 and findall/3 built-in predicates. The defined aggregation operations are counting, computing the sum, minimum, maximum, a bag of solutions and a set of solutions. We first give a simple example, computing the country with the smallest area:
smallest_country(Name, Area) :- aggregate(min(A, N), country(N, A), min(Area, Name)).
- aggregate vs. aggregate_all
- The aggregate predicates use setof/3
(aggregate/4) or bagof/3
dealing with existential qualified variables (Var
^Goal) and providing multiple solutions for the remaining free variables in Goal. The aggregate_all/3 predicate uses findall/3, implicitly qualifying all free variables and providing exactly one solution, while aggregate_all/4 uses sort/2 over solutions that Discriminator (see below) generated using findall/3.
- The Discriminator argument
- The versions with 4 arguments provide a Discriminator argument that
allows for keeping duplicate bindings of a variable in the result. For
example, if we wish to compute the total population of all countries, we
do not want to lose results because two countries have the same
population. Therefore we use:
aggregate(sum(P), Name, country(Name, P), Total)
All aggregation predicates support the following operators below in
Template. In addition, they allow for an arbitrary named compound term,
where each of the arguments is a term from the list below. For example,
r(min(X), max(X)) computes both the minimum and
maximum binding for X.
- Count number of solutions. Same as
- Sum of Expr for all solutions.
- Minimum of Expr for all solutions.
- min(Expr, Witness)
- A term
min(Min, Witness), where Min is the minimal version of Expr over all solutions, and Witness is any other template applied to solutions that produced Min. If multiple solutions provide the same minimum, Witness corresponds to the first solution.
- Maximum of Expr for all solutions.
- max(Expr, Witness)
min(Expr, Witness), but producing the maximum result.
- An ordered set with all solutions for X.
- A list of all solutions for X.
The development of this library was sponsored by SecuritEase, http://www.securitease.com
- [nondet]aggregate(+Template, :Goal, -Result)
- Aggregate bindings in Goal according to Template. The aggregate/3 version performs bagof/3 on Goal.
- [nondet]aggregate(+Template, +Discriminator, :Goal, -Result)
- Aggregate bindings in Goal according to Template. The aggregate/4 version performs setof/3 on Goal.
- [semidet]aggregate_all(+Template, :Goal, -Result)
- Aggregate bindings in Goal according to Template. The aggregate_all/3 version performs findall/3 on Goal.
- [semidet]aggregate_all(+Template, +Discriminator, :Goal, -Result)
- Aggregate bindings in Goal according to Template. The aggregate_all/4 version performs findall/3 followed by sort/2 on Goal.
- foreach(:Generator, :Goal)
- True if conjunction of results is true. Unlike forall/2,
which runs a failure-driven loop that proves Goal for each
creates a conjunction. Each member of the conjunction is a copy of Goal,
where the variables it shares with Generator are filled with
the values from the corresponding solution.
The implementation executes forall/2 if Goal does not contain any variables that are not shared with Generator.
Here is an example:
?- foreach(between(1,4,X), dif(X,Y)), Y = 5. Y = 5 ?- foreach(between(1,4,X), dif(X,Y)), Y = 3. No
- [det]free_variables(:Generator, +Template, +VarList0, -VarList)
- Find free variables in bagof/setof template. In order to handle
variables properly, we have to find all the universally quantified
variables in the Generator. All variables as yet unbound are
universally quantified, unless
free_variables(Generator, Template, OldList, NewList)finds this set using OldList as an accumulator.