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|library(aggregate): Aggregation operators on backtrackable predicates|
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)).
^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.
country(belgium, 11000000)may succeed twice, we can use the following to avoid counting the population of Belgium twice:
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.
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.
min(Expr, Witness), but producing the maximum result.
The development of this library was sponsored by SecuritEase, http://www.securitease.com
max(X,Witness)and Goal has no solutions, i.e., the minumum and maximum of an empty set is undefined.
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. false.
free_variables(Generator, Template, OldList, NewList)
finds this set using OldList as an accumulator.