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SWI-Prolog owl logoaggregate.pl -- Aggregation operators on backtrackable predicates
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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)).

There are four aggregation predicates (aggregate/3, aggregate/4, aggregate_all/3 and aggregate/4), distinguished on two properties.

aggregate vs. aggregate_all
The aggregate predicates use setof/3 (aggregate/4) or bagof/3 (aggregate/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, the term r(min(X), max(X)) computes both the minimum and maximum binding for X.

count
Count number of solutions. Same as sum(1).
sum(Expr)
Sum of Expr for all solutions.
min(Expr)
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.
max(Expr)
Maximum of Expr for all solutions.
max(Expr, Witness)
As min(Expr, Witness), but producing the maximum result.
set(X)
An ordered set with all solutions for X.
bag(X)
A list of all solutions for X.

Acknowledgements

The development of this library was sponsored by SecuritEase, http://www.securitease.com

Compatibility
- Quintus, SICStus 4. The forall/2 is a SWI-Prolog built-in and term_variables/3 is a SWI-Prolog with a different definition.
To be done
- Analysing the aggregation template and compiling a predicate for the list aggregation can be done at compile time.
- aggregate_all/3 can be rewritten to run in constant space using non-backtrackable assignment on a term.
aggregate(+Template, :Goal, -Result) is nondet
Aggregate bindings in Goal according to Template. The aggregate/3 version performs bagof/3 on Goal.
aggregate(+Template, +Discriminator, :Goal, -Result) is nondet
Aggregate bindings in Goal according to Template. The aggregate/4 version performs setof/3 on Goal.
aggregate_all(+Template, :Goal, -Result) is semidet
Aggregate bindings in Goal according to Template. The aggregate_all/3 version performs findall/3 on Goal.
aggregate_all(+Template, +Discriminator, :Goal, -Result) is semidet
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 solution of Generator, foreach/2 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.
false.
bug
- Goal is copied repeatedly, which may cause problems if attributed variables are involved.
free_variables(:Generator, +Template, +VarList0, -VarList) is det
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
  1. they occur in the template
  2. they are bound by X^P, setof/3, or bagof/3

free_variables(Generator, Template, OldList, NewList) finds this set using OldList as an accumulator.

author
- Richard O'Keefe
- Jan Wielemaker (made some SWI-Prolog enhancements)
license
- Public domain (from DEC10 library).
To be done
- Distinguish between control-structures and data terms.
- Exploit our built-in term_variables/2 at some places?
sandbox:safe_meta(+Goal, -Called) is semidet[multifile]
Declare the aggregate meta-calls safe. This cannot be proven due to the manipulations of the argument Goal.