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Choice construct is now available in Souffle to support the notion of non-determinism. With choice, it is now much easier to express worklist algorithm in Souffle by specifying functional dependencies on the relations.

A functional dependency x -> y on relation R(x:number, y:number) means that each x in R will uniquely define a value of y. For example, during the computation, if a set of tuples {(1,1), (1,2), (1,3)} are about to be inserted into R, Souffle only chooses arbitrary one of them (hence the “non-determinism”), since inserting more than one would break the functional dependency.

Functional dependency can be enforced on relation using the keyword choice-domain during relation declaration in Souffle with the following syntax:

<relation-declaration>      ::= ".decl" <relation-name> "(" <attributes> ")" <choice-domain>
<choice-domain>             ::= "" | "choice-domain" <constraint> { "," <constraint>}
<constraint>                ::= <variable> | "(" <variable> { "," <variable> } ")"

Note here, for the sake of brevity, our syntax omits the co-domain (i.e. the right hand side of the arrow). Therefore, a choice-domain D for a relation with attribute set X implicitly defines a functional dependency of D -> X \ D.


Spanning Tree

.decl edge(u:symbol, v:symbol)
.decl st(u:symbol, v:symbol) choice-domain v
.decl startNode(x:symbol)

st("root", x) :- startNode(x).
st(u, v) :- st(_, v), edge(u, v).

The above program calculates a spanning tree on a connected component of an undirected graph. The first rule simply chooses a start node. The second rule states that, an edge (u, v) is in the spanning tree: if v is reachable in the spanning tree and there is an edge between u and v. The choice-domain v on st ensures that the value of v is unique, i.e., each node can be visited only once in the spanning tree.

Eligible advisors

.decl student (s:symbol, majr:symbol, year:number)
.decl professor (s:symbol, majr:symbol)
.decl advisor (s:symbol, year:number, p:symbol) choice-domain (s, year)

advisor(s, y, p) :- student(s, y, m), professor(p, m).

The above program allocates an advisor for each student. The choice-domain (s, year) on advisor makes sure that the combination of (student, year) is unique, i.e., student from each year is assigned to a professor only once.

Total order.

.decl d(x:symbol)
.decl list(prev:symbol, next:symbol) choice-domain prev, next

list(p, n) :- d(n), list(_, p).

Given an unordered set of elements, the above program assigns them into a list, effectively computes a total order over the set. The rule states that, p is before e if p is already assigned into the total order list. With the help of the two choice-domains prev and next: prev makes sure that for each element, there can only be one unique element after it; similarly, next makes sure for each element, there can be only one unique element before it. Those constraints defines the property of a total order set.