In this example, yes.
But in general, rule priority is important, when a suspended constraint be triggered.
Note that constraint store should be seen as a set (instead of ordered list, or something like that), there is no any semantics to determine which constraint should be triggered first.
For example,
Consider that we want to check whether two graphs, G1 and G2 are equal. We do this by removing those edges that are common to both graphs. If there are still edges after reaching a fixed point, then the graphs are different.We represent the edges of graph G1 and G2 by the edge constraints e1/2 and e2/2 respectively. This gives us the following program
s1 @ e1(X,Y) \ e1(X,Y) <=> true pragma priority(1).
s2 @ e2(X,Y) \ e2(X,Y) <=> true pragma priority(1).
rc @ e1(X,Y), e2(X,Y) <=> true pragma priority(2).
Edges obey set semantics, as implemented by the rules s1 and s2.
The rule rc (remove common) removes those edges that appear
both in graph G1 and in graph G2 Now consider the query:
?- e1(X,X), e2(X,Y), e2(Y,X), X = Y.
When executing this query using the refined operational semantics, ignoring the rule priorities, the X = Y built-in constraint causes the sequential activation of all three CHR constraints. If the e1/2 constraint is activated before any of the e2/2 constraints, then rule rc fires before the s2 rule is tried, which results in a final constraint store containing the e2(X,X) constraint which erroneously indicates that the graphs are different.
using rule priorities, the set semantics rules will always be tried before the lower priority rule rc and when the highest priority fireable rule instance is one of priority 2 (or less), then there will be no two syntactically equal e1/2 or e2/2 constraints.
The above example can be implemented correctly using the refined semantics, but this leads to inefficient and unreadable code:
s1@ e1(X,Y) \ e1(X,Y) <=> true.
s2@ e2(X,Y) \ e2(X,Y) <=> true.
s3 @ e1(_,_), e1(X,Y) \ e1(X,Y) <=> true.
s4 @ e1(_,_), e2(X,Y) \ e2(X,Y) <=> true.
s5 @ e2(_,_), e1(X,Y) \ e1(X,Y) <=> true.
s6 @ e2(_,_), e2(X,Y) \ e2(X,Y) <=> true.
PS. The above contents are just about static priorities. The dijkstra’s shortest path uses dynamic priorities, which is more advanced.