This module traverses the data structure maintained by the unifier and offers an occurs check, that is, a cycle detection algorithm. It is parameterized by the term structure S and by the unifier U. Read-only access to the unifier's data structure suffices.
module S : sig ... endmodule U : sig ... endexception Cycle of U.variableCycle v is raised by the occurs check.
val new_occurs_check : (U.data -> bool) -> U.variable -> unitnew_occurs_check is_young initiates a cycle detection phase. It produces a function check that can be applied to a number of roots. The function check visits the vertices that are reachable from some root and that satisfy the user-supplied predicate is_young. If a cycle is detected, then Cycle v is raised, where v is a vertex that participates in the cycle.
The function check has internal state and records the vertices that it has visited, so no vertex is visited twice: the complexity of the occurs check is linear in the size of the graph fragment that is visited.