package unionFind

A store can be thought of as a region of memory in which objects, known as references, can be dynamically allocated, read, and written. Stores are homogeneous: all references in a store of type 'a store have the content type, namely 'a. In general, a store may be persistent or mutable, depending on which data structure is used to implement it.

new_store() creates an empty store.

A reference of type 'a rref can be thought of as (a pointer to) an object that exists in some store.

make s v creates a fresh reference in the store s and sets its content to v. It returns a pair of an updated store and the newly-created reference.

get s x reads the current content of the reference x in the store s. It returns a pair of a possibly-updated store and the current content of the reference.

set s x v updates the store s so as to set the content of the reference x to v. It returns an updated store.

eq s x y determines whether the references x and y are the same reference. It returns a pair of a possibly-updated store and a Boolean result. The references x and y must belong to the store s.

tentatively s f runs the function f within a new transaction on the store s. If f raises an exception, then the transaction is aborted, and all updates performed by f on references in the store s are rolled back. If f terminates normally, then the updates performed by f are committed.

Two transactions on a single store cannot be nested.

A cell that is created during a transaction still exists after the transaction, even if the transaction is rolled back. In that case, its content should be considered undefined.