Derivative

In addition to updating when the underlying Px changes, this will also check its own result and halt updates if reusable.

If this Px contains a function, it can be extracted and the Px dropped from the signature. Every time the function is invoked it will use the latest value of this Px, even if you don't explicitly hold a reference to it anymore.

Example. From a Px[Int => String], an Int => String can be extracted.

Get the last used value without updating.

Revision of its value. Increments when value changes.

Get the latest value, updating itself if necessary.