Phantom type examples in Ocaml

What are phantom types

Phantom types are a way for multiple types to have the same exact underlying representation but still be seen as a distinct type by the compiler. The common example is units of measurements. Feet and meters are both distances and are best represented as an int or float, but it makes no sense to add a foot to a meter. Phantom types allow you to solve this and still represent a foot and a meter exactly the same. One can think of this as the opposite of duck typing. In duck typing, if you only care if the current value you are working with has four legs, a cat a dog and a table are the effectively the same thing. In phantom types you care about what something actually represents and that means they are different, even though underneath they are represented with the same type.