Built-in Variants

Lists

OCaml's built-in list data type is really a recursive, parameterized variant. It is defined as follows:

type 'a list = [] | :: of 'a * 'a list

So list is really just a type constructor, with (value) constructors [] (which we pronounce "nil") and :: (which we pronounce "cons"). The only reason you can't write that definition yourself in your own code is that the compiler restricts you to constructor names that begin with initial capital letters and that don't contain any punctuation (other than _ and ').

Options

OCaml's built-in option data type is really a parameterized variant. It's defined as follows:

type 'a option = None | Some of 'a

So option is really just a type constructor, with (value) constructors None and Some. You can see both list and option defined in the core OCaml library.