Error: This expression has type t/1 but an expression was expected of type
t/2
Hint: The type t has been defined multiple times in this toplevel
session. Some toplevel values still refer to old versions of this
type. Did you try to redefine them?
`type 'a t = Cons of 'a * 'a t | Tail` is defining a type with a constructor `Cons` with two arguments and a constructor `Tail` with zero argument. You write values of type `'a t` as `Cons (a, b)` and `Tail`.
`type 'a u = (::) of 'a * 'a u | []` is defining a type with a constructor `(::)` with two arguments and a constructor `[]` with zero argument. You build values of type `'a u` as `(::) (a, b)` and `[]`.
The rest comes from the special syntax support in OCaml for the `(::)` and `[]` names. Namely, `a :: b` is parsed as `(::) (a, b)`, and `[a; b; ...; z]` is parsed as `a :: b :: ... :: z :: []`, and hence as `(::) (a, (::) (b, (::) (..., (::) (z, []))))`.