How the sequence type is defined in ocaml

问题

type 'a node =
  | Nil
  | Cons of 'a * 'a t

and 'a t = unit -> 'a node

type 'a mappable = 'a t

What does 'a t = unit -> 'a node mean in a type declaration? I thought that in a type declaration in ocaml we can only do an enumeration or call a constructor. thank you

回答1:

unit -> 'a node is the type of a function that takes no argument and returns a 'a node (a node parameterized by 'a). Example :

let f () = Nil;;

type 'a t = unit -> 'a node makes a synonym of the above type which is used in the first type defined in your code.

let l = Cons (4, fun () -> Cons (3, fun () -> Nil));;
let Cons(_,ll) = l;; (* ok , just for example, it returns a warning due to incomplete pattern matching *)
ll ();; (* - : int node = Cons (3, <fun>) *)

来源：https://stackoverflow.com/questions/49636168/how-the-sequence-type-is-defined-in-ocaml