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module List = struct
  type zero
  type _ t =
    | Nil : zero t
    | Cons : 'a * 'b t -> ('a * 'b) t
 
  let head (type s)  : (s * _) t -> s  =
    function Cons (a,b) -> a
 
  let tail (type s) : (_ * s) t -> s t =
    function Cons (a,b) -> b
 
  let rec length : type a . a t -> int = function
    | Nil -> 0
    | Cons (a,b) -> 1 + length b
end
 
let rec from_list: type a b. a list -> b List.t = function
  | []    -> List.Nil
  | x::xs -> List.Cons(x, from_list xs)
 

File "prog.ml", line 19, characters 13-21:
Error: This expression has type List.zero List.t
       but an expression was expected of type b List.t
       Type List.zero is not compatible with type b 

Standard output is empty