[Caml-list] A weird typing error

[bob zhang <bobzhang1988@gmail.com>]

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Hi, list
    I came across a weird type error.
    The error message is  "Error: The implementation type_error_ppo.ml
       does not match the interface (inferred signature):"
    I attached the file
   Many thanks

-- 
-- Bob

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type 'a m_list = | M_nil | M_cons of 'a * 'a m_list

type 'a twice = 'a

type 'a tree =
  | Leaf | Node of ('a tree) m_list * (('a tree) m_list) twice * 'a

class base =
  object method int : int -> int = fun x -> x
    method float : float -> float = fun x -> x
    method string : string -> string = fun x -> x
  end
  
module EQ =
  struct
    class map =
      object ((self : 'self_type))
        inherit base
        method unknown : 'a. 'a -> 'a = fun x -> x
        method tree :
          'a0 'b0. ('self_type -> 'a0 -> 'b0) -> 'a0 tree -> 'b0 tree =
          fun mf_a ->
            function
            | Leaf -> Leaf
            | Node (a0, a1, a2) ->
                Node ((self#m_list (fun self -> self#tree mf_a) a0),
                  (self#twice
                     (fun self -> self#m_list (fun self -> self#tree mf_a))
                     a1),
                  (mf_a self a2))
        method twice :
          'a0 'b0. ('self_type -> 'a0 -> 'b0) -> 'a0 twice -> 'b0 twice =
          fun mf_a -> mf_a self
        method m_list :
          'a0 'b0. ('self_type -> 'a0 -> 'b0) -> 'a0 m_list -> 'b0 m_list =
          fun mf_a ->
            function
            | M_nil -> M_nil
            | M_cons (a0, a1) ->
                M_cons ((mf_a self a0), (self#m_list mf_a a1))
      end
      
  end