[Caml-list] polymorphic stack

[Walter Cazzola <cazzola@dico.unimi.it>]

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Hi all,
thanks a lot for your help in this travel through OCaML but I have still
a question. I have tried to write a polymorphic stack code attached but
I don't understand its behavior:

    # open Stack;;
    # let s = empty;;
    val s : '_a Stack.stack = {c = []}
    # push s 7;;
    - : unit = ()
    # push s 25;;
    - : unit = ()
    # let s1 = empty;;
    val s1 : int Stack.stack = {c = [25; 7]}
    # push s1 "Hello";;
    Error: This expression has type string but an expression was expected of type
              int

Apparently seems that I can have only a variable of type stack and any
other call to its constructor links the new variable to the old one. 
This means that once I have put an int inside I can't have a second
stack for characters or what else? This behavior is completely
unexpected and I can't explain it.

I'm sure I'm doing something wrong but I can't say what it is. Do you
have any idea about?

TIA
Walter

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module Stack = struct
  type 'a stack = { mutable c : 'a list }
  exception EmptyStackException
 
  let empty = { c = [] }

  let push s x = s.c <- x :: s.c
 
  let pop s =
    match s.c with
      hd::tl -> s.c <- tl
    | []     -> raise EmptyStackException
 
  let top s =
    match s.c with
      hd::_ -> hd
    | []    -> raise EmptyStackException
 
  let is_empty s = (s.c = [])
end;;