Re: [Caml-list] polymorphic stack

[pierrchp@free.fr]

Hello,

Your problem here is that when you created your module stack, you wrote

   let empty = { c=[]} ;;

which creates a global variable. Writing

   let s = empty

creates a new stack by copying the value empty,  therefore putting a type
constraint, that s and empty must have the same type. In order to do what you
want, you need to reevaluate the expression { c=[]} every time you create a new
stack, and not just copy its result. This is achieved by using a function
instead of a global variable:

   let empty () = { c=[]}

an create a new stack with

   let s = empty ()

Hope it is not too confusing

Cheers

-Pierre

Selon Walter Cazzola <cazzola@dico.unimi.it>:

> 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
>
> --
> --
> Caml-list mailing list.  Subscription management and archives:
> https://sympa-roc.inria.fr/wws/info/caml-list
> Beginner's list: http://groups.yahoo.com/group/ocaml_beginners
> Bug reports: http://caml.inria.fr/bin/caml-bugs
>
>