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 <[email protected]>:
> 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
>
>
--
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
