Re: [Caml-list] polymorphic stack

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

Hello,
thanks a lot you got the point; I will never see that empty wasn't a
function but just a constant name. Thanks also for the clear
explanation.

Cheers
Walter

On Sat, 24 Sep 2011, pierrchp@free.fr wrote:

> 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
>>
>>
>

--