[Caml-list] polymorphic stack

[Caml-list] polymorphic stack

[Walter Cazzola]

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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;;

Re: [Caml-list] polymorphic stack

[pierrchp]

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
>
>

Re: [Caml-list] polymorphic stack

[Walter Cazzola]

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
>>
>>
>

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