Re: Whether to raise exception or End_of_List function

[Mark Biggar <mark.a.biggar@home.com>]

Preben Randhol wrote:
> 
> Hi
> 
> I have made a simple double linked list that I use in the application I
> develop. Before somebody suggest I use some other libraries I want to
> say that I made this double linked list to learn Ada 95 and get aware of
> different problems/aspects of implementing this, so at least for this
> application I want to keep it for now.
> 
> The list code can be viewed here: http://www.pvv.org/~randhol/Ada95/List/
> Note that the code is still rough, I haven't removed some debugging
> functions/procedures etc...
> 
> I put this into a package:
> 
>    package Examine_List is
>          new Double_Linked_List (Data_Type => Natural);
> 
>    Main_Examine : Examine_List.List_Type := Examine_List.New_List;
> 
> And use the Main_Examine. So far it has worked nicely, but now I came to
> that at one place (at least) in my application I have to go from the
> current position of the iterator until the end of the list. I cannot
> know the current position of the list iterator so I cannot use a
> for-loop. If I want to use a while loop however there are the problem of
> knowing when the end of the list is reached. I guess I can make a
> function End_of_List (which must also work if one try to backwards in
> the list) or I can just raise an exception when the user tries to
> step off the list.
> 
> My question is which is the better solution; Exception or a End_of_List
> function and which would have fewer side effects?

Even though it may seem redundant, you should have both.  You need the
exception to handle illegal list usage notification.  But, as a general
rue of thumb for packages of this type, such a package should provide 
test functions that pre-test for exceptional conditions so a user can
program around the exception.

--
Mark Biggar
mark.a.biggar@home.com