Re: Imperative programming in Caml

[Walid Taha <taha@cs.chalmers.se>]

Dear Markus,

Thanks for the nice rewrite, but it ain't purely functional still! :)

Your point is well-taken.  I *am* a functional programmer, though I
haven't posted to this list before.  I am interested in seeing how easy it
is to explain imperative concepts in Caml.  A bit to my surprise, it is
turning out to be more subtle than I thought.  In some cases, interesting
remedies might exits.  Your rewrite of the example is quite interesting
and I have noted it, but it still uses IO, which is imperative.

I'll summarize the discussions at some point and send them to the list.

Best regards,

Walid.

On Fri, 4 Aug 2000, Markus Mottl wrote:

> On Thu, 03 Aug 2000, Walid Taha wrote:
> > [Apologies in advance for purists that this project might offend.]
> 
> It is not about being "purist" - just about solving problems effectively...
> 
> > The presence of a "while" construct and easy of terminal IO in Caml
> > should help an imperative programmer feel at home.
> 
> I agree that "impurity" can come handy at times (but very seldom!).
> 
> > But I am concerned (and a bit surprised, actually) that the use of "let"
> > bindings and the presence of normal variables in addition to "mutable"
> > variables might make it more difficult to explain this program to a
> > beginer that is *not* interested in the functional aspects.
> 
> It is a pity that he is not interested in the functional aspects (why?)!
> If I wanted to confuse a beginner with a piece of code, I'd take one that
> is relatively long, does not use many abstractions and ... is imperative.
> 
> Yours (excuse my criticism, I hope you understand the positive intention)
> comes quite close to working as a repellent for beginners. - Even I have
> troubles understanding what problem the code tries to solve!
> 
> > If any one has suggestions for making this program more "imperative", I
> > would appreciate it.
> 
> If you absolutely need to make it imperative, please introduce some
> additional functions (procedures) with intuitiv names so that the meaning
> is clear.
> 
> In any case, the reason for using functional programming is not to annoy
> beginners: in fact, since functional programming is on a somewhat higher
> level compared to imperative programming, it is easier to reason about
> "what" to solve rather than "how", which is what a beginner should be
> concerned about. If he (she?) is already "biased" towards imperative
> programming, I'd rather ask for hints on how to explain the merits of
> functional style to motivate the beginner to try it.
> 
> Just compare the functional version with the imperative one (see below): I
> can impossibly imagine that anybody would rather feel attracted by the much
> more difficult imperative solution. I hope you'll succeed in "seducing"
> your beginner to the elegance of functional style!
> 
> Regards,
> Markus Mottl
> 
> ---------------------------------------------------------------------------
>   let rec read_ints () =
>     print_string "Enter a number: ";
>     let number = read_int () in
>     if number <> 0 then number :: read_ints ()
>     else []
> 
>   let squareMany () =
>     print_string "\nPlease enter zero (0) to stop.\n\n";
>     let lst = read_ints () in
>     print_string "Here are the squares of the numbers you entered: ";
>     List.iter (fun n -> print_int (n*n); print_char ' ') lst;
>     print_string "\n\nGood bye!\n\n"
> ---------------------------------------------------------------------------
> 
> > let squareMany () =
> >  print_string "\nPlease enter zero (0) to stop.\n\n"; 
> >  let finished = ref false 
> >  and list = ref Empty in 
> >  let here = ref list in
> >  while not(!finished) do
> >         print_string "Enter a number : ";
> >         let number = read_int () in
> >         if number<>0 
> >          then begin
> >                let new = ref Empty in
> >                !here := Cell (number, new);
> >                here := new;
> >               end
> >          else begin
> >                finished:=true;
> >               end 
> >        done;
> >  print_string "Here are the squares of the numbers you entered: ";
> >  while (!list)<>Empty do
> >        let (Cell(number, rest)) = !list in
> >            print_int (number*number);
> >            list := !rest;
> >            print_string " ";
> >        done;
> >  print_string "\n\nGood bye!\n\n";;
> 
> -- 
> Markus Mottl, mottl@miss.wu-wien.ac.at, http://miss.wu-wien.ac.at/~mottl
>