Re: [Caml-list] [ANN] The Missing Library

[John Goerzen <jgoerzen@complete.org>]

On Thu, Apr 29, 2004 at 10:54:09AM +1000, skaller wrote:
> > > if (!start <> 0) then print "]"
> > > 
> > > and please don't tell me that isn't a total mess compared with
> > 
> > Of course, your example ignores nicer features available for
> > this sort of thing. 
> 
> Indeed. If I may be so bold to observe .. you've control
> inverted. You're now driving the iteration. You've
> got rid of the HOF List.iter and implemented your own
> special one.

Isn't fold_left an iterator too, just one that works with slightly
different semantics?

Of course, your example could be simplified to:

let print_list l =
  let count = ref 0 in
  let printfunc item = 
    print_string (if !count = 0 then "[" else ";");
    print_string item; count := !count + 1 in
  List.iter printfunc l;
  if !count > 0 then print_string "]";;

> The point of the code I displayed was that it
> control inverts *without* getting rid of the 
> driver loop.
> 
> Also .. a small bug in your translation: it prints "[]"
> for an empty list. The original prints "".

No.  Again, my code was:

let print_list l =
  let printfunc count item =
    print_string (if count = 0 then "[" else ";");
    print_string item; count + 1 in
  if (List.fold_left printfunc 0 l) > 0 then print_string "]";;

Try it.  If the list is empty, printfunc is never called since there is
nothing to iterate over.  The return value from fold_left is zero since
the count was never incremented, so the if statement evalutes to false
and "]" is never called.

-- John

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