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

[Jon Harrop <jdh30@cam.ac.uk>]

On Thursday 29 April 2004 4:00 am, skaller wrote:
> On Thu, 2004-04-29 at 11:56, Jon Harrop wrote:
> > Can you give me a simple example where you are forced to use mutable
> > state?
> ...
> The inverse is a supplier of list elements:
>
> let make_supplier lst =
>   let lst = ref lst in
>   let get () = function
>
>   | h :: t -> lst :=t; h
>   | [] -> raise Not_found

But can't this be implemented without mutable state as:

let make_supplier lst =
  let rec get lst () = match lst with [] -> raise Not_found | h::t -> (h, fun 
() -> get t ()) in
  get lst

so it returns the next element and a function to "supply" the rest.

> ...
> let iter f = function
> | h::t -> f h; iter f t
> | [] ->()
>
> This is the desirable form.

Why is that desirable? To me, it looks as though this form is necessarily more 
verbose and, therefore, undesirable.

> Traditionally, algorithms had to be factored
> so one of the cooperating pair of routines
> had to be unnatural: one had to be master,
> and the other slave.

Is mutual recursion not an example of having two "master" functions.

> Felix control inverts arbitrary procedural code
> mechanically: that is, you write code that reads
> data and it is translated into a callback.

What is your favourite example of this code morphing being useful?

> Wouldn't you rather program an *active* agent
> for your complex GUI widgets that a passive callback
> driven one?

I don't know. I just brush my hands over the keyboard and programs come 
out. :-)

Cheers,
Jon.

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