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Date: Fri, 22 Jul 2005 11:58:43 -0700
From: Stephane Glondu <Stephane.Glondu@crans.org>
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To: Christophe Dehlinger <christophe.dehlinger@ifp.fr>
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Subject: Re: [Caml-list] How to do this properly with OCaml?
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Hi,

Christophe Dehlinger wrote:
> I have my own array-related problem, so I'll plug it here :)
> A couple of times, I would have liked to build arrays of functions whose
> body uses the array itself, like:
> 
> #let rec foo = Array.init 5 (fun n () -> if n=0 then 0 else foo.(n/2) ())
> This kind of expression is not allowed as right-hand side of `let rec'
> 
> Why is this kind of construct forbidden ?

In general, you cannot define a recursive value as a result of a
computation which uses the being-defined value. Intuitively, I explain
it in that way: here, the second argument references sth which is
created by Array.init itself, so the compiler has to know the result of
the call before compiling the argument function, which is clearly
impossible.

You have to do sth like this:

let foo =
  let bar = Array.make 5 (fun () -> 0) in
  for n = 1 to 4 do
    bar.(n) <- (fun () -> bar.(n/2) ())
  done;
  bar;;

However, you can do this:

let rec foo = [| (fun () -> 0); (fun () -> foo.(0) ()) |];;

(here, the array is created by the compiler itself, so it knows its
location).

I hope this is clear.


Stephane Glondu.