Re: [Caml-list] question: restrictions on `let rec`

[Goswin von Brederlow <goswin-v-b@web.de>]

On Tue, Jul 12, 2016 at 09:37:27PM -0000, Hongbo Zhang (BLOOMBERG/ 731 LEX) wrote:
> Hi all,
>    I have a question simplified as below:
> 
>  let rec fib = fun x -> fib (x - 1);;                                                                                                                       
> val fib : int -> 'a = <fun>                                                                                                                                  
> # let rec fib = (Obj.magic (fun x -> fib (x - 1)) : int -> 'a);;                                                                                             
> Characters 15-47:                                                                                                                                            
>   let rec fib = (Obj.magic (fun x -> fib (x - 1)) : int -> 'a);;                                                                                             
>                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^                                                                                                            
> Error: This kind of expression is not allowed as right-hand side of `let rec'  
> 
> I know `fib` is a legal expression, is there any way to work around it?
>  Thanks -- Hongbo

The problem is in the evaluation order:

    let rec fib = fun x -> fib (x - 1);;

is a function that will be evaluated when a parameter is passed. At
that later date "fib" is well defined.

    let rec fib = (Obj.magic (fun x -> fib (x - 1)) : int -> 'a);;

Here fib is a value and Obj.magic will be evaluated right there. At
this point "fib" is not yet well defined, you are still defining it.
"fib" is not legal there.

What you have to do is make it a function again:

# let rec fib = fun x -> (Obj.magic (fun x -> fib (x - 1)) : int -> 'a) x;;
val fib : int -> 'a = <fun>

MfG
	Goswin