############################## librep ĒŠ ############################## 
#!/usr/bin/rep
!#

(defun deriv (f)
    (let (
	    (dx 1e-12)
	)
	(lambda (x) (/
	    (- (f (+ x dx)) (f (- x dx)))
	    (* 2 dx)
	))
    )
)

(defun newton (f guess)
    (let (
	    (adjust (/ (f guess) ((deriv f) guess)))
	)
	(if (< (abs (/ adjust guess)) 1e-12)
	    guess
	    (newton f (- guess adjust))
	)
    )
)

(format standard-output "cubic root of 2 is: %d\n"
    (newton (lambda (x) (- (* x x x) 2)) 1.0)
)

############################## perl ĒŠ ############################## 
#!/usr/bin/perl -w

sub deriv {
    my ($f) = @_;
    my ($dx) = 1e-6;
    return sub { my($x)=@_; (&$f($x+$dx)-&$f($x-$dx))/2/$dx; }
}

sub newton {
    my ($f, $guess) = @_;
    my ($df, $adjust) = deriv($f);
    do {
	$adjust = &$f($guess) / &$df($guess);
	$guess -= $adjust;
    } while (abs($adjust/$guess) > 1e-6);
    return $guess;
}

printf("cubic root of 2 is: %f\n",
    newton(sub { my ($x)=@_; return $x*$x*$x-2; }, 1)
);