(*---------------------------------------------------------------------*)
(* define pi as a constant *)
(*---------------------------------------------------------------------*)
val pi = 3.14159;

(*---------------------------------------------------------------------*)
(* convert yards to feet *)
(*---------------------------------------------------------------------*)
fun yards2feet (yards:real) =
   yards * 3.0;

(*---------------------------------------------------------------------*)
(* square a real number *)
(*---------------------------------------------------------------------*)
fun square_real (x:real) =
   x*x;

(*---------------------------------------------------------------------*)
(* convert square feet to acres *)
(*---------------------------------------------------------------------*)
fun feetsq2acres (feetsq:real) =
   feetsq / 43560.0;

(*---------------------------------------------------------------------*)
(* compute the area of a circle in acres, given its diameter in yards  *)
(*---------------------------------------------------------------------*)
fun circle_area1 (diameter_yds:real) =
   feetsq2acres ( pi * square_real ( yards2feet ( diameter_yds/2.0 )));

(*---------------------------------------------------------------------*)
(* compute the area of a circle in acres, given its diameter in yards  *)
(*---------------------------------------------------------------------*)
fun circle_area2 (diameter_yds:real) =
let
   val radius_yds = diameter_yds / 2.0;
   val radius_ft = yards2feet(radius_yds);
   val radius_sq = square_real(radius_ft);
   val area_ftsq = pi * radius_sq;
in
   feetsq2acres (area_ftsq)
end;


(*---------------------------------------------------------------------*)
(* compute the area of a rectangle in acres, given its dimensions in *)
(* yards  *)
(*---------------------------------------------------------------------*)
fun rect_area (width:real, length:real) = 
let
   val width_ft = yards2feet(width);
   val length_ft = yards2feet(length);
   val area_ftsq = width_ft * length_ft;
in
   feetsq2acres (area_ftsq)
end;