(* Code of Figure 9.2, page 365 from
   Kenneth C. Louden, Programming Languages
   Principles and Practice 2nd Edition
   Copyright (C) Brooks-Cole/ITP, 2003
   (Test code from page 366 added, too.)
*)

abstype Complex = C of real * real
with
	fun makecomplex (x,y) = C (x,y);
	fun realpart (C (r,i)) = r;
	fun imaginarypart (C (r,i)) = i;
	fun +: ( C (r1,i1), C (r2,i2) ) = C (r1+r2, i1+i2);
	infix 6 +: ;
	(* other operations *)
end;

val z = makecomplex (1.0,2.0);
val w = makecomplex (2.0,~1.0); (* ~ is negation *)
val x = z +: w;
realpart x;
imaginarypart x;