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Hidden Data Structures

• Ada:

  package body localpoint is
     point: Point;
     procedure makePoint(x, y: Real); ...
     function x_coord return Real; ...
     function y_coord return Real; ...
  end localpoint

• Fun:

  value localpoint =
     let p: Point = ref((0,0)) in
     {makepoint = fun(x: Real, y: Real) p := (x, y),
        x_coord = fun() fst(p)
        y_coord = fun() snd(p)}

• First-order information hiding:
  • Use let construct to restrict scoping at value level (hide record components).