-- File stacks.ads
--  Specification for a simple stack, implemented as a linked list
--  The stack is intended to be used in lab4, to help implement an
--  infix to postfix algorithm
package Stacks is

  type Stack is private;                                          

-- constructors

  procedure Copy (Source : in Stack;
                  Destination : in out Stack);

  procedure Clear (AStack : in out Stack);

  procedure Push (Item : in Character;
                  OnStack : in out Stack);

  procedure Pop (AStack : in out Stack);
                  
-- selectors

  function Equal (Left, Right : in Stack) return Boolean;

  function Depth (OfStack : in Stack) return Natural;

  function Empty (AStack : in Stack) return Boolean;

  function Top (OfStack : in Stack) return Character;
                 
-- exceptions

  Overflow, Underflow : exception;

-- hidden part

private
  type Node;
  type Stack is access Node;
  type Node is
     record
       Datum : Character;
       Tail : Stack;
     end record;

end Stacks;