If nested subprograms in the same PL/SQL block invoke each other, then one requires a forward declaration, because a subprogram must be declared before it can be invoked.

A forward declaration declares a nested subprogram but does not define it. You must define it later in the same block. The forward declaration and the definition must have the same subprogram heading.

In Example 9-8, an anonymous block creates two procedures that invoke each other.

DECLARE
  -- Declare proc1 (forward declaration):
  PROCEDURE proc1(number1 NUMBER);

  -- Declare and define proc2:
  PROCEDURE proc2(number2 NUMBER) IS
  BEGIN
    proc1(number2);
  END;

  -- Define proc 1:
  PROCEDURE proc1(number1 NUMBER) IS
  BEGIN
    proc2 (number1);
  END;

BEGIN
  NULL;
END;
/