CREATE OR REPLACE PACKAGE pkg 
   AUTHID DEFINER 
IS 
   FUNCTION b 
      RETURN BOOLEAN; 
END; 

CREATE OR REPLACE PACKAGE BODY pkg 
IS 
   FUNCTION b 
      RETURN BOOLEAN 
   IS 
   BEGIN 
      RETURN TRUE; 
   END; 
END; 

SELECT pkg.b () b FROM DUAL 

CREATE OR REPLACE PACKAGE pkg 
   AUTHID DEFINER 
IS 
   c_true    CONSTANT VARCHAR2 (1) := 'T'; 
   c_false   CONSTANT VARCHAR2 (1) := 'F'; 
 
   FUNCTION b 
      RETURN BOOLEAN; 
 
   FUNCTION b_in_sql 
      RETURN VARCHAR2; 
END; 

CREATE OR REPLACE PACKAGE BODY pkg 
IS 
   FUNCTION b 
      RETURN BOOLEAN 
   IS 
   BEGIN 
      RETURN TRUE; 
   END; 
 
   FUNCTION b_in_sql 
      RETURN VARCHAR2 
   IS 
   BEGIN 
      RETURN CASE b () 
                WHEN TRUE THEN c_true 
                WHEN FALSE THEN c_false 
                ELSE NULL 
             END; 
   END; 
END; 

SELECT pkg.b_in_sql () b FROM DUAL