Hi Tom,
As per Oracle docs on CAST function,

http://docs.oracle.com/javadb/10.8.3.0/ref/rrefsqlj33562.html

A BOOLEAN value can be cast explicitly to any of the string types. The result is 'true', 'false', or null. Conversely, string types can be cast to BOOLEAN. However, an error is raised if the string value is not 'true', 'false', 'unknown', or null. Casting 'unknown' to boolean results in a null value.

But,

When I tried to do so it will throw an error

DECLARE
V_FLAG BOOLEAN := TRUE;
V_RESULT VARCHAR2(10);
BEGIN

SELECT CAST(v_flag AS VARCHAR(10))
INTO V_RESULT
FROM DUAL;

DBMS_OUTPUT.put_line('Value for Flag is '||V_RESULT);
END;
/

Error report:
ORA-06550: line 6, column 19:
PLS-00382: expression is of wrong type
06550. 00000 - "line %s, column %s:\n%s"
*Cause: Usually a PL/SQL compilation error.
*Action:

Would you please reach out whether there is deficiency in oracle Docs or the concept I understand.
Please acknowledge,

Thanks,
Amar Borishaha

my_varchar_variable := case when boolean_var then 'true' else 'false' end;