select case p1 when 't' then 'yes' else 'no'    end
 from t_cast ;

create function bool_to_char(val boolean) returning varchar( 5 )
if val then
     begin
       return 'y';
     end;
   else
      return 'n';
    end if;
end function;

CREATE IMPLICIT CAST (boolean AS varchar with bool_to_char) 

echo "Create temp table t_cast (p1 boolean) with no log;
insert into t_cast values ('t');
insert into t_cast values ('f');
select cast(p1 as varchar) from t_cast ;" |dbaccess bt

Database selected.
Temporary table created.
1 row(s) inserted.
1 row(s) inserted.
(expression)
y
n
2 row(s) retrieved.
Database closed.

echo "Create temp table t_cast (p1 boolean) with no log;
insert into t_cast values ('t');
insert into t_cast values ('f');
select p1||'' from t_cast ;" |dbaccess bt

Database selected.
Temporary table created.
1 row(s) inserted.
1 row(s) inserted.
(expression)  y
(expression)  n
2 row(s) retrieved.
Database closed.