How to return argument data type from sql function

PostgreSQL 12.2+ function is defined as

     create FUNCTION torus(eevarus text) returns text immutable AS $f$
      select translate( $1, U&'\00f8\00e9', U&'\0451\0439' );
     $f$ LANGUAGE SQL ;

This function is called as CHAR(n) or text columns like

     create temp table test (
     charcol char(10),
     textcol text );

     insert into test values ('test', 'test');

     select torus(charcol), torus(textcol), charcol

torus(charcol) returns text column and loses original column width. How 
to force torus() to return argument type:

if char(n) column is passed as argument, torus() should also return 
char(n) data type.
I tried to use bpchar instead on text

     create or replace FUNCTION torusbpchar(eevarus bpchar) returns 
bpchar immutable AS $f$
      select translate( $1, U&'\00f8\00e9', U&'\0451\0439' );
     $f$ LANGUAGE SQL ;

torusbpchar(charcol) still returns text data type.

npgsql DataReader is used to get data.

Andrus.