create function problem - Mailing list pgsql-admin

I want to index my table using the year of the date. So I create the
following function.

create function to_year(date) returns varchar as '
declare
  str varchar;
  begin
    str := to_char($1, "YYYY");
    return str;
  end;
' language 'plpgsql' with (iscachable, isstrict);

But I face the problem as follow
select to_year(current_date);
WARNING:  Error occurred while executing PL/pgSQL functions to_year
WARNING:  line 4 at assignment
ERROR:  Attribute "YYYY" not found

Anybody can help? Thanks.

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