Is it possible to write a function in postgresql which directly writes its parameters to a text file?
Using COPY, this boils down to one simple SQL command:

COPY (SELECT 'a','b','c','d','e') TO PROGRAM 'cat &gt;&gt; /path/to/file/my.csv'; 


You can wrap it into a function if required:

CREATE OR REPLACE FUNCTION public.f_dump_row(varchar,varchar,varchar,varchar,varchar)
  RETURNS void LANGUAGE plpgsql AS
$func$
BEGIN
EXECUTE format($$COPY (SELECT %L,%L,%L,%L,%L) TO PROGRAM 'cat &gt;&gt; /path/to/file/my.csv'$$
             , $1,$2,$3,$4,$5); 
END
$func$;


Why the complication with EXECUTE?


How to set column DEFAULT using value of a variable


cat is a standard UNIX utility (shell command).

Your Postgres role needs appropriate privileges. The manual:


  COPY naming a file or command is only allowed to database superusers
  or users who are granted one of the default roles
  pg_read_server_files, pg_write_server_files, or
  pg_execute_server_program, since it allows reading or writing any file
  or running a program that the server has privileges to access.


If you need this for under-privileged roles, consider a SECURITY DEFINER function. But do it properly as instructed in the manual to avoid misuse.

And the target file must be writable by the system user running the Postgres process, typically postgres.

Related:


copy command in postgresql to append data