Postgres - how to 'print' a variable within a function (similar to python)?

How to print(var) in postgres like python?

Given the following code:

CREATE OR REPLACE FUNCTION custom_sum(c1 integer, c2 integer)RETURNS integer AS $$    SELECT c1 + c2;$$ LANGUAGE SQL;CREATE aggregate agg_custom_sum(integer) (    sfunc = custom_sum,    stype = integer,    initcond = 0);DROP TABLE IF EXISTS aggcheck;CREATE TABLE aggcheck AS SELECT x FROM generate_series(1, 3) AS g(x);SELECT agg_custom_sum(x) FROM aggcheck;

How can I print the values of c1, c2 to the terminal when running?

Eg (I'm aware this won't work - it's python syntax - hopefully communicates the sort of usage I'm after though):

CREATE OR REPLACE FUNCTION custom_sum(c1 integer, c2 integer)RETURNS integer AS $$    print(f"c1 = {c1}, c2 = {c2}");     SELECT c1 + c2;$$ LANGUAGE SQL;

If it makes much difference - I'm calling this using \i script.sql from within a psql session.

If there are any links to debugging approaches more generally (eg breakpoing, try/catch, writing to a file) they would be appreciated, though not necessary to answer this question.