Postgres 11

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CREATE TABLE reps (id int, fname text, lname text, fullname text);
INSERT INTO reps VALUES (1, 'jane', 'fonda', 'jane fonda'), (1, 'john', 'doe', 'john doe');

CREATE FUNCTION fullname(reps)
  RETURNS text AS
$func$
   SELECT concat_ws(' | ', $1.fname, $1.lname);  -- no outer parentheses needed
$func$  LANGUAGE sql STABLE;  -- can be stable

CREATE TABLE

INSERT 0 2

CREATE FUNCTION

SELECT r.fullname FROM reps r;

fullname

jane fonda

john doe

SELECT 2

SELECT fullname(r) FROM reps r;

fullname

jane | fonda

john | doe

SELECT 2

SELECT function_misspelled(r) FROM reps r;

ERROR:  function function_misspelled(reps) does not exist
LINE 1: SELECT function_misspelled(r) FROM reps r;
               ^
HINT:  No function matches the given name and argument types. You might need to add explicit type casts.

SELECT r.column_misspelled FROM reps r;

ERROR:  column r.column_misspelled does not exist
LINE 1: SELECT r.column_misspelled FROM reps r;
               ^