How to CAST empty array value of an ANYARRAY-function to ANYARRAY? - arrays

Question

I am using pgv10. The function that I need seems this wrong function:
CREATE FUNCTION array_coalesce(ANYARRAY) RETURNS ANYARRAY AS $f$
SELECT CASE WHEN $1 IS NULL THEN array[]::ANYARRAY ELSE $1 END;
$f$ language SQL IMMUTABLE;
Curiosity
... I started to simplify a complex problem, and arrives in the test select coalesce(null::text[], array[]::text[]) that not worked... So it was a good question, how to implement it? But sorry, I do something workng, COALESCE(array,array) is working fine (phew!).
So, "coalesce problem" is merely illustrative/didatic. What I really want to understand here is: How to use ANYARRAY?
PS: other curiosity, the string concat(), || and other concatenation operators in PostgreSQL do some "coalescing",
select concat(NULL::text, 'Hello', NULL::text); -- 'Hello'
select null::text[] || array[]::text[]; -- []
select array[]::text[] || null::text[]; -- []

Answer 1

How to use anyarray?
It's an interesting issue, in the context of the usage described in the question. The only way I know is to use an argument as a variable. It's possible in plpgsql (not in plain sql) function:
create or replace function array_coalesce(anyarray)
returns anyarray as $f$
begin
if $1 is null then
select '{}' into $1;
end if;
return $1;
end
$f$ language plpgsql immutable;
select array_coalesce(null::int[]);
array_coalesce
----------------
{}
(1 row)
By the way, you can simply use coalesce() for arrays:
select coalesce(null::text[], '{}'::text[]);
coalesce
----------
{}
(1 row)