Syntax error at or near "array_append" in recursive function - sql

Question

I'm newbie in plpgsql. What is wrong in my function declaration?
It's rises the error:
ERROR: syntax error at or near "array_append". LINE 19:
array_append(parent.childs , get_childs( ROW( child.id_pk, c... ***
The function:
CREATE TYPE category_return AS (
id_pk INT,
parent_id_fk INT,
name varchar,
path ltree,
childs json[]
);
------------------------
CREATE OR REPLACE FUNCTION get_childs ( parent category_return )
RETURNS category_return AS
$BODY$
DECLARE
child record;
BEGIN
FOR child IN
SELECT * FROM categories c WHERE c.parent_id_fk = parent.id_pk
LOOP
array_append(parent.childs , get_childs( ROW( child.id_pk, child.parent_if_fk, child.name, child.path, '[]'::json[]) ) );
END LOOP
RETURN row_to_json(parent);
END;
$BODY$ LANGUAGE plpgsql;

Answer 1

The cause for the error at hand is that you would have to assign the result of the array_append() to a variable. Like:
some_var := array_append(parent.childs , get_childs( ... ));
But the whole function is twisted in various ways and still wouldn't work.
No mention of what you are trying to achieve, so here is an educated guess (updated with recursive version):
CREATE OR REPLACE FUNCTION get_children (_id_pk int)
RETURNS TABLE (id_pk int, parent_if_fk int, name text, path int[]) AS
$func$
WITH RECURSIVE cte AS (
SELECT c.id_pk, c.parent_if_fk, c.name, ARRAY[c.id_pk] AS path
FROM categories c
WHERE c.parent_if_fk = $1
UNION ALL
SELECT c.id_pk, c.parent_if_fk, c.name, cte.path || c.id_pk
FROM cte
JOIN categories c ON c.parent_if_fk = c.id_pk
)
SELECT *
FROM cte
ORDER BY path
$func$ LANGUAGE sql;
Returns all rows in the hierarchy below the starting ID.
Use a recursive CTE in a plain SQL function. No plpgsql necessary.