\ take a list (array) and flatten it:

: (flatten)  \ a -- a
	(
		\ is it a number?
		dup >kind ns:n n:= if
			\ yes.  so add to the list
			r> swap a:push >r
		else
			\ it is not, so flatten it
			(flatten)
		then
		drop
	) a:each drop ;
	
: flatten \ a -- a
	[] >r (flatten) r> ;

[[1], 2, [[3,4], 5], [[[]]], [[[6]]], 7, 8, []]
dup . cr
flatten
. cr
bye