On a page https://shakti.com/tutorial/ I've found the solution, it is
#:'z / counts each list
The same solution was mentioned in https://code.kx.com/v2/learn/startingkdb/language/ by switching to k mode in q:
q) #:'(1 2;"abc") / equivalent k expression
2 3
Why this expression #:' counts the number?
# counts' is an each Adverb: means in this case? This is not an assignment, right?On a page http://www.math.bas.bg/bantchev/place/k.html they mentioned that:
:within|:is used to force the verb|to be interpreted as a monad, as by default ambiguities are resolved in favour of dyads
Also here http://web.archive.org/web/20050504070651/http://www.kx.com/technical/documents/kreflite.pdf noted about the same:
Note that whenever Each is applied to the monad of a primitive verb, as in
!:'for Enumerate-Each, the monadic case must be made explicit by modifying the verb with colon. The dyadic case is assumed if no modifier is present.
And that's make sense:
/ # want's to act as dyadic verb
#' (1 2; "abc")
#'[(1 2;"abc")]
/ make use of dyadic # behavior
5 6 #' (1 2; "abc")
(1 2 1 2 1;"abcabc")
/ monadic case
#:' (1 2; "abc")
2 3