Ḣ - Head (First)
This atom is monadic, which means it takes one argument.
Pop and return the first element of a list (modifies the list).
If the list is empty, returns zero. This mutates the original object itself, so
1,2,3 Ḣ,⁸Ɗ yields
[1, [2, 3]], not
[1, [1, 2, 3]]. If you do not want to modify the list, you can access the first element using
1ị$; however, a clever trick you can employ is
ḷ/ (as long as your list isn't empty). This is technically less efficient because you reduce the whole list whereas random access is constant, but can save you a byte over
1ị$ if you would need the
$ to group those links.