how is function() supposed to work?

[DryPringleSoup]

This is what I've tried so far and the documentation doesn't really help much either

[hanna-kn]

function(total(\x), [1, 2], 0)
function() was not updated with support for use of x, y, and z without backslash (as elsewhere), and vector as single argument required use of a second bogus argument (otherwise the vector were interpreted as two separate arguments).

This has now been fixed (which makes function(total(x), [1 2]) a functional example) in commits 4ee767b and 851c3e1. An example was also added.

Functionally similar expressions (although in this simple example it would of course be easier to use total() directly instead):
replace(total(x), x, [1 2])
total(x) where x=[1 2]