I am going through a tutorial on q. Here is the initial setup.
buys:2 1 4 3 5 4
sells:2 4 3 2
I then defined the following function
f:{x & (sums buys)}
I tried to use it in
f (sums sells)
But I get the following error
'length
[1] f:{x & (sums buys)}
I note that if instead I do this, it works
q)f each (sums sells)
2 2 2 2 2 2
2 3 6 6 6 6
2 3 7 9 9 9
2 3 7 10 11 11
However, the following unary function outputting a list has no problem being applied
q){(x*x;x)} (1 2 3)
1 4 9
1 2 3
The type signature of the 2 functions seems to be the same to me. Why does one get evaluated while the other raises an error?
It's detailed somewhat in q for mortals here: https://code.kx.com/q4m3/4_Operators/#45-greater-and-lesser
Specifically "Being atomic they operate item-wise on lists...."
Generally, kdb/q wants to do list operations itemwise, e.g.
q)1 2+2 3
3 5
If you give it lists of different length:
q)1 2+1 2 3
'length
[0] 1 2+1 2 3
^
it doesn't know if you want to do
q)1 2+\:1 2 3
2 3 4
3 4 5
or
q)1 2+/:1 2 3
2 3
3 4
4 5
Same applies to & in your case.
Your other function is not comparable because the only itemwise operation it does x*x is on lists of the same length. So that's ok.