What is the difference between a functional apply approach .[0!y;();xcols x], seen in xkey:
xkey
k){(#x)!.[0!y;();xcols x]}
and a simple function calling: x xcols 0!y.
Why is it the function apply preferred in xkey?
And a second question about a call to xcols from the inside of k) context - how is it ever works? I could not do the same for some reason:
t:([]a:`a`s`d;b:1 2 3;c:4 5 6)
k).[0!t;();xcols `a`b]
ERROR: 'xcols
(attempt to use variable xcols without defining/assigning first (or user-defined signal))
I think the functional apply may be for when passing the table by name
t:([]a:`a`s`d;b:1 2 3;c:4 5 6)
.[0!t;();xcols[`a`b]] /this works
`a`b xcols 0!t /equivalent non functional form works
q)`a`b xcols 0!`t /this fails when passing `t
'type
[0] `a`b xcols 0!`t
q).[0!`t;();xcols[`a`b]] /though functional form still works with `t
q)`a`b xkey `t /hence you can use xkey in place
`t
Edit: To include my comment in the answer for completeness, when inside k you should prefix q commands with .q to access them, as this is the namesapce they reside in
q)xcols[`a`b]
k){(x,f@&~(f:cols y)in x)#y}[`a`b]
q)\
.q.xcols`a`b
k){(x,f@&~(f:cols y)in x)#y}[`a`b]
Edit 2: To address your comment, xcols for your example comes down to
`a`b`c#t
And
`a`b`c#`t
doesn't work. I don't have much of an explanation for why it doesn't work, except for that is how it is designed. The functional form is telling q to modify the table by applying the take function. But the example above is not an accepted syntax for redefining a table as a re-ordering of its columns.