Array subtraction after select?

[philomory]

I'm trying to do array subtraction after select, and I'm getting some fairly bewildering results. I've managed to reduce the example down to a very minimal one (it doesn't even actually use select anymore), but, I'm still not 100% sure if I've found a bug, or if I'm just failing to understand what I'm looking at. Can anyone clarify?

If you start with an array, and you subtract another array, you get exactly (and only) the results you'd expect, e.g.:

$ cat foo.yml
- a
- b
$ yq e '. - ["b"]' foo.yml
- a

But if you unwrap and later rewrap the array (perhaps to use a select), and then perform array subtraction, you get... the results you'd expect, sort of, except in place of every removed array element, there's a pair of brackets as if an empty array went there. For example:

cat foo.yml
- a
- b
$ yq e '.[] | [.] | . - ["b"]' foo.yml
- a
[]

(Obviously in a real use case you'd never literally write .[] | [.], but that's the minimal reproduction; the real use case it more like .[] | select(...) | [.])

Can anyone explain this behavior?

[philomory]

Nevermind, I figured out that the correct syntax would be [.[]] or, more specifically, [.[] | select(...)]. .[] | [.] doesn't give you ["a","b"], it gives you ["a"] and ["b"] as separate items, so when you do array subtraction afterwards you are left with ["a"]" and [].