[ruby-core:98097] [Ruby master Feature#16822] Array slicing: nils and edge cases

Issue #16822 has been updated by shevegen (Robert A. Heiler).


I do not have a strong preference here either way; I guess one can reason in
favour for both behaviour types/styles, and I think a primary point in the 
suggestion is that it refers to startless/endless situations, such as "5..",
which I don't use myself, but one slight concern is this one:

    a[-1..-2] # => [] 
    a[-10..-9] # => nil 

Is this certain to not break a lot of code? I have not checked myself and I 
rarely use #slice anyway, but I do use a lot of [] in general. It's one of
my favourite method calls in general, in ruby. :)

Admittedly I actually don't remember off-hand having ever used two negative
indices here ... for some reason, I seem to use 0 or positive numbers a lot
more.

No idea how/if other ruby users use or rely on that behaviour though but I
think it would be important to get some specific overview about any potential
effect (or side-effect) of proposed changes, even if the reasoning given is
ok.

----------------------------------------
Feature #16822: Array slicing: nils and edge cases
https://bugs.ruby-lang.org/issues/16822#change-85332

* Author: zverok (Victor Shepelev)
* Status: Open
* Priority: Normal
----------------------------------------
(First of all, I understand that the proposed change can break code, but I expect it not to be a large amount empirically.)

I propose that methods that slice an array (`#slice` and `#[]`) and return a sub-array in the normal case, should **never** return `nil`. E.g.,

```ruby
ary = [1, 2, 3]
```

* 1. Non-empty slice--how it works currently

```ruby
a[1..2] # => [2, 3]
a[1...-1] # => [2]
```

* 2. Empty slice--how it works currently

```ruby
a[1...1] # => []
a[3...] # => []
a[-1..-2] # => [] 
```

* 3. Sudden nil--what I am proposing to change

```ruby
a[4..] # => nil 
a[-10..-9] # => nil 
```

I believe that it would be better because the method would have cleaner "type definition" (If there is nothing in the array at the requested address, you'll have an empty array).

Most of the time, the empty array doesn't require any special handling; thus, `ary[start...end].map { ... }` will behave as expected if the requested range is outside of the array boundary.

It is especially painful with off-by-one errors; for an array of three elements, if `ary[3...]` (just outside the boundary) is `[]` while `a[4...]` (one more step outside) is `nil`, it typically results in some nasty `NoMethodError for NilClass`.

A similar example is `ary[1..].reduce { }` (everything except the first element--probably the first element was used to construct the initial value for reducing) with `ary` being non-empty 99.9% of the times. Then you meet one of the 0.1% cases, and instead of no-op reducing nothing, `NoMethodError` is fired.



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