Issue #16822 has been updated by Dan0042 (Daniel DeLorme).


Slicing returns nil when the index is out of bounds, and that can be a useful signal that something is wrong and we should fail fast. Having that nil return value provides information that is not present if it's auto-converted to an empty array, and it's easy to disregard that information by using `.to_a`

`arr[1..]`
Takes all items after the first one, but if there's no first item it can be argued this is an invalid input and returning nil is safer (fail fast) than pretending everything is as expected.

`arr[-5..-1]`
Takes the last 5 items but if the array has less than 5 items it's an invalid input and we return nil (unlike `arr.last(5)`). If this proposal is accepted I'm not sure that returning an empty array makes sense here.

Now, all that being said... personally I don't remember ever having depended on array slicing returning a nil for out-of-bounds checking, but I _do_ remember adding a bunch of `.to_a` or `&.each` to my code to account for this case. So I am tentatively positive about the idea. But caution is required.

----------------------------------------
Feature #16822: Array slicing: nils and edge cases
https://bugs.ruby-lang.org/issues/16822#change-85373

* 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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