Feature #16822

Array slicing: nils and edge cases

Added by zverok (Victor Shepelev) 2 months ago. Updated about 2 months ago.

Target version:


(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.,

ary = [1, 2, 3]
  • 1. Non-empty slice--how it works currently
a[1..2] # => [2, 3]
a[1...-1] # => [2]
  • 2. Empty slice--how it works currently
a[1...1] # => []
a[3...] # => []
a[-1..-2] # => [] 
  • 3. Sudden nilwhat I am proposing to change
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.


Updated by sawa (Tsuyoshi Sawada) 2 months ago

  • Description updated (diff)

Updated by shevegen (Robert A. Heiler) 2 months ago

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

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

Updated by Dan0042 (Daniel DeLorme) 2 months ago

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

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.

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.


Updated by zverok (Victor Shepelev) 2 months ago

  • Description updated (diff)

Updated by marcandre (Marc-Andre Lafortune) 2 months ago

I'm strongly against this, for compatibility reasons and because current choice is a consistent convention.

Before proposing any incompatible change, especially for an API that is very much in use, please provide a compelling use case. If you write ary[1..].reduce { }, you must give a context (what contains ary, why would you want to skip the first value, why not use values_at(1..), etc.).

Updated by matz (Yukihiro Matsumoto) about 2 months ago

I don't think the benefit of changing outweighs the pain of incompatibility. Rejected.



Updated by mrkn (Kenta Murata) about 2 months ago

  • Status changed from Open to Rejected

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