> On 4 Jul 2017, at 16:05, Aleks-Daniel Jakimenko-Aleksejev via RT
> <perl6-bugs-follo...@perl.org> wrote:
> On 2017-07-04 05:29:20, comdog wrote:
>> Accessing a List element beyond the end of the List returns Nil,
>> although accessing an element before the beginning returns an out of
>> bounds failure. I think there's two things that can be better here
>> since we know the size of the List.
>>
>> my $list = < a b c >;
>> put "I have a {$list.^name}";
>>
>> First, in the "before" case, we have more information than the error
>> message lets on. The index should be from 0 to 2:
>>
>> {
>> my $i = -1;
>> $list[$i]; # Index out of range. Is: -1, should be in 0..^Inf
>> }
>>
>> But this requires the change I think is more helpful. Since the List
>> size won't change, we can have the same out-of-bounds error on
>> accesses past the end. At the moment it's no warning:
>>
>> {
>> my $i = $list.end + 1;
>> $list[$i]; # No warning
>> }
>>
>> This would then be the error for assigning into a position beyond the
>> end. The existing error doesn't say what went wrong even though Perl 6
>> has enough information to figure that out:
>>
>> {
>> my $i = $list.end + 1;
>> $list[$i] = 5; # Cannot modify an immutable Nil
>> }
> Hm. Wouldn't that make behavior of Lists and Arrays different?
No, because Lists are supposed to be immutable wrt to the number of elements.
That said, a List may not always be completely reified already. So logically,
a List may have 100 elements, it could well be that only 42 of these elements
exist already. Which means that the underlying NQP array, which *is* mutable,
only has 42 elements. But it cannot know offhand whether those are all
possible elements, as that depends on the iterator that is being used to fill
the NQP array.
A complicating factor is that this is a very hot code path, so any changes
there could affect general performance significantly. My initial tests to
generate a Failure on out of bounds value immediately results in 2 internal
errors trying to generate backtrace :-(
Anyways, I agree with brian’s feelings on this. The challenge is now to make
it so without making everything significantly slower.
FWIW, the code in question lives around line 480 in List.pm.
Liz
