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bluss opened this issue Jul 29, 2013 · 7 comments
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Implement RandomAccessIterator where possible #8108

bluss opened this issue Jul 29, 2013 · 7 comments
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C-enhancement Category: An issue proposing an enhancement or a PR with one.

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@bluss
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bluss commented Jul 29, 2013

Adaptors like Enumerate, Map, Zip, Take etc need to implement Random access.

I'm working on this.
@huonw
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huonw commented Jul 29, 2013

One has to be careful with Map because it will mean that the mapped function can get evaluated multiple times (if you access a specific element more than once).

@bluss
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bluss commented Jul 29, 2013

That's correct. Since the introduction of double-ended iterator, you can't be sure of the order the map function is invoked either (could be alternating front and back). I don't know how to best handle this.

@bluss
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bluss commented Jul 29, 2013

We could provide a separate adaptor that only takes an extern fn. That Map adaptor will be clonable and random access, and the standard one will not? In the end we don't have any purity guarantees either way.

@huonw
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huonw commented Jul 29, 2013

Not sure that such a restriction is worth it, either no functions or all functions (at least until we get an effects system); since there's no guarantee that extern fn's are idempotent/pure (e.g. fn increment_and_get_network_resource, or something using TLS), and many closure would be (e.g. |&x| x).

@bluss bluss mentioned this issue Jul 30, 2013
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bors added a commit that referenced this issue Jul 30, 2013
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auto merge of #8120 : blake2-ppc/rust/iterator-fixes, r=thestinger
e94e4d5 
Implement RandomAccessIterator (RAI) where possible, for Iterator adaptors such as Map, Enumerate, Peek, Skip, Take, Cycle, where the adapted iterator is already RAI, and for collections where it is relevant (ringbuf and bitv).

After discussion with thestinger, remove the RAI impl for VecMutIterator, we cannot soundly provide mutable access with this trait.

Implement Extendable everywhere FromIterator is already implemented.

Fixes issue #8108.
@emberian
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emberian commented Apr 7, 2014

Hasn't been fixed entirely, #8120 added a bunch of impls.

@bluss
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bluss commented Aug 24, 2014

"RAI" seems to never have reached usefulness. I'm still not sure if it should even be in rust.

@steveklabnik
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This ticket isn't particularly actionable, if anything is still missing, please open a new one with specifics, thanks.

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