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iter: use Option::map() in struct Iterater::map() #22958

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Mar 4, 2015
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iter: use Option::map() in struct Iterater::map() #22958

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19 changes: 3 additions & 16 deletions src/libcore/iter.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -1565,24 +1565,13 @@ pub struct Map<I, F> {
f: F,
}

impl<I: Iterator, F, B> Map<I, F> where F: FnMut(I::Item) -> B {
#[inline]
fn do_map(&mut self, elt: Option<I::Item>) -> Option<B> {
match elt {
Some(a) => Some((self.f)(a)),
_ => None
}
}
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<B, I: Iterator, F> Iterator for Map<I, F> where F: FnMut(I::Item) -> B {
type Item = B;

#[inline]
fn next(&mut self) -> Option<B> {
let next = self.iter.next();
self.do_map(next)
self.iter.next().map(|a| (self.f)(a))
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Can this just be replaced with self.iter.next().map(self.f)?

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self.f can not be moved.
if we have "impl FnMut for &FnMut", then we can use &self.iter.next().map(&self.f)

I'm curious that it is not implemented yet.
I'm also curious that fn(Arg) is not trait Fn(Arg). If it is implemented, it also helps (in other code).

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I believe this should be possible with self.iter.next().map(&mut self.f). The fn types are distinct from the Fn traits because fn is a specific type for a function pointer (but implements the Fn trait).

We cannot implement FnMut for &FnMut, but it is implemented for &mut FnMut (as the mutable borrow is required).

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Due to the auto implmeting among the Fn, FnMut, FnOnce, we have some constrains.

Among these 3 types: &mut T where T: Fn, &mut T where T:FnMut, &mut T where T:FnOnce, we can only impl one Fn* trait to them, otherwise they will conflict. The only choice is impl FnMut for &mut T where T:FnMut, other choices make no sense.

The same, Among these 3 types: &T where T: Fn, &T where T:FnMut, &T where T:FnOnce, we can only impl one Fn* trait to them, otherwise they will conflict. The best choice is impl Fn for &T where T:Fn, other choices will cover less situation.

Could I implement the above?

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Ah sorry I was under the impression that this was possible when it in fact isn't! I've opened #23015 on the topic.

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Ok, I will leave this and look forward to your implementing.

}

#[inline]
Expand All @@ -1597,8 +1586,7 @@ impl<B, I: DoubleEndedIterator, F> DoubleEndedIterator for Map<I, F> where
{
#[inline]
fn next_back(&mut self) -> Option<B> {
let next = self.iter.next_back();
self.do_map(next)
self.iter.next_back().map(|a| (self.f)(a))
}
}

Expand All @@ -1613,8 +1601,7 @@ impl<B, I: RandomAccessIterator, F> RandomAccessIterator for Map<I, F> where

#[inline]
fn idx(&mut self, index: usize) -> Option<B> {
let elt = self.iter.idx(index);
self.do_map(elt)
self.iter.idx(index).map(|a| (self.f)(a))
}
}

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