Add `array` lang item and `[T; N]::map(f: FnMut(T) -> S)` #75212

JulianKnodt · 2020-08-06T07:53:27Z

This introduces an array lang item so functions can be defined on top of [T; N]. This was previously not done because const-generics was not complete enough to allow for this. Now it is in a state that is usable enough to start adding functions.

The function added is a monadic (I think?) map from [T; N] -> [S; N]. Until transmute can function on arrays, it also allocates an extra temporary array, but this can be removed at some point.

r? @lcnr

library/core/src/array/mod.rs

library/core/tests/array.rs

library/core/src/array/mod.rs

lcnr

The implementation looks good to me, leaving this to:

@rust-lang/lang for the new lang item array which allows us to implement methods for [T; N].
@rust-lang/libs for the new unstable method fn array::map.

pickfire · 2020-08-07T06:01:37Z

Looks like this may lead to impl Iterator for [T; N].

pickfire

Looks good to me. I wonder if the doc changes requires doc team to review. CC @jyn514

lcnr · 2020-08-07T06:06:34Z

Looks like this may lead to impl Iterator for [T; N].

I don't think we want to implement Iterator of arrays, but I am not really sure what you mean with "lead to" here. This seems more closely related to Option::map than Iterator.

We do want to implement IntoIterator(#65819) and FromIterator(#69985) for arrays but this method is still a lot more concise than arr.into_iter().map(func).collect::<Result<_, _>>().unwrap().

JulianKnodt · 2020-08-07T06:06:46Z

@pickfire ah I'm not sure if impl Iterator for [T; N] makes sense and it's really only map that is useful. fold doesn't require outputting to a fixed size(vector -> scalar sort of thing) so it can just be used with an into_iter(), and other things such as filter cannot know the output size. map is one of the few that makes a lot of sense because if we use a normal iterator we have to try_convert or other shenanigans to fixed size, which is annoying, so I think the whole point of map is to get around that.

library/core/src/array/mod.rs

bors · 2020-08-13T12:43:17Z

⌛ Testing commit af32db2 with merge a0c290f...

bors · 2020-08-13T15:03:32Z

☀️ Test successful - checks-actions, checks-azure
Approved by: LukasKalbertodt
Pushing a0c290f to master...

leonardo-m · 2020-08-15T08:28:56Z

I can't find one use case for this in my codebase :-(

JulianKnodt · 2020-08-15T08:43:24Z

Mm, one case that is particularly useful to me is in numerical computing and performing vectorized operations with fixed size arrays. In this case, it's nice to have a method that retains the size of the array but allows performing some operation on each item in the inner type.

I would keep it in mind when looking to operate on something that is a known size at compile that must be transmuted into something that is the same size. With that said, it's not useful for a lot of cases, because I imagine that if the code is not performance sensitive and/or must be dynamically allocated, it'll just use Vec, which this wouldn't apply to.

scottmcm · 2020-08-16T02:42:25Z

@leonardo-m Given how annoying to use arrays have historically been, I'm not that surprised.

This is one of those things that gets more useful as new things that produce arrays start showing up -- for example, from the array_chunks(_mut) iterators introduced in #74373 and #75021.

leonardo-m · 2020-08-16T07:51:55Z

In my Nightly-based codebase I have probably few thousand arrays, tiny ones, small ones, and larger boxed ones. And currently in about 100% of the case I don't want to create a new array from another one, but read the contents of an array or but mutate an existing array.

…-to-array, r=Mark-Simulacrum Add `[T; N]::as_[mut_]slice` Part of me trying to populate arrays with a couple of basic useful methods, like slices already have. The ability to add methods to arrays were added in rust-lang#75212. Tracking issue: rust-lang#76118 This adds: ```rust impl<T, const N: usize> [T; N] { pub fn as_slice(&self) -> &[T]; pub fn as_mut_slice(&mut self) -> &mut [T]; } ``` These methods are like the ones on `std::array::FixedSizeArray` and in the crate `arraytools`.

electroCutie · 2020-09-11T08:48:00Z

The function added is a monadic (I think?)

It is not Monadic. That would be more like mapping from elements intro arrays and then concatenating them as the result
This appears to be a Functor

Added [T; N]::zip() This is my first PR to rust so I hope I have done everything right, or at least close :) --- This is PR adds the array method `[T; N]::zip()` which, in my mind, is a natural extension to rust-lang#75212. My implementation of `zip()` is mostly just a modified copy-paste of `map()`. Should I keep the comments? Also am I right in assuming there should be no way for the `for`-loop to panic, thus no need for the dropguard seen in the `map()`-function? The doc comment is in a similar way a slightly modified copy paste of [`Iterator::zip()`](https://doc.rust-lang.org/beta/std/iter/trait.Iterator.html#method.zip) `@jplatte` mentioned in [rust-lang#75490](rust-lang#75490 (comment)) `zip_with()`, > zip and zip_with seem like they would be useful :) is this something I should add (assuming there is interest for this PR at all :))

Added [T; N]::zip() This is my first PR to rust so I hope I have done everything right, or at least close :) --- This is PR adds the array method `[T; N]::zip()` which, in my mind, is a natural extension to rust-lang#75212. My implementation of `zip()` is mostly just a modified copy-paste of `map()`. Should I keep the comments? Also am I right in assuming there should be no way for the `for`-loop to panic, thus no need for the dropguard seen in the `map()`-function? The doc comment is in a similar way a slightly modified copy paste of [`Iterator::zip()`](https://doc.rust-lang.org/beta/std/iter/trait.Iterator.html#method.zip) ``@jplatte`` mentioned in [rust-lang#75490](rust-lang#75490 (comment)) `zip_with()`, > zip and zip_with seem like they would be useful :) is this something I should add (assuming there is interest for this PR at all :))

Added [T; N]::zip() This is my first PR to rust so I hope I have done everything right, or at least close :) --- This is PR adds the array method `[T; N]::zip()` which, in my mind, is a natural extension to rust-lang#75212. My implementation of `zip()` is mostly just a modified copy-paste of `map()`. Should I keep the comments? Also am I right in assuming there should be no way for the `for`-loop to panic, thus no need for the dropguard seen in the `map()`-function? The doc comment is in a similar way a slightly modified copy paste of [`Iterator::zip()`](https://doc.rust-lang.org/beta/std/iter/trait.Iterator.html#method.zip) ```@jplatte``` mentioned in [rust-lang#75490](rust-lang#75490 (comment)) `zip_with()`, > zip and zip_with seem like they would be useful :) is this something I should add (assuming there is interest for this PR at all :))

Added [T; N]::zip() This is my first PR to rust so I hope I have done everything right, or at least close :) --- This is PR adds the array method `[T; N]::zip()` which, in my mind, is a natural extension to rust-lang#75212. My implementation of `zip()` is mostly just a modified copy-paste of `map()`. Should I keep the comments? Also am I right in assuming there should be no way for the `for`-loop to panic, thus no need for the dropguard seen in the `map()`-function? The doc comment is in a similar way a slightly modified copy paste of [`Iterator::zip()`](https://doc.rust-lang.org/beta/std/iter/trait.Iterator.html#method.zip) `@jplatte` mentioned in [rust-lang#75490](rust-lang#75490 (comment)) `zip_with()`, > zip and zip_with seem like they would be useful :) is this something I should add (assuming there is interest for this PR at all :))

…ds, r=dtolnay Add `[T; N]::each_ref` and `[T; N]::each_mut` This PR adds the methods `each_ref` and `each_mut` to `[T; N]`. The ability to add methods to arrays was added in rust-lang#75212. These two methods are particularly useful with `map` which was also added in that PR. Tracking issue: rust-lang#76118 ```rust impl<T, const N: usize> [T; N] { pub fn each_ref(&self) -> [&T; N]; pub fn each_mut(&mut self) -> [&mut T; N]; } ```

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Tracking Issue for array_map #75243

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bors added the merged-by-bors This PR was explicitly merged by bors. label Aug 13, 2020

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This was referenced Aug 13, 2020

Default for arrays via const generics #74254

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Add array_windows fn #75026

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Add [T; N]::each_ref and [T; N]::each_mut #75490

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Add [T; N]::as_[mut_]slice #76120

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lcnr added A-const-generics Area: const generics (parameters and arguments) F-const_generics `#![feature(const_generics)]` labels Nov 14, 2020

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Tracking issue for const generics (RFC 2000) #44580

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Added [T; N]::zip() #79451

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Add 'core::array::from_fn' and 'core::array::try_from_fn' #75644

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Add `array` lang item and `[T; N]::map(f: FnMut(T) -> S)` #75212

Add `array` lang item and `[T; N]::map(f: FnMut(T) -> S)` #75212

JulianKnodt commented Aug 6, 2020

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JulianKnodt commented Aug 7, 2020

bors commented Aug 13, 2020

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leonardo-m commented Aug 15, 2020

JulianKnodt commented Aug 15, 2020

scottmcm commented Aug 16, 2020

leonardo-m commented Aug 16, 2020

electroCutie commented Sep 11, 2020

Add array lang item and [T; N]::map(f: FnMut(T) -> S) #75212

Add array lang item and [T; N]::map(f: FnMut(T) -> S) #75212

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electroCutie commented Sep 11, 2020

Add `array` lang item and `[T; N]::map(f: FnMut(T) -> S)` #75212

Add `array` lang item and `[T; N]::map(f: FnMut(T) -> S)` #75212

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