Add `empty` for `AbstractArray`s and their types #33482

janEbert · 2019-10-06T14:06:17Z

empty now works on all AbstractArrays and the corresponding call
using Type{<:AbstractArray}.
The method specialized on AbstractVector remains for inlining
purposes. I'm not sure whether that is actually necessary but I'm sure you can tell. (And maybe also explain how to test for that myself. That would be amazing!)

`empty` now works on all `AbstractArray`s and the corresponding call using `Type{<:AbstractArray}`. The method specialized on `AbstractVector` remains for inlining purposes.

rfourquet · 2019-10-06T14:28:27Z

The motivation to make empty work on types (instead of instances) is not clear here. Also, empty is defined for other kind of containers, so the same would have to be done for those too.

janEbert · 2019-10-06T17:09:21Z

Assume you have a Vector{Vector{Int}} of sequence data. You cannot construct an empty element for it using eltype.
Sorry, not looking for other methods of empty was a silly oversight of me. I will fix it.

rfourquet · 2019-10-06T17:28:19Z

You cannot construct an empty element for it using eltype.

At least In this particular case you can do eltype(Vector{Vector{Int}})()

Sorry, not looking for other methods of empty was a silly oversight of me. I will fix it.

It's a good starting point, and I would wait for more feedback before spending time on updating this PR to other methods.

janEbert · 2019-10-06T17:49:43Z

Hehe, you are right. I was just confused when it didn't work and wanted to patch the hole since most methods usually take types rather than instances (or both).

It's a good starting point, and I would wait for more feedback before spending time on updating this PR to other methods.

Thanks for saying that, I will not continue for now, then. However, I had already started and found that writing every method for instances and types was not good design.

If this seems worthwhile, maybe having a generic

#= @inline ? =# empty(a, types...) = empty(typeof(a), types...)

just like for eltype would be a good idea.

Keno · 2019-10-13T00:40:14Z

base/abstractarray.jl

@@ -687,6 +688,10 @@ julia> empty([1.0, 2.0, 3.0], String)
 ```
 """
 empty(a::AbstractVector{T}, ::Type{U}=T) where {T,U} = Vector{U}()


change this method also then?

I left that in for inlining purposes due to all the existing code depending on it. If that is not necessary, then I'll change it as well.

janEbert · 2019-10-13T10:50:55Z

I also encountered another use case. Pre-allocating a Vector of Arrays of high dimensionality requires you to write an array comprehension with the awkward Array{Int, 25}(undef, Tuple(Iterators.repeated(0, 25))) constructor.
Even for saner dimensionalities, this quickly leads to unreadable code where empty(Array{Int, 25}) is much quicker to process.

mbauman · 2019-10-15T16:01:55Z

I have a few questions for which I don't have solid answers:

Does it make sense for the other methods of empty (the non-array ones) to allow types, too?
Should we have a fallback for empty(x) = empty(typeof(x)) a la eltype? What would the default empty(::Type) be? Maybe just have this fallback for ::AbstractArray?
Are there any cases where you need the object itself to determine what its empty looks like (and the type doesn't have enough information)?

nalimilan · 2019-10-18T10:59:25Z

* Does it make sense for the other methods of `empty` (the non-array ones) to allow types, too?

I'd say yes.

* Should we have a fallback for `empty(x) = empty(typeof(x))` a la `eltype`?  What would the default `empty(::Type)` be?  Maybe just have this fallback for `::AbstractArray`?

Yes, makes sense.

* Are there any cases where you need the object itself to determine what its `empty` looks like (and the type doesn't have enough information)?

Hopefully not. Actually that's a question we should also try to address for similar, as having a version of it taking a type rather than an instance would be useful in other situations (#20815).

nalimilan · 2019-10-18T10:48:35Z

base/abstractarray.jl

@@ -672,9 +672,10 @@ similar(::Type{T}, shape::Tuple{Union{Integer, OneTo}, Vararg{Union{Integer, One
 similar(::Type{T}, dims::Dims) where {T<:AbstractArray} = T(undef, dims)

 """
-    empty(v::AbstractVector, [eltype])
+    empty(a::AbstractArray, [eltype])
+    empty(a::Type{AbstractArray}, [eltype])


Suggested change

empty(a::Type{AbstractArray}, [eltype])

empty(a::Type{<:AbstractArray}, [eltype])

nalimilan · 2019-10-18T10:55:27Z

base/abstractarray.jl

@@ -687,6 +688,10 @@ julia> empty([1.0, 2.0, 3.0], String)
 ```
 """
 empty(a::AbstractVector{T}, ::Type{U}=T) where {T,U} = Vector{U}()
+function empty(a::Type{<:AbstractArray{T,N}}, ::Type{U}=T) where {T,N,U}
+    convert(AbstractArray{U}, a(undef, Tuple(Iterators.repeated(0,N))))


Unfortunately I'm not sure T(undef, dims) is a required part of the AbstractArray API. The operation of creating a new array from a type but with a different element type could potentially be handled by similar (see #20815).

Also this should be faster:

Suggested change

convert(AbstractArray{U}, a(undef, Tuple(Iterators.repeated(0,N))))

convert(AbstractArray{U}, a(undef, ntuple(_ -> 0, N)))

BTW, better use the short function syntax as for other definitions.

Thanks for the ntuple correction! I actually found that function some days ago and was gonna modify the PR accordingly but you were faster. :)

About the T(undef, dims), yeah, sorry; that was stupid since similar has the dims argument. I completely forgot about it.
I also love your referenced PR; just today I ran into a case where it would have been useful.

BTW, better use the short function syntax as for other definitions.

Do you mean I should always use empty(x) = ...? I didn't use it as the functions were getting too wide (over 92 columns) and I thought the short syntax should be avoided when the function spans multiple lines.

About the T(undef, dims), yeah, sorry; that was stupid since similar has the dims argument. I completely forgot about it.
I also love your referenced PR; just today I ran into a case where it would have been useful.

As I noted, the problem with similar is that it requires an instance, not a type.

Do you mean I should always use empty(x) = ...? I didn't use it as the functions were getting too wide (over 92 columns) and I thought the short syntax should be avoided when the function spans multiple lines.

Yes it's fine to break after = (see examples in this file).

As I noted, the problem with similar is that it requires an instance, not a type.

That was the problem with it, I remember... So first we will have to keep the constructor by type, if I'm seeing this correctly? Until #20815 is merged.

Yes it's fine to break after = (see examples in this file).

I was thinking more in an aesthetic/readability sense. Only because it has been done in the past does not imply it has to be done again. But sure, I will change it!

Actually I'm not even sure calling the constructor like that is guaranteed to work. Somebody else will have to confirm.

StefanKarpinski · 2019-10-28T14:13:32Z

It's usually quite a bad idea to introduce methods that work on both types and instances of types since types are values in Julia so this tends to produce an inherent ambiguity. However, in this case the empty function only makes sense for collections and types are not collections, so it seems safe and unambiguous to me:

if the argument is a type, try to create an empty instance of that type
if the argument is not a type, type to create an empty instance of the value's type

janEbert · 2019-11-01T12:02:06Z

Hey Stefan, so you are in favour of something like
empty(a, types...) = empty(typeof(a), types...)?
I'm sorry this hasn't moved along at all, I'm a bit busy this month but once I find the time, I'll get back to this taking all the feedback into account! Thank you all.

StefanKarpinski · 2019-11-27T16:10:02Z

Sure, I think that would be fine.

fingolfin · 2024-02-15T09:05:41Z

@janEbert Sorry for neglecting this PR for 4 years. It now has conflicts. If you are still interested and would be willing to update it, I think it could be merged soon?

janEbert · 2024-02-15T09:11:04Z

Hi Max, sorry from my side; after all I promised to add stuff to the PR but never did. I haven't used Julia deeply in a while but if you are fine with waiting a bit more for me to catch up again, I'll gladly update the PR according to all the 4 amazing years of development. ;)

fingolfin · 2024-02-15T22:19:50Z

@janEbert not your fault. thank you for being willing to pick this up again -- and sure, take your time

Add empty for AbstractArrays and their types

38b85d9

`empty` now works on all `AbstractArray`s and the corresponding call using `Type{<:AbstractArray}`. The method specialized on `AbstractVector` remains for inlining purposes.

Keno reviewed Oct 13, 2019

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kshyatt added the arrays [a, r, r, a, y, s] label Oct 15, 2019

nalimilan reviewed Oct 18, 2019

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Add `empty` for `AbstractArray`s and their types #33482

Add `empty` for `AbstractArray`s and their types #33482

janEbert commented Oct 6, 2019

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fingolfin commented Feb 15, 2024

janEbert commented Feb 15, 2024

fingolfin commented Feb 15, 2024

	empty(a::Type{AbstractArray}, [eltype])
	empty(a::Type{<:AbstractArray}, [eltype])

	convert(AbstractArray{U}, a(undef, Tuple(Iterators.repeated(0,N))))
	convert(AbstractArray{U}, a(undef, ntuple(_ -> 0, N)))

Add empty for AbstractArrays and their types #33482

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Add empty for AbstractArrays and their types #33482

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Add `empty` for `AbstractArray`s and their types #33482

Add `empty` for `AbstractArray`s and their types #33482

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