# What about an \`undefs\` function in \`Base\`?
**URL:** https://discourse.julialang.org/t/what-about-an-undefs-function-in-base/92690
**Category:** General Usage
**Tags:** feature-request
**Created:** [January 9, 2023, 2:18am UTC](https://discourse.julialang.org/t/what-about-an-undefs-function-in-base/92690 "2023-01-09T02:18:50Z")
**Posts on this page:** 5
**Page:** 4
### Author:  [@mkitti](https://discourse.julialang.org/u/mkitti)
#### Post date: [January 20, 2023, 3:15am UTC](https://discourse.julialang.org/t/what-about-an-undefs-function-in-base/92690/61 "2023-01-20T03:15:24Z")
Perhaps there should be a `@CuArray` macro if there is not one already. StaticArrays.jl has a `@SArray` macro.
```julia
julia> @SArray [x for x in 1:5]
5-element SVector{5, Int64} with indices SOneTo(5):
1
2
3
4
5
```
---
### Author:  [@uniment](https://discourse.julialang.org/u/uniment)
#### Post date: [January 20, 2023, 3:58am UTC](https://discourse.julialang.org/t/what-about-an-undefs-function-in-base/92690/62 "2023-01-20T03:58:40Z")
Funny that `SA` doesn’t work with comprehensions.
```julia
julia> SA[1, 2, 3]
3-element SVector{3, Int64} with indices SOneTo(3):
1
2
3
julia> SA[x for x=1:3]
ERROR: MethodError: Cannot `convert` an object of type
Int64 to an object of type
SA
```
So for comprehensions, you have to use the macro? Sidenote, the space between the macrocall and the brackets is optional:
```julia
julia> SArray[@SArray[1,2,3] for _=1:4]
4-element Vector{SArray}:
[1, 2, 3]
[1, 2, 3]
[1, 2, 3]
[1, 2, 3]
julia> @SArray[@SArray[1,2,3] for _=1:4]
4-element SVector{4, SVector{3, Int64}} with indices SOneTo(4):
[1, 2, 3]
[1, 2, 3]
[1, 2, 3]
[1, 2, 3]
```
---
### Author:  [@sijo](https://discourse.julialang.org/u/sijo)
#### Post date: [January 20, 2023, 8:03am UTC](https://discourse.julialang.org/t/what-about-an-undefs-function-in-base/92690/63 "2023-01-20T08:03:39Z")
> [@uniment](#):
>
> Is there a syntax for array comprehensions of different array types, such as `CuArray`? Maybe such a thing would diminish the need for pre-allocation and `for` loops to begin with. Something like this, perhaps?
>
> ```julia
> Type{CuArray{Int}}[i*j for i=1:2, j=1:2]
>
> ```
I think custom array types can define constructors that accept generators, e.g.
```julia
CuArray(Int, (i*j for i=1:2, j=1:2))
```
---
### Author:  [@uniment](https://discourse.julialang.org/u/uniment)
#### Post date: [January 20, 2023, 8:13am UTC](https://discourse.julialang.org/t/what-about-an-undefs-function-in-base/92690/64 "2023-01-20T08:13:02Z")
Should that be part of `AbstractArray` interface?
```julia
CuArray{Int}(i*j for i=1:2, j=1:2)
```
---
### Author:  [@mkitti](https://discourse.julialang.org/u/mkitti)
#### Post date: [January 24, 2023, 10:58am UTC](https://discourse.julialang.org/t/what-about-an-undefs-function-in-base/92690/65 "2023-01-24T10:58:38Z")
Undefs.jl is now registered:
> [@Undefs.jl: Convenience and Experiment](https://discourse.julialang.org/t/undefs-jl-convenience-and-experiment/93451):
>
> Undefs.jl I created a registered package called Undefs.jl that exports three names: undefs(A = Array, T = Float64, dims) = A{T}(undef, dims): a convenience method similar to ones, zeros, trues, and falses. undef!(array::Array, index=1): An experimental method to reset an element to an #undef state. This also works on Base.RefValue. @undef! A[1]: A macro to make the use of the undef! easier. Demonstration of the convenience constructor, undefs julia\> using Undefs julia\> mutable struct Foo …
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