Why doesn't `recursivecopy!` allow to copy a vector into a matrix?

[albertomercurio]

Is your feature request related to a problem? Please describe.

I need to copy the content of a vector into a matrix. This is handled in pure Julia through the copyto! function

x_mat = rand(10, 10)
y_mat = rand(100, 100)
y_vec = view(y_mat, :, 1)

copyto!(x_mat, y_vec)

but it is not supported by the recursivecopy! function

using RecursiveArrayTools

recursivecopy!(x_mat, y_vec)

ERROR: MethodError: no method matching recursivecopy!(::Matrix{Float64}, ::SubArray{Float64, 1, Matrix{Float64}, Tuple{Base.Slice{…}, Int64}, true})
The function `recursivecopy!` exists, but no method is defined for this combination of argument types.

Closest candidates are:
  recursivecopy!(::AbstractArray{T, N}, ::AbstractArray{T2, N}) where {T<:Number, T2<:Number, N}
   @ RecursiveArrayTools ~/.julia/packages/RecursiveArrayTools/9sjwU/src/utils.jl:73
  recursivecopy!(::AbstractVectorOfArray, ::AbstractVectorOfArray)
   @ RecursiveArrayTools ~/.julia/packages/RecursiveArrayTools/9sjwU/src/utils.jl:97
  recursivecopy!(::T, ::T) where T<:JumpProcesses.ExtendedJumpArray
   @ JumpProcesses ~/.julia/packages/JumpProcesses/MF0Pb/src/extended_jump_array.jl:143
  ...

Stacktrace:
 [1] top-level scope
   @ Untitled-1:41
Some type information was truncated. Use `show(err)` to see complete types.

Describe the solution you’d like

Perhaps changing the current method

function recursivecopy!(
        b::AbstractArray{T, N},
        a::AbstractArray{T2, N}
    ) where {T <: Number, T2 <: Number, N}
    return copyto!(b, a)
end

to

function recursivecopy!(
        b::AbstractArray{T, N},
        a::AbstractArray{T2, M}
    ) where {T <: Number, T2 <: Number, N, M}
    return copyto!(b, a)
end

I don't know if we can also change the other methods that do not have T <: Number.

[ChrisRackauckas]

It requires matching shapes. do recursivecopy!(vec(a),vec(b)) if you don't care about shape: the vec won't allocate so it's fine. I think it's good to shape check.

[albertomercurio]

Ok. Is there a reason why it requires shape matching if the copy function would still work?

[ChrisRackauckas]

copyto! doesn't work

[albertomercurio]

Doesn't work on VectorOfArray? Because on AbstractArray it works.

[ChrisRackauckas]

When it matches shape it works.

[albertomercurio]

The example I gave in the first comment uses copyto! from a Vector to a Matrix. So it works even if the shapes don't match.

[ChrisRackauckas-Claude]

The relevant Julia norm here is copy!, not copyto!. From the Base.copy! docstring: "If dst and src are multidimensional arrays, they must have equal axes." copyto! is the shape-independent linear-copy version; copy! is the shape-checking version.

recursivecopy! is named after copy!, and the docstring already says "like copy! on arrays of scalars", so the intent has always been shape-checking semantics. The current (::AbstractArray{T,N}, ::AbstractArray{T2,N}) signature enforces matching dimensionality, which is a partial enforcement of that — relaxing it to allow Matrix ← Vector would push the function toward copyto! semantics and away from its name.

For your use case, just call copyto!(x_mat, y_vec) directly — that's exactly what copyto! is for. Or recursivecopy!(vec(x_mat), y_vec) if you want to keep going through this function (vec is allocation-free for dense arrays).

If anything, the existing method should be tightened to require full axes matching rather than only matching N, to actually honor the copy!-like contract the docstring promises.

[albertomercurio]

But the recursivecopy! function internally calls copyto! and not copy!.

I have this problem because I have to re-initialize a ODEProblem that has u0 as a Matrix but I'm doing reinit(prob; u0 = some_vec). This raises the following error above.

[ChrisRackauckas]

I think it seems appropriate that would need a matrix?

[albertomercurio]

Yes I can for sure reshape it, but then studying the error, I found it was caused by recursivecopy! which internally uses copyto!`, which allows for different shapes arrays.

So I was just wondering if this error can be fixed fo free by just relaxing this condition, as copyto! doesn't have this constraint.

[ChrisRackauckas]

#591 should be a fine solution.