How to trick Array{T} into behaving covariantly?

General Usage
2

How important is performance accessing the vectors of “mixed” data?
Do the different types have common fields?

If the performance of accessing the “blocks” field (which will have to essentially hold pointers to boxed VTKData types) isn’t so important, then the following (using the new syntax) should work:

abstract type AbstractVTK end

mutable struct VTKUnstructuredData <: AbstractVTK
end

mutable struct VTKStructuredData <: AbstractVTK
end

mutable struct VTKMultiblockData <: AbstractVTK
    blocks::Vector{AbstractVTK}
end

mutable struct VTKTimeSeriesData{T<:AbstractVTK} <: AbstractVTK
    data::Vector{T}
end

The VTKTimeSeriesData is parameterized so the “data” field will be a homogenous vector of a single subtype.

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