Consider adding an attrs-like converter macro?

[singularitti]

I was using attrs in Python. It shares a similar functionality as Parameters.jl. However, one thing it has is not supported by Parameters.jl now---the default converter. It is often the case that users do not supply the right type of parameters to the constructor. I want that we can handle this if they do not give the right type instead of throwing an error.
Here is an example:

struct A
   mass::Int
   velocity::String
   coordinate::Vector{Pair{Int, Float64}}
   time::NTuple{3, Float64}
   frequency::Float64
end

For some reason, I want to handle if a user supplies a Vector{Float64} or a NTuple{N,Float64} to the coordinate field, the construct A could directly converts it to a Vector{Pair{Int, Float64}}, by calling a function

convertfield(x::Vector{Pair{Int, Float64}}) = x
function convertfield(x::Union{AbstractVector{<: Real}, NTuple{N, <: Real} where {N}})
    return collect(pairs(Float64.(x)))
end

on it. The syntax could be like

@with_kw struct A
   mass::Int
   velocity::String
   coordinate::Vector{Pair{Int, Float64}} = ones(3); @convert convertfiled(coordinate); @assert length(coordinate) == 3
   time::NTuple{3, Float64}
   frequency::Float64
end

where @convert is a macro that takes a function. I could write something like

@with_kw struct A
   mass::Int
   velocity::String
   coordinate::Vector{Pair{Int, Float64}} = collect(pairs(ones(3)))
   time::NTuple{3, Float64}
   frequency::Float64
end

But it is just a default value, not a converter.

[mauro3]

I am not really interested in supporting this as it is too much of a feature creep to me.

Maybe some sort of plug-in system should be made, so you can do this personally. Not sure what this could look like.