[julia-users] Composite type with dependent fields (using Base.setindex!)

[Spencer Lyon]

I have a very simple composite type:

type DiscreteRv{T <: Real}
    q::Vector{T}
    Q::Vector{T}
end

DiscreteRv{T <: Real}(x::Vector{T}) = DiscreteRv(x, cumsum(x))

Here q represents a probability vector for a discrete random variable. Q is 
the associated cdf. 

I want to make sure that the relationship Q = cumsum(q) is preserved 
whenver q is updated.

I presume I need to overload Base.setindex!, but I haven’t been able to 
figure it out. Any suggestions?
------------------------------

In the end I would like this example to work:

julia> d = DiscreteRv([.1, .9])
DiscreteRv{Float64}([0.1,0.9],[0.1,1.0])

julia> d.q = [.3, .7]; d
DiscreteRv{Float64}([0.3,0.7],[0.3,1.0])

​