[julia-users] Union in composite type

[David van Leeuwen]

Hello, 

For a long lime I've been having a bad feeling for having a Union in a 
composite type, as in 

type GMM{T:<FloatingPoint}
...
 kind::Symbol
 μ::Matrix{T}
 Σ::Union(Matrix{T},Vector{Matrix{T}})
...
end

where the choice in `Σ` depends on whether `kind==:diag` or `kind==:full`. 
 I have the feeling it is better to have defined something along the lines 
of

typealias  DiagCov{T}  Matrix{T}
typealias  FullCov{T}  Vector{Matrix{T}}

type GMM{T:<FloatingPoint,CT}
...
 μ::Matrix{T}
 Σ::CT
end

function kind(g::GMM) 
    T = eltype(g.μ)
    if isa(g.Σ, FullCov{T})
        return :full
    else
        return :diag
    end
end


In the second case, the `kind` is inferred from the type instance, rather 
than stored as a type variable.  

What is in your expert's opinion a better way of defining such a type?

Thanks, 

---david