As per discussion in https://groups.google.com/forum/#!topic/julia-users/alavN8tRdyI :
Let e.g.

Array{<:Real}

act like RArray given by

typealias RArray{T<:Real} Array{T}

This convenient shortcut should go a long way in those cases where people would want covariant types.

The type parameter of Array{<:Real} might become anonymous or hidden; it probably doesn't matter so much since you would rarely re-provide it, e.g. Array{<:Real}{Int}.