[julia-users] Re: Type inheritance question

[Tim Wheeler]

Julia only supports concrete leaves in the type tree - all non-leaves 
(parents) must be abstract types.

You should be able to do:

abstract AbstractDataStream{T <: Number}

type DirectedDataStream{T <: Number} <: AbstractDataStream{T}
    data::Vector{T}
    stuff
end

type ReversedDataStream{T <: Number} <: AbstractDataStream{T}
    data::Vector{T}
    stuff
end

Another thing you can do that still leverages multiple dispatch is define 
the reversal as a separate type:

abstract Direction
type ForwardDirection <: Direction end
type BackwardDirection <: Direction end

type MyDataStream{T<:Number, D<:Direction}
    data::Vector{T}
    direction::D
    stuff
end

Base.plus{T<:Number, S<:Number, D<:ForwardDirection}(a::MyDataStream{T,D}, b
::MyDataStream{S,D}) ...
Base.plus{T<:Number, S<:Number, D<:BackwardDirection}(a::MyDataStream{T,D}, 
b::MyDataStream{S,D}) ...