Over at LightGraphs we have been discussing support for collections indexed
by integers that can be either dense like Vector{T} or sparse like
Dict{Int,T}.
The idea is that you want to collect some T's together indexed by a range
of integers 1:n. Often you will have a value for all or most of the indices
and should use a Vector{T} for that case. But sometimes you will only have
a few values and don't want to allocate O(n) storage just to hold a much
smaller amount of data.
What is the best way to express this in types? Does
Union{Associative{Int,T}, AbstractArray{T}} express this with no
performance penalty? Is there any reason to avoid Union types?
Thanks,
James
