Ismael, although ASCIIString is a subtype of String, Array{ASCIIString,1}
is not a subtype of Array{String,1}. As far as I understand Julia has been
designed this way for practical reasons. You can read about it in the
manual's section: Types.
You might want to rewrite your function as
function Pkg.add{T<:String}(pkgs::Array{T, 1})
Pkg.update()
for pkg in pkgs
Pkg.add(pkg)
end
end
