default_dict.jl

immutable DefaultDict{K, V} <: Associative{K, V}
  dict::Dict{K, V}
  default::V
end

function DefaultDict(val)
  return DefaultDict(Dict(), val)
end

function Base.getindex{K, V}(dd::DefaultDict{K, V}, key::K)
  if haskey(dd.dict, key)
    getindex(dd.dict, key)
  else
    dd.default
  end
end

Base.convert(::Type{Associative}, dd::DefaultDict) = dd.dict
Base.promote_rule(::Type{Associative}, ::Type{DefaultDict}) = Associative

#= Why don't we get these for free with `convert` and `promote_rule`?
Base.setindex!{K, V}(dd::DefaultDict{K, V}, key::K, val::V) = setindex!(dd.dict, key, val)
Base.length(dd::DefaultDict) = length(dd.dict)
=#