LayerDict is an Associative type that wraps a series of other associatives (e.g. Dicts).
When performing a lookup, a LayerDict will look through its associatives in the order they were passed to the constructor until it finds a match.
LayerDicts are immutable—you cannot call setindex! on them.
However, you can update its wrapped associatives and those changes will be reflected in future lookups.
using LayerDicts
dict1 = Dict{Symbol, Int}(:foo => 1, :bar => 1)
dict2 = Dict{Symbol, Int}()
dict3 = Dict{Symbol, Int}(:bar => 3, :baz => 3)
ld = LayerDict([dict1, dict2, dict3])
@assert ld[:foo] == 1
@assert ld[:bar] == 1
@assert ld[:baz] == 3
ld[:quuz] # throws a KeyError