KeyedTuple and inference

Specific Domains Data
7

Ok, well, this seems like it works on 0.7

struct Key{T <: Any} end
Key(x) = Key{x}()

Base.@pure argtail(x, rest...) = rest
tail(x) = argtail(x...)

Base.show(io::IO, ::Key{T}) where T = begin
    print(io, ".")
    print(io, T)
end

next_item(t::Tuple, reference_key::T, next_key::T) where T <: Key =
    first(t)
next_item(t::Tuple{}, reference_key::T, next_key::T) where T <: Key =
    error("No item after key $reference_key")
next_item(t::Tuple, reference_key::Key, next_key) = 
    get_key(t, reference_key)
next_item(t::Tuple{}, reference_key::Key, next_key) = 
    error("Key $reference_key not found")

get_key(t::Tuple, k::Key) = next_item(tail(t), k, first(t))
get_key(t::Tuple{}, k::Key) = error("Can't index an empty tuple")

Base.getproperty(t::Tuple, k::Symbol) = get_key(t, Key(k))

function test()
    x = (Key(:a), 1, Key(:b), 2)
    x.b
end
@code_warntype test()

Involves a bit of type piracy, but