KeyedTuple and inference

Specific Domains Data
9

Maybe this implementation is slightly nicer:

struct Key{K} end
Key(s::Symbol) = Key{s}()
function Base.show(io::IO, k::Key{K}) where {K}
    print(io, ".")
    print(io, K)
end

struct Keyed{K, V}
    value::V
end
Base.@pure Keyed(key::Symbol, value::V) where V = Keyed{key, V}(value)
function Base.show(io::IO, k::Keyed{K, V}) where {K, V}
    print(io, K)
    print(io, " = ")
    print(io, k.value)
end

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

key_is(keyed::Keyed{K, V}, key::Key{K}) where {K, V} = true
key_is(any, k::Key) = false

value(k::Keyed) = k.value

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

get_key(t::Tuple{}, k::Key) = error("Key $k not found")
function get_key(t::Tuple, k::Key) where {K, V}
    first_item = first(t)
    if key_is(first(t), k)
        value(first_item)
    else
        get_key(tail(t), k)
    end
end

function test()
    x = (Keyed(:a, 1), Keyed(:b, "b"))
    x.a
end
@code_warntype test()