Using a `?` as a nullable typename

Maybe.jl

module Maybe

export ?

immutable ?{T}
    hasvalue::Bool
    value::T

    ?() = new(false)
    ?(x::T) = new(true, x)
end

?{T}(x::T) = ?{T}(x)
?{T}(::Type{T}) = ?{T}()
Base.:*(::Type{?}, x) = ?(x) # I'm confused... ? is a unary in the Scheme file

function Base.show{T}(io::IO, x::?{T})
    if x.hasvalue
        print(io, x.value)
    else
        print(io, "?(", T, ")")
    end
end

end # module

Interesting. Though it would still be good to have a real name for that type, with ? only an alias. Else, searches are going to be hard, and for dispatch x::?{T} looks really weird.