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

Kind of amusing. E.g.

julia> using Maybe

julia> x = 3
3

julia>  ?x
3

julia> typeof(?x)
Maybe.?{Int64}

julia>  ?Int
?(Int64)

On this note we have ?, !, ¿ and ¡ (spanish style inverted question/exclamation marks). And unicode defines a reverse question mark ⸮ which wikipedia describes as the irony mark. Between these you could develop a whole language for get, isnull/hasvalue, construction, type names, module names, and so-on.

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.