I'm not sure if I'm confused, or if there's a problem here, and I don't
know what any fix would be anyway, so apologies for the open-ended post
but...
I cannot find on "option" type in Julia that I can dispatch on, so that I
have a method call different functions, depending on whether a value is
present or not.
What I think I need is:
typealias Maybe{T} Union(T,Nothing}
That allows me to do:
julia> foo(::Int) = "int"
foo (generic function with 1 method
julia> foo(::Nothing) = "nothing"
foo (generic function with 2 methods)
julia> bar(x::Maybe{Int}) = foo(x)
bar (generic function with 1 method)
julia> foo(1)
"int"
julia> foo(nothing)
"nothing"
julia> bar(nothing)
"nothing"
julia> bar(1)
"int"
which seems like what I want.
HOWEVER, there's also Nullable, described at
http://julia.readthedocs.org/en/latest/manual/types/#nullable-types-representing-missing-values
and also at
http://docs.julialang.org/en/latest/manual/faq/#nothingness-and-missing-values
which seems like what I "should" be using. But I can't see how to dispatch
on it.
So it seems like one of the following is true
1 - There is a way to dispatch on Nothing, and please someone explain it to
me
2 - Nullable should be changed so that it can be dispatched on
3 - We need Maybe in Base as well as Nothing.
1 or 2 sounds fine (although I personally don't see how you can do 2). 3
seems like the worst option, but still is presumably better than everyone
and their dog defining this type in their own code.
Any enlightenment appreciated,
Andrew
PS More references
https://github.com/JuliaLang/julia/issues/3332
https://github.com/JuliaLang/julia/issues/1134
https://github.com/JuliaLang/julia/pull/8152
