typealias Maybe{T} Union(T,Nothing)

[StefanKarpinski]

Can this be made to work? Currently it's an error:

julia> typealias Maybe{T} Union(T,Nothing)
type error: typealias: expected Type{T<:Top}, got TypeVar
 in anonymous at no file

[JeffBezanson]

This has various problems. If Maybe{T} is used as an argument type, nothing can't match it because we don't know what T is. Or, we could let T be Nothing in that case, but then the maybe isn't doing anything for you.

These kinds of unions are ambiguous. Matching Foo{S} against Union(T,Foo{T}) could mean T is Foo{S} or S.

One problem is that we use these "type constructors" also as types; you expect to say Maybe and have it mean anything that could come from one of those. In this case it is equivalent to Any. If we didn't allow this, and insisted that these be applied to actual types before they could be used, then we wouldn't have these problems, but then you also couldn't use them in argument patterns. They would be equivalent to functions that return types.

[StefanKarpinski]

Got it. I'll just close.

[pao]

Fortunately, I'm not sure that the Maybe type is all that useful in Julia. You could always add a method for the nothing case:

# like Maybe{Int}
foo(x::Int) = x^2
foo(x::Nothing) = NaN

I can think of a few things Maybe should be able to do that this can't, though. Was there a motivating case?

[JeffBezanson]

Sometimes it is useful to define a Nil{T} singleton, as for lists.

[pao]

You have that pattern in list.jl without the assistance of a Maybe-like type.

[StefanKarpinski]

Yeah, this wasn't for a particular reason. I'm content not to have this — as Jeff points out, it doesn't really work for us.

[andrewcooke]

for people coming from googling "julia maybe type" - there's now Nullable which seems to be similar http://julia.readthedocs.org/en/latest/manual/types/#nullable-types-representing-missing-values

[link2xt]

@andrewcooke Update: https://docs.julialang.org/en/latest/manual/types/#man-nullable-types-1

[nalimilan]

Update to the update: #22682 and #23642.