What happened to `T?`

If memory serves me, once upon a time there was a proposal to simplify syntax for missing values:

T? == Union{T, Missing}

Currently I have to handle a lot of missing values and find the extra typing quite annoying.

  • Is there still a plan to integrate this into julia?
  • How do other people handle this?
m(T) = Union{T, Missing}
macro m(T)
    :(Union{$T, Missing})

don’t work in function definitions

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You can write f(x::@?(Int), y::Int) = x+y with tkf’s clever macro, from this earlier discussion:


cool macro, scares me to use it in production code :fearful:

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of course without the random Nothing/Missing switch

Why? If you are living on the edge already… :wink:

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Would it make sense to add this at the beginning of your project?

for T in collection_of_types
    @eval const $(Symbol(T, :_)) = Union{$T, Missing}

(with collection_of_types being a collection of all the types with possible missing values you are working with, and replacing :_ by any suitable symbol you want to append to the type names for aliasing them).

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There is no consensus whether it should mean Union{T, Nothing} or Union{T, Missing}.

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I’d rather have one of them (which ever) than none at all… I guess we could scan all the registered packages and see what it more common, Missing or Nothing…?

T? for one, T?? for the other?

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I’m pretty sure I’ve mentioned this elsewhere, but I just want to repeat that I think that T? is really ugly syntax. The first reason for this is that there are very few right unary operators in Julia. The only exception I am aware of is ', which I think is a much better case because the notation for adjoint or transpose as a right unary operator is universal (something that cannot be said for a short-hand of Union{T,Missing}). The second reason is that the logical implication syntax a ? b : c is valid and often used in Julia, and when I see T? my first instinct is that I’m looking at a broken ternary operation.

I recall that @Tamas_Papp had some nice suggestion for an alternative, but I can’t remember what it was at the moment, it might have been something like U(T,Missing). I also think it would be nice to have a notation that might be a little bit of extra typing, but is both explicit about whether it is using Nothing, Missing or something else, and general enough to be extensible to all binary type unions.


I like this one:

const U = Union
const M = Missing
U{Float64, M}
  • No magic macros
  • Explicit
  • Short enough

M seems very mysterious and ad hoc to me. I find the T? notation to be fantastic, and the other suggestions verbose and cumbersome.


T U Missing makes a lot of sense to me. Simply having Union as an infix operator makes it a lot prettier.

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That rings a bell, I think @Tamas_Papp’s original suggestion may have been

∪(T::Type, S::Type) = Union{T,S}

As in [1,2,3] ∪ [4,5,6] the union (small ‘u’) of two types is a Union (capital ‘U’) type of the two. No idea what Tamas said :slight_smile:

I keep defining

const Maybe{T} = Union{T, Nothing}

And I’m OK with that.

How about Meh{T} instead? Shorter and funnier.


To handle both missing and nothing perhaps:

const Maybe{T} = Union{T, Missing}
const Naybe{T} = Union{T, Nothing}

Neh works then for Nothing…


I can’t recall anything, sorry. In fact, I think I was against any clever syntax, I find the explicit Union{...} much cleaner. For frequent application one can always define an alias.

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