# Status of question mark syntax for missing values

**URL:** <https://discourse.julialang.org/t/status-of-question-mark-syntax-for-missing-values/27189>\
**Category:** Internals & Design\
**Created:** [August 5, 2019, 9:42pm UTC](https://discourse.julialang.org/t/status-of-question-mark-syntax-for-missing-values/27189 "2019-08-05T21:42:33Z")\
**Posts on this page:** 1\
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**Author:** ![StefanKarpinski](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/stefankarpinski/32/24_2.png) [@StefanKarpinski](https://discourse.julialang.org/u/StefanKarpinski)\
**Post date:** [August 5, 2019, 10:00pm UTC](https://discourse.julialang.org/t/status-of-question-mark-syntax-for-missing-values/27189/2 "2019-08-05T22:00:29Z")

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For a while I was pretty ambivalent about whether we should use `T?` for `Union{T, Nothing}` or for `Union{T, Missing}`. The code I read and write has much more `Union{T, Nothing}` in it than `Union{T, Missing}`. However, I’ve come back around to wanting it to mean `Union{T, Missing}` for one very key reason: we need a way to express lifting of functions to the `Union{T, Missing}` domain. What I mean by that is that instead of every couple of days getting a new feature request for missing support for the `atanh` function or whatever, I’d like to just make `atanh?` lift the `atanh` function so that it returns `missing` when any of its arguments are `missing`. We don’t need this feature for `nothing` since the whole point of `nothing` is that it doesn’t lift, whereas the point of `missing` is to lift. We might also want a `@?` macro similar to the `@.` macro, which annotates all function calls with `?` lifting.

How do `f?` and `T?` mesh? Quite well, actually, if you view the `T` constructor as a function that returns values of type `T`—then `T?` is a function that returns values of type `Union{T, Missing}` which is exactly what function lifting is supposed to do. Of course, you still want this to be represented as the union type, `Union{T, Missing}`, so you just need to make sure that the type, when called, does the lifted version of `T`. @jameson had some thoughts on this and might be able to say a bit more.

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