Please don't use ? as a unary operator?

[carlobaldassi]

See JuliaLang/julia#6286 (comment) and following comment and related discussions.
It's an old discussion, but it doesn't seem things have changed since (? could be possibly used a part of an identifier name in the future, not by its own).

Basically, the fact that using ? as unary works is an accident, and it makes things really difficult to parse.

[ararslan]

It's a temporary hack until we can steal T? in Base to mean Union{T, Null}.

[StefanKarpinski]

I don't see what the big problem with using ?T as a unary operator is. Sure, it's a nasty hack now that works by accident and awkwardly, but we could make it a true unary operator in the future.

[ExpandingMan]

I personally find this to be a major annoyance. It totally screws up the parsing of julia-vim which makes it very unpleasant for me to work with it. For the time-being, can we use (almost literally) any other unicode character that is not already a binary operation of some kind?

[nalimilan]

The long-term (1.0 or 1.x) plan is to use T? instead of ?T, which is consistent with other major languages like C# and Swift. See JuliaLang/julia#22523.

[carlobaldassi]

I don't see what the big problem with using ?T as a unary operator is

One problem is that if you try to enter it in the REPL it goes in help mode.

As for parsing: right now, omitting a space in front of ? in ternary expressions is deprecated, but not afterwards. Which means that parsing ? as a unary operator correctly is still awfully context-dependent (I'm not just thinking about parsing in julia-vim, which I have kind of fixed, it's hard even for a human).

I believe that using ? by itself as an identifier should just be disallowed (mostly because of the REPL issue). Otherwise, at least spaces should be required on both sides to use it as a ternary operator, such that one can understand at a glance what's going on.

[nalimilan]

One problem is that if you try to enter it in the REPL it goes in help mode.

That does not apply to T?, which has been retained as the long-term solution. Several other languages use that convention for a long time, and Swift recently adopted it, so it does not seem to cause big problems in practice.

[carlobaldassi]

Yes, I was not speaking about T?, which doesn't cause any problems now that it's deprecated as a ternary expression. My points were specifically about expressions that start with ?, including using ? as an identifier/operator.