Change ? to another symbol

[nightscape]

It seems like Dotty will get native type lambdas. The plan there is to use F[_] as a short-hand syntax for the type lambda [X] => F[X] and F[?] to be the wildcard syntax which unfortunately is different to how the ? is used in kind-projector.
In scala/scala3#5379 there's a discussion of doing the swap incrementally in Scala 3.0 - 3.3, but imo that would lead to unnecessary churn and confusion, so I was wondering if kind-projector could preemptively replace the ? by another symbol so that until Dotty is out the ? is free to use for wildcards.
Maintainers' opinions are welcome here or in the above ticket 😃

[TomasMikula]

Thank you for letting us know! I think the proposed syntax (F[?] for existential, F[_] for [X] => F[X]) is more intuitive. It will probably be a long and painful transition for projects, but I view it positively overall.

The question for kind projector is, what symbol do we use (alongside ?) in shorthand syntax for type lambdas.

In math, – (en-dash) is commonly used for this purpose, as in

Hom(A,–) maps each object X in C to the set of morphisms, Hom(A, X).

The problem is that the en-dash is not a valid Scala identifier. Also, it is not easily typed on a keyboard.

So I suggest to use -- (double minus), which is also the TeX way to typeset an en-dash.

[LukaJCB]

How about using a star *? I'm not sure if double minus is a great idea, since a single minus already means contravariance.

I think something like Either[Error, *] reads relatively nice.

[SystemFw]

👍 to *

- would read nice, but double minus is not as nice as star I would say.

Agreed on both the pain it will take to translate, and the fact that it's worth it, btw.

[kubukoz]

Let's rename to ∃ :D

/s * sounds like a good idea

[TomasMikula]

See #91 where I went with *.