Change to (using x: T) => E for context functions

[odersky]

Change syntax of context functions to (using x: T) => E. The previous
syntax (x: T) ?=> E is still supported, but the docs have been changed
to using.

[odersky]

As a trial, implement a suggestion @Kaishh on Scala contributors to align context parameter syntax of methods and function values.

[odersky]

My takeaway from the conversion:

• Unlike the other contextual abstractions, context function literals are quite rare.
• The rewrites do improve readability (to some degree) and regularity (significantly).

[arturopala]

context (implicit) function literals are the most common way to define actions in PlayFramework, are they? i.e. {implicit request => ... }

[odersky]

context (implicit) function literals are the most common way to define actions in PlayFramework, are they? i.e. {implicit request => ... }

I was not aware of that! We should get PlayFramework into the community build, that would give us better insights. But U=I fear it will be a while until that can happen.

[mdedetrich]

Unlike the other contextual abstractions, context function literals are quite rare.

I would be cautious of making such conclusions. Just because something isn't in the community build doesn't mean its rare.

[odersky]

@mdedetrich Understood. But there is another argument why context closures will be rare in Scala 3: They are automatically inferred if your target type is a context function type. So it's good style not to write them explicitly. For instance, a new version of the Play framework would probably just use {response} instead of {implicit request => response} since the expected type of such expressions would be Request ?=> Response.

[nicolasstucki]

One advantage of this notation is that it composes naturally with erased function types

(using x: T) => E
(erased x: T) => E
(using erased x: T) => E

vs

(x: T) ?=> E
(erased x: T) => E
(erased x: T) ?=> E

[nicolasstucki]

Should we also have (using A) => B?

[odersky]

No, let's have one way todo things.

[jdegoes]

Wonderful to see increased regularity during the polishing phase!