Generalizable Function Modifier Syntax

[Brendan Eich]

Kevin Smith wrote:
>     async function af() {}
>     { async af() { } }

This lines up with what Luke Hoban presented at the last TC39 meeting. 
So far, so good (but not a done deal by any means, of course). The main 
bone of contention is the use of ! in promises future-proposed syntax.

> The hard part is arrow functions.  One possibility would be to use 
> another cover grammar and place the modifier to the left of the arrow 
> function:
>
>     modifier [NoNewLine] Identifier =>
>     modifier [NoNewLine] ArgumentList =>

We could do this.

> We could probably reuse some of the existing arrow function parsing 
> strategy to back up the input stream or transform the AST.

It smells, but we're used to it!

> Another possibility would be to place the modifier to the right of the 
> argument list:
>
>     Identifier [NoNewLine] modifier => ...
>     ArgumentList [NoNewLine] modifier => ...
>
> e.g.
>
>     x async=> {}
>     (a, b, c) async=> {}
>
> This would be easier to parse, and would align with a potential 
> "generator arrow":
>
>     x *=> {}
>
> (Presuming that we can give up on arrows with empty parameter lists.)

Bletch, and don't multiple risks unnecesasrily by entangling with 
opposition to empty arrow param list elision.

/be