Precedence in parsing types

[elbarzil]

It looks to me like this should parse, but it doesn't:

var x: number | (string)=>void;

It works with added parens:

var x: number | ((string)=>void);

But if the parser is not too hacky, then maybe it's easy to make the first work?

(FWIW, I ran into this in a rest argument, where it would be nice to avoid the extra parens in

function foo(...xs: (number | ()=>void)[]): void { }

)

[RyanCavanaugh]

It's still going to be problematic because types can be parenthesized (i.e. var x: string | (number) is valid). In general you should always be parenthesizing arrow function types like that because it's very difficult to guess what var x: n => number | string means.

Side note: type names in parameters are not optional; you must write (s: string)=>void (see https://github.com/Microsoft/TypeScript/wiki/FAQ#why-dont-i-get-type-checking-for-number--string-or-t--t)

[elbarzil]

I'd likely read that as (n => number) | string, so I'd definitely parenthesize the number | string if that was my intention... So I guess that I'm naturally assuming that the precedence of => is higher than | and &. Is this described somewhere in the docs?

And re the side note: is that new? I see that

function foo(f: (number) => number): number { return f(0); }
foo("foo");

works as expected now, and it looks like there's a good pile of such things in our code...

[RyanCavanaugh]

Is this described somewhere in the docs?

It's in the grammar. I don't think we have explicitly written down the precedence order anywhere.

And re the side note: is that new? I see that

It's been that way since forever. That declaration is for a callback that takes a parameter named number of the implicit type any. I'd recommend compiling with --noImplicitAny to find these.

[elbarzil]

So at least make this into a feature request to describe the precedence for parsing types somewhere...?

(And bah, I messed up that example, sorry.)

[chiumax]

@RyanCavanaugh @mhegazy How may I help with this?