Add support for parsing binding patterns in arrow function parameters

[rbuckton]

Fixes #2678, adds some additional tests to the isParenthesizedArrowFunctionExpressionWorker lookahead to allow for array and object patterns in the first parameter of a parenthesized arrow function.

[mhegazy]

can we add a --target=es6 test as well

[DanielRosenwasser]

Rather, take all of these tests and duplicate them as ES6 tests.

[rbuckton]

Already done.

[JsonFreeman]

👍

[rbuckton]

I simplified the look-ahead to go no more than 3 tokens to find some cases where we can shortcut out with Tristate.False.

[CyrusNajmabadi]

Thse seem so rare as to not need special handling. I would just do: if (second === OpenBracket || second === OpenBrace) ... return TriState.Unknown

[JsonFreeman]

True, I picture arrow functions and destructuring parameters to not be very common in conjunction.

[DanielRosenwasser]

I picture arrow functions and destructuring parameters to not be very common in conjunction.

I tried writing one 2 days ago for the record.

[rbuckton]

I'm seeing it often in conjunction with Promise.all:

Promise.all([a, b, c]).then(([a, b, c]) => {
});

[CyrusNajmabadi]

But won't your new code return ' unknown' on this case? I'm curious what cases were Will see that are common where we'll be able to effectively bail out early with this code b

-- Cyrus

---

From: Ron Bucktonmailto:notifications@github.com
Sent: ‎4/‎10/‎2015 9:20 AM
To: Microsoft/TypeScriptmailto:TypeScript@noreply.github.com
Cc: Cyrus Najmabadimailto:cyrusn@microsoft.com
Subject: Re: [TypeScript] Add support for parsing binding patterns in arrow function parameters (#2680)

In src/compiler/parser.tshttps://github.com//pull/2680#discussion_r28158593:

@@ -3221,6 +3221,37 @@ module ts {
}
}

•            // If encounter "([", this could be the start of an array binding pattern.
  

•            // Examples:
  

•            //      ([ x ]) => { }
  

•            //      ([ x ])
  

•            if (second === SyntaxKind.OpenBracketToken) {
  

I'm seeing it often in conjunction with Promise.all:

Promise.all([a, b, c]).then(([a, b, c]) => {
});

—
Reply to this email directly or view it on GitHubhttps://github.com//pull/2680/files#r28158593.

[rbuckton]

I went ahead and just made OpenBrace/OpenBracket return Tristate.Unknown. The tryParse will already be able to detect this after reading two tokens, so its not much of an added cost to be worth it.