const VS features detection

Andreas Rossberg rossberg at
Tue Jan 7 07:04:04 PST 2014

On 6 January 2014 17:59, Allen Wirfs-Brock <allen at> wrote:
> The major new complication of do-expressions is that they allow for the occurrence of break/continue/return abrupt completions in contexts such as for loop heads where they could not perviously occur.  However, do-expressions where still on the table when I did the spec. work for  "completion reform"  so the ES6 draft already deals with these abrupt completions in those contexts. Even though there is currently no way to produce them.

I agree that's a complication, which is why I would propose to
disallow them, at least for the time being. Motivation:

- YAGNI -- I have a hard time coming up with a use case that isn't
obfuscated code (even considering generated code).

- They complicate the semantics and implementation -- for example, you
would have to roll back non-empty expression stacks (in a stack
machine implementation).

- They destroy nice equivalences -- in particular, I'd like "do {...}"
to be equivalent to "(() => {...})()", e.g. to minimise refactoring

- We can always allow them later, if the need should ever arise.

Dave, I remember you were in favour of allowing these. Do you have
specific use cases in mind?


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