Bound instance-function vending (was RE: Arrow binding)

Brendan Eich brendan at
Sun Apr 29 01:28:17 PDT 2012

Axel Rauschmayer wrote:
> I am not saying that the transition from the old rules to the new rules will be entirely painless, but if the new rules are simple, that pain is worth it, IMHO.

There are no new rules. Some functions ignore |this| or bind it rather 
than using it. These differences in kind do not change due to arrows.

>   Library code might need to go to extra lengths to help normal developers with the transition (error messages, different behavior, tool functions, etc.) and – to be explicit – might need a predicate such as `isArrowFunction` (which should only ever be used under the hood and thus would not increase confusion for library*users*).

There's no point in such a misnamed predicate. Today we have functions 
that ignore or bind |this|, as well as those that use |this| passed by 
the caller. Code requires and assumes one kind or another and does not 
test (and most programmers wouldn't cover all cases, and shouldn't have to).

Part of an API, an essential part of the contract, is any function 
parameter's |this| binding. Arrows make it easier to use callbacks 
wanting lexical |this| or not using |this| at all. This covers a large 
cohort. Code invoking the arrow is not going to test well or at all and 
do something helpful, or unhelpful such as throwing an exception.


More information about the es-discuss mailing list