How would shallow generators compose with lambda?

[Waldemar Horwat]

Brendan Eich wrote:
> On May 14, 2009, at 6:04 PM, Waldemar Horwat wrote:
> 
>> Brendan Eich wrote:
>>> On May 14, 2009, at 1:47 PM, Waldemar Horwat wrote:
>>>> This whole thing is another nail in the coffin of generators.  
>>>> Generators are the root of the troublesome longjmp's which don't 
>>>> play well with others.
>>> Are you talking about generators, or lambdas?
>>
>> Generators.  They cause control flow to jump around between two 
>> independent functions.
>>
>> THe same problem occurs without lambdas:
>>
>> function gen(arg) {
>> try {
>>   yield 1;
>>   yield 2;
>> } finally {
>>   alert("Finally called up to three times?!");
>> }
>> }
>>
>> That finally will be called either 1, 2, or 3 times depending on what 
>> the caller does, which violates the principle that finally is called 
>> exactly once, no matter how you leave a scope.
> 
> Did you actually test this? Firefox 2 and up work the same.
> 
> js> g = gen()
> [object Generator]
> js> g.next()
> 1
> js> g.next()
> 2
> js> g.next()
> typein:6: ReferenceError: alert is not defined
> js> alert=print
> function print() {
>     [native code]
> }
> js> g = gen(42)
> [object Generator]
> js> g.next()
> 1
> js> g.next()
> 2
> js> g.next()
> Finally called up to three times?!
> uncaught exception: [object StopIteration]
> 
> Exactly one finally.

That behavior is wrong in the case that g gets forgotten after calling next just once.  You're silently exiting through a finally clause without calling it.

    Waldemar