From: Charles Oliver Nutter <charles.nutter@...>
Date: 2008-01-03T15:35:20+09:00
Subject: Re: Inconsistency in rescuability of "return"

Gary Wright wrote:
> 
> On Jan 2, 2008, at 7:15 PM, Jano Svitok wrote:
> 
>> On Jan 3, 2008 12:53 AM, Gary Wright <gwtmp01@mac.com> wrote:
>>>
>>> On Jan 2, 2008, at 6:31 PM, Charles Oliver Nutter wrote:
>>>> But when eval'ing a break outside a while, it does propagate an
>>>> error. So the code under eval is changing its behavior depending on
>>>> what structures surround it. That seems wrong to me, and it means
>>>> that any implementations wanting to emulate it will have to
>>>> maintain state across calls, so eval knows it's being called within
>>>> a loop.
>>>
>>> I don't see how this is any different than the other examples.  When
>>> "eval 'break'" is inside the while loop, 'break' doesn't cause an
>>> error, it simply breaks out of the loop.  If you put "eval 'break'"
>>> at the top level you've got your local jump error again, right?
>>
>> The issue is that eval result depends on one more variable - whether
>> the call is inside a loop or not.
>> So it's a function of it's parameters, data, AND previous calls (or
>> how you would call it).
> 
> Yes, but the behavior of 'break' itself is dependent on the context.  So 
> it is the semantics of 'break' that seems to be in question in this 
> example, not the semantics of 'eval' and the semantics of break in this 
> example are 'normal' as far as I can tell:
> 
> A) break (via eval or not) outside of a loop context causes an exception
> B) break (via eval or not) inside a loop context causes the loop to be 
> terminated
> 
> If this is problematic, what behavior would you expect for A and B?

The problem with this logic is that in case A, the exception also causes 
the loop to be terminated as normal:

while true
   eval 'break'
end

So there's obviously some duality behind break.

1. A bare break can't be cause as an exception, and terminates an 
enclosing loop
2. A break inside an eval can be caught as an exception, and also 
terminates an enclosing loop like a normal break.

- Charlie