Luke Tierney wrote:
> On Fri, 19 May 2006, Duncan Murdoch wrote:
>
>> On 5/19/2006 9:54 AM, Roger D. Peng wrote:
>>> I noticed something recently that I thought was odd:
>>>
>>> delayedAssign("x", { Sys.sleep(5); 1 })
>>> x ## Hit Ctrl-C within the first second or 2
>>>
>>> gives me:
>>>
>>> > delayedAssign("x", { Sys.sleep(5); 1 })
>>> > x ## Hit Ctrl-C within the first second or two
>>>
>>> > x
>>> Error: recursive default argument reference
>>> >
>>>
>>> My only problem here is that now I'm stuck---there's no way to
>>> recover whatever
>>> 'x' was supposed to be (i.e. 1).
>>>
>>> In reality, I want 'x' to be a promise to load a moderately large
>>> data object.
>>> But if I (or a user) Ctrl-C's during the load I'll have to start from
>>> scratch.
>>> Is there anyway to recover the promise (or the value of the
>>> expression) in case
>>> of an interrupt?
>>
>> I don't know of one. Normally substitute(x) is supposed to retrieve the
>> promise expression, but by a strange quirk of history, it does not
>> work when x is in .GlobalEnv.
>>
>> I'd say the behaviour you're seeing is a bug. If I do
>>
>> > x <- 2
>> > x <- {Sys.sleep(1); 1} # Break before complete
>>
>> > x
>> [1] 2
>>
>> nothing is changed about x. I would think the same thing should happen
>> when x is a promise: if the evaluation of the promised expression
>> fails, the promise should not be changed.
>
> I don't think this is a clear as you make it out--given that these
> uses of promises often have side effects, and some of those side
> effects may have occurred prior to an error, it isn't clear that
> pretending like no evaluation had happened is the right way to go.
>
> It should not be too hard to write a delayedAssignmentReset function
> if that is really useful; alternatively a user of delayedAssign should
> be able to arrange via tryCatch to chatch interrupts and re-install
> the delayed assignment if one occurs.
This was my original thought, and I think it would be possible to use tryCatch
to reinstall the delayed assignment. However, I couldn't figure out how to
have
the reinstalled expression to be able to catch interrupts without getting into
some infinite expression.... Perhaps I need to investigate it further.
>
> It might not be a bad idea for us to look into the promise evaluation
> internals and see if we should/can separate the promise black-holing
> from detection of recursive default argument references to get more
> reasonable error messages in these situations and maybe allow
> resetting more gnerally. But anything done here had better keep
> efficiency in mind since this is prety core to R function call
> evaluation. I may try to look into this when I get back to workign on
> R internals.
>
> luke
>
>
>
--
Roger D. Peng | http://www.biostat.jhsph.edu/~rpeng/
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