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.
Duncan Murdoch
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