R devel - May 2006 - delayedAssign and interrupts

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?

-roger
-- 
Roger D. Peng  |  http://www.biostat.jhsph.edu/~rpeng/

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

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?
Here's the code that causes this:

static SEXP forcePromise(SEXP e)
{
   if (PRVALUE(e) == R_UnboundValue) {
     SEXP val;
     if(PRSEEN(e))
       errorcall(R_GlobalContext->call,
		_("recursive default argument reference"));
     SET_PRSEEN(e, 1);
     val = eval(PRCODE(e), PRENV(e));
     SET_PRSEEN(e, 0);
     SET_PRVALUE(e, val);
   }
   return PRVALUE(e);
}

The idea is that you don't want to get into an infinite loop by having 
something like

f <- function(x = x)

or

delayedAssign(x, x)

so the PRSEEN bit is set before trying to evaluate the promise. 
However, if the eval() aborts it jumps right back up to the top level, 
the bit never gets reset to 0, and you get the spurious error message 
you saw.

I'll look into fixing this for 2.3.1.

Duncan Murdoch