From kbare at andrew.cmu.edu  Wed Jul 29 19:50:16 2009
From: kbare at andrew.cmu.edu (kbare at andrew.cmu.edu)
Date: Wed, 29 Jul 2009 19:50:16 +0200 (CEST)
Subject: [Rd] Strange Interaction Between Promises and Closures (PR#13861)
Message-ID: <20090729175016.731F8282C76A@mail.pubhealth.ku.dk>

Full_Name: Keith Bare
Version: 2.7.1
OS: Linux
Submission from: (NULL) (128.2.134.48)


I observed unexpected behavior attempting to use lapply to vary parameters in
generated closures.  All the generated closures ran with the last parameter
value in the list.

Here's a simple example:

> funcs <- lapply(c("alpha", "beta", "gamma", "delta"), function(x) function()
print(x))
> funcs[[1]]()
[1] "delta"
> funcs[[2]]()
[1] "delta"
> funcs[[3]]()
[1] "delta"
> funcs[[4]]()
[1] "delta"


What appears to be happening, is that the unevaluated promise for x is getting
stored in the generated closure.  This promise references a variable in the
local environment for the lapply call.  However, that variable gets clobbered by
subsequent processing by lapply.


This may be a language "feature" rather than a bug.  But IMO, the observed
behavior is very non-intuitive.  If so, maybe it deserves mention in the
documentation for lapply, "function", or the language reference section on
functions.


For now, I've found that adding a force in the function that generates the
returned closure is a workaround.  E.g.:

> funcs <- lapply(c("alpha", "beta", "gamma", "delta"), function(x) { force(x);
function() print(x) })
> funcs[[1]]()
[1] "alpha"
> funcs[[2]]()
[1] "beta"
> funcs[[3]]()
[1] "gamma"
> funcs[[4]]()
[1] "delta"


And here's my version output (I'm running the version in Debian lenny):

> version
               _                           
platform       x86_64-pc-linux-gnu         
arch           x86_64                      
os             linux-gnu                   
system         x86_64, linux-gnu           
status                                     
major          2                           
minor          7.1                         
year           2008                        
month          06                          
day            23                          
svn rev        45970                       
language       R                           
version.string R version 2.7.1 (2008-06-23)


--Keith



[Rd] Strange Interaction Between Promises and Closures (PR#13861)
kbare at andrew.cmu.edu