[Rd] single assignment affecting multiple sub-structures (PR#7924)

[tplate at blackmesacapital.com]

I'm trying to create a language structure that is a call to a function 
with a number of arguments that is only known at run time.  I do this by 
using repeated indices to expand out a call with a single argument. 
However, when I change one of the arguments, all are changed.

I don't see the same behavior when I initially create a call with 
multiple arguments.

Even more strangely, calling identical(call1, call2) before any attempts 
to modify the structures changes the subsequent behavior to be correct.

[If there are better methods to create and modify calls, please let me 
know!]

 > # Example of commands that give incorrect results
 > call1 <- Quote(f(arg[[1]], arg[[1]], arg[[1]]))
 > call2 <- Quote(f(arg[[1]]))[c(1,2,2,2)]
 > call1
f(arg[[1]], arg[[1]], arg[[1]])
 > call2
f(arg[[1]], arg[[1]], arg[[1]])
 > call1[[3]][[3]] <- 2
 > call2[[3]][[3]] <- 2
 > call1
f(arg[[1]], arg[[2]], arg[[1]])
 > # note that all the arguments of call2 have changed!
 > call2
f(arg[[2]], arg[[2]], arg[[2]])
 > identical(call1, call2)
[1] FALSE
 >
 > # if we do 'identical(call1, call2)' directly
 > # after creation, then everything behaves correctly !??
 > call1 <- Quote(f(arg[[1]], arg[[1]], arg[[1]]))
 > call2 <- Quote(f(arg[[1]]))[c(1,2,2,2)]
 > identical(call1, call2)
[1] TRUE
 > call1
f(arg[[1]], arg[[1]], arg[[1]])
 > call2
f(arg[[1]], arg[[1]], arg[[1]])
 > call1[[3]][[3]] <- 2
 > call2[[3]][[3]] <- 2
 > call1
f(arg[[1]], arg[[2]], arg[[1]])
 > call2
f(arg[[1]], arg[[2]], arg[[1]])
 > identical(call1, call2)
[1] TRUE
 >
 > # The same thing happens when the call is created using 'call()'
 > call3 <- call("f", call("[[", as.name("arg"), 1))[c(1,2,2,2)]
 > call3
f(arg[[1]], arg[[1]], arg[[1]])
 > call3[[3]][[3]] <- 2
 > call3
f(arg[[2]], arg[[2]], arg[[2]])
 >
 > version
          _
platform i386-pc-mingw32
arch     i386
os       mingw32
system   i386, mingw32
status
major    2
minor    1.0
year     2005
month    04
day      18
language R
 >


I also see the same behavior in the development release of R:
 > version
          _
platform i386-pc-mingw32
arch     i386
os       mingw32
system   i386, mingw32
status   Under development (unstable)
major    2
minor    2.0
year     2005
month    06
day      07
svn rev  34588
language R
 >

-- Tony Plate