[R] [RE: [Rd] why does parent.frame() cycle when called from inside capture.output()?]

[frederik at ofb.net]

Thanks, Mark - I'm taking up your invitation to forward your message
to the list just because it gives us some valuable data on (1) how
long the behavior has been around, and (2) how many other people
(don't) understand the behavior, and (3) how we might fix or work
around it.

I notice some other people also seem to be diffident about posting on
R-devel; perhaps I should conclude that bugs like this are below the
radar for busy statisticians.

FWIW, after playing around a bit with mvbutils::mvb.parent.frame, I
now have a working "desubN" (see the attachment on my original
message, and this one). I don't really have a good understanding of
*why* it now works, and why the original version didn't work...

Thanks again Mark. Also, nice hacking.

Frederick

----- Forwarded message from Mark.Bravington at data61.csiro.au -----

Date: Mon, 12 Dec 2016 06:20:17 +0000
From: Mark.Bravington at data61.csiro.au
To: frederik at ofb.net
Subject: RE: [Rd] why does parent.frame() cycle when called from inside
	capture.output()?
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Hi Frederik

[ I'm replying off-list in case you, or the rest of R-devel, don't find this reply useful... please fwd to the list if it does help you ]

I'm the author of the 'debug' package. When I wrote it--- many years ago now--- I encountered some fairly strange behaviour with frames in the call stack, which is reminiscent of what you're seeing. The debug package still works fine, so I presume nothing has changed much.

If you load debug and then do ?mvb.sys.parent (and look at the code), you might get *some* idea of what's going on. TBH I can't remember the details now...

HTH
Mark

Mark Bravington
CSIRO Marine Lab
Hobart
Australia

________________________________________
From: R-devel [r-devel-bounces at r-project.org] on behalf of frederik at ofb.net [frederik at ofb.net]
Sent: 12 December 2016 16:54
To: r-devel at r-project.org
Cc: r-help at r-project.org
Subject: [Rd] why does parent.frame() cycle when called from inside capture.output()?

Hello R devel/help,

I ran into this strange behavior:

    # showstack is supposed to walk through the stack of parent
    # environments when it is called:
    showstack = function() {
      env = environment()
      for(i in 1:12) {
        env = do.call(parent.frame, list(), env=env)
        print(env)
      }
    }

    # a simple chain of functions:
    g3=function(t) showstack()
    g2=function(w) g3(w)
    g1=function(z) g2(z)
    g=function(y) g1(y)

    g()
    # outputs:
    # <environment: 0xb5ef810>
    # <environment: 0xb5ef6f8>
    # <environment: 0xb5ef5e0>
    # <environment: 0xb5ef4c8>
    # <environment: R_GlobalEnv>
    # <environment: R_GlobalEnv>
    # <environment: R_GlobalEnv>
    # <environment: R_GlobalEnv>
    # <environment: R_GlobalEnv>
    # ...

    cat(capture.output(g()),sep="\n")
    # outputs:
    # <environment: 0x8106a30>
    # <environment: 0x8106918>
    # <environment: 0x8106800>
    # <environment: 0x81066e8>
    # <environment: R_GlobalEnv>
    # <environment: 0x8107458>
    # <environment: 0x81071b8>
    # <environment: 0x80c6a08>
    # <environment: R_GlobalEnv>
    # <environment: 0x8107458>
    # <environment: 0x81071b8>
    # <environment: 0x80c6a08>

The strange thing of course is that the second call doesn't stay with
R_GlobalEnv, but in fact goes into a loop of period 4. I'm not so
surprised that the parent frame of the global environment is itself,
as in the first call, but it seems weird to have a loop of period 4...
Using `ls()` shows that two of the "loop" environments belong to
capture.output() and eval().

But if capture.output is really evaluating its input in the parent
frame, as it appears to be doing from its source code, then I would
have expected the output to be the same as the output I get by
evaluating the same expression in this frame.

I was trying to debug a function which attempts to be a multi-frame
version of deparse(substitute(...)). I'm attaching this function in
case anyone is curious. Perhaps my attachment can shed more light on
the problem I'm having.

Apologies if this is not a bug - I wasn't sure which mailing list to
send this to, and I took a guess.

Thanks,

Frederick


----- End forwarded message -----
-------------- next part --------------
# We can't just use `mvb.parent.frame` as a replacement for
# `parent.frame`, because the former throws an error when there is no
# parent frame - while we had relied on the latter gracefully giving
# us the global environment. So here's a wrapper which gives
# mvb.parent.frame the behavior we want:

my_mvb_parent=function() {
  tryCatch(
    mvb.parent.frame(2),
    error=function(e) { globalenv()})
}

desubN <- function(y,n=1) {
  env=environment();
  for(i in 1:n) {
    y = do.call(substitute, list(substitute(y)), env=env)
    env = do.call(my_mvb_parent, list(), env=env)
  }
  deparse(y)
}
g2=function(t) {
  for(i in 1:5) {
    print(desubN(t,i))
    print(capture.output(desubN(t,i)))
  }
}
g1=function(z) g2(z)
g=function(y) g1(y)

g(log)

# now capture.output seems to give the same results as the bare
# expression
## [1] "t"
## [1] "[1] \"t\""
## [1] "z"
## [1] "[1] \"z\""
## [1] "y"
## [1] "[1] \"y\""
## [1] "log"
## [1] "[1] \"log\""
## [1] "log"
## [1] "[1] \"log\""