[Rd] Behavior or as.environment in function arguments/call (and force() behaviors...)

[Duncan Murdoch]

On 13-01-01 4:35 PM, Jeff Ryan wrote:
> Happy 2013!
>
> Can someone with more knowledge of edge case scoping/eval rules explain
> what is happening below?  Happens in all the versions of R I have on hand.

Even though it is used as a default in a number of places, the pos==-1 
value is really poorly documented.  You need to look in the source, in 
particular src/main/envir.c, function pos2env.  There you'll see that 
pos==-1 is special cased to be the environment from which pos.to.env
(or as.environment in your case) was called.  For non-negative values, 
it indexes the search list (i.e. the list returned by search().)  Other 
values are errors.

The trouble in your examples is that this location varies.  In Fn1, it 
is being called in the ls() call.  In Fn2, it is in the force() call. 
In Fn3 and Fn4, it's the Fn3/Fn4 call.

In spite of what the docs say in ?get, I would rarely if ever use a pos 
argument to as.environment.  Use an environment and pass it as envir.

Duncan Murdoch

>
> Behavior itself is confusing, but ?as.environment also provides no clue.
>   The term used in that doc is 'search list', which is ambiguous, but the
> see also section mentions search(), so I would *think* that is what is
> intended.  Either way Fn1() below can't really be explained.
>
> Major question is what in the world is Fn1 doing, and why is Fn2 not equal
> to Fn3? [ Fn3/Fn4 are doing what I want. ]
>
>
> Fn1 <- function(x="test",pos=-1,env=as.environment(pos)) {
> ls(env)
> }
>
> Fn2 <- function(x="test",pos=-1,env=as.environment(pos)) {
> force(env)
> ls(env)
> }
>
> Fn3 <- function(x="test",pos=-1,env=as.environment(pos)) {
> # should be the same as force() in Fn2, but not
> # ?force
> # Note:
> #
> #    This is semantic sugar: just evaluating the symbol will do the
> #    same thing (see the examples).
> env
> ls(env)
> }
>
> Fn4 <- function(x="test",pos=-1,env=as.environment(pos)) {
> # same as Fn3
> env <- env
> ls(env)
> }
>
> Fn1()
> Fn2()
> Fn3()
> Fn4()
> ls()
>
> ###################### output #########################
>> Fn1()
> [1] "doTryCatch" "expr"       "handler"    "name"       "parentenv"
>
>> Fn2()
> [1] "env" "pos" "x"
>
>> Fn3()
> [1] "Fn1" "Fn2" "Fn3" "Fn4"
>
>> Fn4()
> [1] "Fn1" "Fn2" "Fn3" "Fn4"
>
> ### .GlobalEnv
>> ls()
> [1] "Fn1" "Fn2" "Fn3" "Fn4"
>
>> R.version
>                 _
> platform       x86_64-apple-darwin11.2.0
> arch           x86_64
> os             darwin11.2.0
> system         x86_64, darwin11.2.0
> status
> major          2
> minor          15.1
> year           2012
> month          06
> day            22
> svn rev        59600
> language       R
> version.string R version 2.15.1 (2012-06-22)
> nickname       Roasted Marshmallows
>
>> R.version
>                 _
> platform       x86_64-apple-darwin11.2.0
> arch           x86_64
> os             darwin11.2.0
> system         x86_64, darwin11.2.0
> status         Under development (unstable)
> major          3
> minor          0.0
> year           2012
> month          12
> day            28
> svn rev        61464
> language       R
> version.string R Under development (unstable) (2012-12-28 r61464)
> nickname       Unsuffered Consequences
>