function doesn't exists but still runs..... (akshay kulkarni)

________________________________
From: Jorgen Harmse <JHarmse at roku.com>
Sent: Friday, January 20, 2023 9:34 PM
To: r-help at r-project.org <r-help at r-project.org>; akshay_e4 at hotmail.com <akshay_e4 at hotmail.com>; williamwdunlap at gmail.com <williamwdunlap at gmail.com>
Subject: Re: function doesn't exists but still runs..... (akshay kulkarni)


It may help to expand a bit on Bill Dunlap's answer. I think that library does something like this:



Create a new environment for all the package objects. This environment will not be directly visible from .GlobalEnv, and ancestor environments may not be directly visible either. It may contain functions & other objects that are not exported, and it may use objects in ancestor environments that .GlobalEnv doesn't see directly. On the other hand, functions in the package will still see external functions in the way the package author intended instead of seeing functions with the same name that are visible to .GlobalEnv.



Run the source code in the private environment (using source(local=private environment, ....)?). Most package source code just defines functions, but the source code could build other objects that the package needs for some reason, or it could use delayedAssign to build the objects lazily. By default, the environment of any function defined by the source code is the private environment, so the function has access to private objects and to anything in ancestor environments.



Create a second new environment whose parent is parent.env(.GlobalEnv). For every export, assign the corresponding object from the private environment into the corresponding name in the public environment. Note that the environment of any function is still the private environment in which it was created. (I think that a function is mostly determined by its environment, its formals, and its body. A function call creates a new environment whose parent is the environment of the function. Thus whoever wrote the function can control the search for anything that isn?t passed in or created by the function itself.)



Reset parent.env(.GlobalEnv) to be the public environment. This makes all the exported objects (usually functions) available at the command line and allows the user to see everything that was available before (usually by name only, but by scope-resolved name if necessary). As noted by Bill Dunlap and in more detail above, package functions can use functions & other objects that are not directly visible to the user. As he also showed, you can (usually) pierce the privacy as long at least one function is exported. environment(package_function) is the private environment, so you can use it to see all the private objects and everything in the ancestor environments. You can repeat the trick to see private environments of packages you didn't directly pull in. I think you can even unlock bindings and do ghastly things to the package's private environment.



Regards,

Jorgen Harmse.

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Message: 17
Date: Thu, 19 Jan 2023 16:02:31 -0800
From: Bill Dunlap <williamwdunlap at gmail.com>
To: akshay kulkarni <akshay_e4 at hotmail.com>
Cc: R help Mailing list <r-help at r-project.org>
Subject: Re: [R] function doesn't exists but still runs.....
Message-ID:
       <CAHqSRuTCBqh84FHg7=Xmd9qHxUiAN3Y4zBJbkAR3dRWhggu89Q at mail.gmail.com>
Content-Type: text/plain; charset="utf-8"

Look into R's scoping rules.  E.g.,
https://bookdown.org/rdpeng/rprogdatascience/scoping-rules-of-r.html.

* When a function looks up a name, it looks it up in the environment in
which the function was defined.
* Functions in a package are generally defined in the package's environment
(although sometimes they are in a descendent of the parent's environment).
* When one searches an environment for a name, if it is not found in the
environment the search continues in the parent environment of that
environment, recursively until the parent environment is the empty
environment.

with(environment(wdman::selenium), java_check)
function ()
{
   javapath <- Sys.which("java")
   if (identical(unname(javapath), "")) {
       stop("PATH to JAVA not found. Please check JAVA is installed.")
   }
   javapath
}
<bytecode: 0x000001fd0ab826a8>
<environment: namespace:wdman>

-Bill

On Thu, Jan 19, 2023 at 2:28 PM akshay kulkarni <akshay_e4 at hotmail.com>
wrote:

dear members,
                            I am using the RSelenium package which uses
the function selenium() from the wdman package. The selenium function
contains the function java_check at line 12. If I try to run it, it throws
an error:

  javapath <- java_check()
Error in java_check() : could not find function "java_check"

Also:

exists("java_check")
[1] FALSE

But when I run selenium(), it works fine....

How do you explain this conundrum? You can refer to this link:
https://github.com/ropensci/wdman/issues/15

Specifically what concept of R explains this weird behaviour?

Thanking you,
Yours sincerely,
AKSHAY M KULKARNI


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