[Rd] experiments with slot functions and possible problems NOTE

[Duncan Murdoch]

On 1/21/2008 8:30 AM, Thomas Petzoldt wrote:
> Hello,
> 
> first of all, thanks to LT for \pkg{codeutils}. I agree that it is
> indeed very useful to identify errors and also to encourage re-thinking
> past solutions. My problem:
> 
> I want to compare different sets of related sub-functions which should
> be used alternatively by the same top-level function. Sets of related
> functions should be bound together (as lists) and the workspace should
> be as clean as possible.
> 
> Finally, these functions are to be called by top-level functions that
> work with such sets.
> 
> What's the best way to do this?
> 
> - clutter the workspace with lots of functions?
> OR:
> - ignore "notes about possible problems"
> OR:
> - a third way?
> 
> Thanks in advance
> 
> Thomas P.
> 
> 
> 
> An example:
> 
> ##=============================================================
> ## 1) One possible "set of functions"
> flistA <- list(
>    foo = function() {
>     1:10
>    },
>    bar = function() {
>      log(foo())
>    }
> )
> 
> ## .. we may also have alternative sets,
> ##    e.g. flistB, flistC, ... etc
> 
> ## 2) Now we try to construct closures
> 
> ## 2a) non-nested
> makefun1 <- function(flist) {
>    with(flist,
>      function() foo()
>    )
> }
> 
> ## 2b) nested call
> makefun2 <- function(flist) {
>    with(flist,
>      function() bar()
>    )
> }
> ## 2c) or use an alternative way with a special function
> ##  addtoenv, suggested by Gabor Grothendieck some times ago:
> addtoenv <- function(L, p = parent.frame()) {
>    for(nm in names(L)) {
>      assign(nm, L[[nm]], p)
>      environment(p[[nm]]) <- p
>    }
>    L
> }
> 
> makefun3 <- function(flist) {
>    addtoenv(flist)
>    function() bar()
> }
> 
> ## 3) now we create the "top-level" functions
> ##    with one particular "set of functions"
> m1 <- makefun1(flistA)
> m2 <- makefun2(flistA)
> m3 <- makefun3(flistA)
> 
> m1()
> ## this was no problem, trivial
> 
> m2()
> # Error in bar() : could not find function "foo"

That's because the environment of bar was the evaluation frame in effect 
at the time it was created, and foo wasn't in that.  bar looks in its 
environment for non-local bindings.

Gabor's function edits the environment.
> 
> m3()
> # works, but even in that case we get problems
> # if we do this in a package:
> 
> # * checking R code for possible problems ... NOTE
> # bar: no visible global function definition for 'foo'

This is a spurious error:  codetools can't follow the strange stuff 
you're doing.

I'd say the best approach would be to use lots of little functions, and 
a namespace to hide them.  Then codetools will be happy.  For example,

Afoo <- function() {
      1:10
}
Abar <-function() {
       log(Afoo())
}
fListA <- list(foo = Afoo, bar = Abar)

This won't allow global references to foo or bar to escape the watchful 
eye of codetools; if you want those, you'd do something like

foo <- function() stop("foo not initialized")
bar <- function() stop("bar not initialized")

and later have

foo <- makefun1(fListA)
bar <- makefun2(fListA)

Duncan Murdoch