RFC: getifexists() {was [Bug 16065] "exists" ...}

On Thu, 8 Jan 2015, Michael Lawrence wrote:

 If we do add an argument to get(), then it should be named consistently

with the ifnotfound argument of mget(). As mentioned, the possibility of a
NULL value is problematic. One solution is a sentinel value that indicates
an unbound value (like R_UnboundValue).

A null default is fine -- it's a default; if it isn't right for a
particular case you can provide something else.

But another idea (and one pretty similar to John's) is to follow the
SYMSXP
design at the C level, where there is a structure that points to the name
and a value. We already have SYMSXPs at the R level of course (name
objects) but they do not provide access to the value, which is typically
R_UnboundValue. But this does not even need to be implemented with SYMSXP.
The design would allow something like:

binding <- getBinding("x", env)
if (hasValue(binding)) {
 x <- value(binding) # throws an error if none
 message(name(binding), "has value", x)
}

That I think it is a bit verbose but readable and could be made fast. And
I
think binding objects would be useful in other ways, as they are
essentially a "named object". For example, when iterating over an
environment.

This would need a lot more thought. Directly exposing the internals is
definitely not something we want to do as we may well want to change
that design. But there are lots of other corner issues that would have
to be thought through before going forward, such as what happens if an
rm occurs between obtaining a binding object and doing something with
it. Serialization would also need thinking through. This doesn't seem
like a worthwhile place to spend our efforts to me.

Adding getIfExists, or .get, or get0, or whatever seems fine. Adding
an argument to get() with missing giving current behavior may be OK
too. Rewriting exists and get as .Primitives may be sufficient though.

Best,

luke


 Michael

On Thu, Jan 8, 2015 at 6:03 AM, John Nolan <jpnolan at american.edu> wrote:

 Adding an optional argument to get (and mget) like

val <- get(name, where, ..., value.if.not.found=NULL )   (*)

would be useful for many.  HOWEVER, it is possible that there could be
some confusion here: (*) can give a NULL because either x exists and
has value NULL, or because x doesn't exist.   If that matters, the user
would need to be careful about specifying a value.if.not.found that
cannot
be confused with a valid value of x.

To avoid this difficulty, perhaps we want both: have Martin's
getifexists(
)
return a list with two values:
  - a boolean variable 'found'  # = value returned by exists( )
  - a variable 'value'

Then implement get( ) as:

get <- function(x,...,value.if.not.found ) {

  if( missing(value.if.not.found) ) {
    a <- getifexists(x,... )
    if (!a$found) error("x not found")
  } else {
    a <- getifexists(x,...,value.if.not.found )
  }
  return(a$value)
}

Note that value.if.not.found has no default value in above.
It behaves exactly like current get does if value.if.not.found
is not specified, and if it is specified, it would be faster
in the common situation mentioned below:
     if(exists(x,...)) { get(x,...) }

John

P.S. if you like dromedaries call it valueIfNotFound ...

 ..............................................................
 John P. Nolan
 Math/Stat Department
 227 Gray Hall,   American University
 4400 Massachusetts Avenue, NW
 Washington, DC 20016-8050

 jpnolan at american.edu       voice: 202.885.3140
 web: academic2.american.edu/~jpnolan
 ..............................................................


-----"R-devel" <r-devel-bounces at r-project.org> wrote: -----
To: Martin Maechler <maechler at stat.math.ethz.ch>, R-devel at r-project.org
From: Duncan Murdoch
Sent by: "R-devel"
Date: 01/08/2015 06:39AM
Subject: Re: [Rd] RFC: getifexists() {was [Bug 16065] "exists" ...}

On 08/01/2015 4:16 AM, Martin Maechler wrote:

In November, we had a "bug repository conversation"
with Peter Hagerty and myself:

  https://bugs.r-project.org/bugzilla/show_bug.cgi?id=16065

where the bug report title started with

 --->>  "exists" is a bottleneck for dispatch and package loading, ...

Peter proposed an extra simplified and henc faster version of exists(),
and I commented

    > --- Comment #2 from Martin Maechler <maechler at stat.math.ethz.ch>

---

    > I'm very grateful that you've started exploring the bottlenecks

of
loading

    > packages with many S4 classes (and methods)...
    > and I hope we can make real progress there rather sooner than

later.

    > OTOH, your `summaryRprof()` in your vignette indicates that

exists() may use

    > upto 10% of the time spent in library(reportingTools),  and your

speedup

    > proposals of exist()  may go up to ca 30%  which is good and well

worth

    > considering,  but still we can only expect 2-3% speedup for

package loading

    > which unfortunately is not much.

    > Still I agree it is worth looking at exists() as you did  ... and
    > consider providing a fast simplified version of it in addition to

current

    > exists() [I think].

    > BTW, as we talk about enhancements here, maybe consider a further

possibility:

    > My subjective guess is that probably more than half of exists()

uses are of the

    > form

    > if(exists(name, where, .......)) {
    >    get(name, whare, ....)
    >    ..
    > } else {
    >     NULL / error() / .. or similar
    > }

    > i.e. many exists() calls when returning TRUE are immediately

followed by the

    > corresponding get() call which repeats quite a bit of the lookup

that exists()

    > has done.

    > Instead, I'd imagine a function, say  getifexists(name, ...) that

does both at

    > once in the "exists is TRUE" case but in a way we can easily keep

the if(.) ..

    > else clause above.  One already existing approach would use

    > if(!inherits(tryCatch(xx <- get(name, where, ...),

error=function(e)e), "error")) {

    >   ... (( work with xx )) ...

    > } else  {
    >    NULL / error() / .. or similar
    > }

    > but of course our C implementation would be more efficient and

use
more concise

    > syntax {which should not look like error handling}.   Follow ups

to this idea

    > should really go to R-devel (the mailing list).

and now I do follow up here myself :

I found that  'getifexists()' is actually very simple to implement,
I have already tested it a bit, but not yet committed to R-devel
(the "R trunk" aka "master branch") because I'd like to get
public comments {RFC := Request For Comments}.

I don't like the name -- I'd prefer getIfExists.  As Baath (2012, R
Journal) pointed out, R names are very inconsistent in naming
conventions, but lowerCamelCase is the most common choice.  Second most
common is period.separated, so an argument could be made for
get.if.exists, but there's still the possibility of confusion with S3
methods, and users of other languages where "." is an operator find it a
little strange.

If you don't like lowerCamelCase (and a lot of people don't), then I
think underscore_separated is the next best choice, so would use
get_if_exists.

Another possibility is to make no new name at all, and just add an
optional parameter to get() (which if present acts as your value.if.not
parameter, if not present keeps the current "object not found" error).

Duncan Murdoch

My version of the help file {for both exists() and getifexists()}
rendered in text is

---------------------- help(getifexists) ------------------------------

-

Is an Object Defined?

Description:

     Look for an R object of the given name and possibly return it

Usage:

     exists(x, where = -1, envir = , frame, mode = "any",
            inherits = TRUE)

     getifexists(x, where = -1, envir = as.environment(where),
                 mode = "any", inherits = TRUE, value.if.not = NULL)

Arguments:

       x: a variable name (given as a character string).

   where: where to look for the object (see the details section); if
          omitted, the function will search as if the name of the
          object appeared unquoted in an expression.

   envir: an alternative way to specify an environment to look in, but
          it is usually simpler to just use the 'where' argument.

   frame: a frame in the calling list.  Equivalent to giving 'where' as
          'sys.frame(frame)'.

    mode: the mode or type of object sought: see the 'Details' section.

inherits: should the enclosing frames of the environment be searched?

value.if.not: the return value of 'getifexists(x, *)' when 'x' does not
          exist.

Details:

     The 'where' argument can specify the environment in which to look
     for the object in any of several ways: as an integer (the position
     in the 'search' list); as the character string name of an element
     in the search list; or as an 'environment' (including using
     'sys.frame' to access the currently active function calls).  The
     'envir' argument is an alternative way to specify an environment,
     but is primarily there for back compatibility.

     This function looks to see if the name 'x' has a value bound to it
     in the specified environment.  If 'inherits' is 'TRUE' and a value
     is not found for 'x' in the specified environment, the enclosing
     frames of the environment are searched until the name 'x' is
     encountered.  See 'environment' and the 'R Language Definition'
     manual for details about the structure of environments and their
     enclosures.

     *Warning:* 'inherits = TRUE' is the default behaviour for R but
     not for S.

     If 'mode' is specified then only objects of that type are sought.
     The 'mode' may specify one of the collections '"numeric"' and
     '"function"' (see 'mode'): any member of the collection will
     suffice.  (This is true even if a member of a collection is
     specified, so for example 'mode = "special"' will seek any type of
     function.)

Value:

     'exists():' Logical, true if and only if an object of the correct
     name and mode is found.

     'getifexists():' The object-as from 'get(x, *)'- if 'exists(x, *)'
     is true, otherwise 'value.if.not'.

Note:

   With 'getifexists()', instead of the easy to read but somewhat
   inefficient

       if (exists(myVarName, envir = myEnvir)) {
         r <- get(myVarName, envir = myEnvir)
         ## ... deal with r ...
       }

   you now can use the more efficient (and slightly harder to read)

       if (!is.null(r <- getifexists(myVarName, envir = myEnvir))) {
         ## ... deal with r ...
       }

References:

     Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988) _The New S
     Language_.  Wadsworth & Brooks/Cole.

See Also:

     'get'.  For quite a different kind of "existence" checking, namely
     if function arguments were specified, 'missing'; and for yet a
     different kind, namely if a file exists, 'file.exists'.

Examples:

     ##  Define a substitute function if necessary:
     if(!exists("some.fun", mode = "function"))
       some.fun <- function(x) { cat("some.fun(x)\n"); x }
     search()
     exists("ls", 2) # true even though ls is in pos = 3
     exists("ls", 2, inherits = FALSE) # false

     ## These are true (in most circumstances):
     identical(ls,   getifexists("ls"))
     identical(NULL, getifexists(".foo.bar.")) # default value.if.not =

NULL(!)

----------------- end[ help(getifexists) ]

-----------------------------

        [[alternative HTML version deleted]]

--
Luke Tierney
Ralph E. Wareham Professor of Mathematical Sciences
University of Iowa                  Phone:             319-335-3386
Department of Statistics and        Fax:               319-335-3017
   Actuarial Science
241 Schaeffer Hall                  email:   luke-tierney at uiowa.edu
Iowa City, IA 52242                 WWW:  http://www.stat.uiowa.edu