[Rd] save() saves extra stuff if object is not evaluated

[Luke Tierney]

On Thu, 25 May 2006, Henrik Bengtsson wrote:

> Hi,
>
> it looks like save() is saving all contents of the calling
> environments if the object to be saved is *not* evaluated, although it
> is not that simple either.

No, it's exactly that simple.  Serialization follows and writes out
all reachable environments.  Unevaluated promises contain the
environments in which their evaluations are to occur; evaluated ones
have this field set to R_NilValue to eliminate this no longer needed
reference.

There are two environments involved: the calling environment in which
saveCache is called and the callee environment of the call to
saveCache where the body of saveCache is evaluated.  Because of
lexical scope the enclosing environment of the callee environment is
the closure environment of saveCache, which is .GlobalEnv.

The call to saveCache creates a promise for evaluating the default
value for 'source' _in the callee environment_. In the case with y the
callee environment includes a value of y which is a promise
referencing the calling environment (either .GlobalENv or the
environment of the call to main).  In the calls without y the value of
y in the calling environment is the missing value indicator, not a
promise.  So only with y and no eval is there a reference to the
calling environment that serialization then has to write out.

Best,

luke


> After many hours of troubleshooting, I'm
> still confused.  Here is a reproducible example (also attached) with
> output.  I let the code and the output talk for itself:
>
> peek <- function(file, from=1, to=500) {
> cat("--------------------------------------\n")
> cat(sprintf("%s: %d bytes\n", file, file.info(file)$size))
> bfr <- suppressWarnings(readBin(file, what="character", n=to))
> bfr <- gsub("(\001|\002|\003|\004|\005|\016|\020|\036|\a|\n|\t)", "", bfr);
> bfr <- bfr[nchar(bfr) > 0];
> cat(bfr, sep="", "\n");
> }
>
> saveCache <- function(file, y, sources=NULL, eval=FALSE) {
> if (eval)
>   dummy <- is.null(sources)
> base::save(file=file, sources, compress=FALSE)
> }
>
> aVariableNotSaved <- double(1e6)
>
> main <- function() {
> # This 'big' variable is saved in case 1 below!
> big <- rep(letters, length.out=1e5)
> identifier <- "This string will be saved too!"
>
> y <- 1
>
> file <- "a.RData"
> saveCache(y, file=file)
> peek(file)
>
> file <- "a-eval.RData"
> saveCache(y, file=file, eval=TRUE)
> peek(file)
>
> file <- "b-noy.RData"
> saveCache(file=file)
> peek(file)
>
> file <- "b-noy-eval.RData"
> saveCache(file=file, eval=TRUE)
> peek(file)
> }
>
>
> # 1. Call saveCache() outside main()
> eval(body(main))
> # --------------------------------------
> # a.RData: 238 bytes
> # RDX2Xsources²filea.RData y²n $  n¦$eval¦¦¦n¦
> # --------------------------------------
> # a-eval.RData: 58 bytes
> # RDX2Xsources¦¦
> # --------------------------------------
> # b-noy.RData: 230 bytes
> # RDX2Xsources²file?b-noy.RData ¶yv$  n¦$eval¦¦¦n¦
> # --------------------------------------
> # b-noy-eval.RData: 58 bytes
> # RDX2Xsources¦¦
>
> # 2. Call saveCache() from within main()
> main()
> # --------------------------------------
> # a.RData: 900412 bytes
> # RDX2Xsources²filea.RData y² a.RData ?=identifierThis
> # string will be saved too!bigåáabcdefghijklmnopqrstuv
> # wxyzabcdefghijklmnopqrstuvwxyzabcdefg
> # --------------------------------------
> # a-eval.RData: 58 bytes
> # RDX2Xsources¦¦
> # --------------------------------------
> # b-noy.RData: 230 bytes
> # RDX2Xsources²file?b-noy.RData ¶yv$  n¦$eval¦¦¦n¦
> # --------------------------------------
> # b-noy-eval.RData: 58 bytes
> # RDX2Xsources¦¦
>
> What is going on?
>
> I get this on both R v2.3.0 patched (2006-04-28 r37936) and R v2.3.1
> beta (2006-05-23 r38179) on my WinXP (with Rterm --vanilla).
>

-- 
Luke Tierney
Chair, Statistics and Actuarial Science
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 at stat.uiowa.edu
Iowa City, IA 52242                 WWW:  http://www.stat.uiowa.edu