I was trying to create a function with a value computed at creation time,
using substitute(), but I got results I don't understand:
> this.is.R
Error: Object "this.is.R" not found
> substitute(this.is.R <- function() X,
list(X=!is.null(options("CRAN")[[1]])))
this.is.R <- function() TRUE
> # the above expression as printed is what I want for the function
definition
> eval(substitute(this.is.R <- function() X,
list(X=!is.null(options("CRAN")[[1]]))))
> this.is.R
function() X
> this.is.R()
[1] TRUE
> X
Error: Object "X" not found
> rm(this.is.R)
> # Try again a slightly different way
> substitute(this.is.R <- function() X,
list(X=!is.null(options("CRAN")[[1]])))
this.is.R <- function() TRUE
> .Last.value
this.is.R <- function() TRUE
> eval(.Last.value)
> this.is.R
function() X
> this.is.R()
[1] TRUE
> rm(this.is.R)
>
Why is the body of the function "X" when I substituted a different
expression for X? Also, given that the body of the function is X, how does
the function evaluate to TRUE since X is not defined anywhere (except in a
list that should have been discarded.)
This happens with both R 1.7.1 and R 1.8.0 (under Windows 2000).
(yes, I did discover the function is.R(), but I still want to discover
what's going here.)
-- Tony Plate
PS. In S-plus 6.1, things worked as I had expected:
> substitute(this.is.R <- function() X,
list(X=!is.null(options("CRAN")[[1]])))
this.is.R <- function()
F
> eval(substitute(this.is.R <- function() X,
list(X=!is.null(options("CRAN")[[1]]))))
function()
F
> this.is.R
function()
F
> this.is.R()
[1] F
>
Tony Plate <tplate at blackmesacapital.com> writes:> Why is the body of the function "X" when I substituted a different > expression for X? Also, given that the body of the function is X, how > does the function evaluate to TRUE since X is not defined anywhere > (except in a list that should have been discarded.) > > This happens with both R 1.7.1 and R 1.8.0 (under Windows 2000). > > (yes, I did discover the function is.R(), but I still want to discover > what's going here.)I think you are defeating the keep.source mechanism. What you're seeing is not the actual function but its source attribute. Try attributes(this.is.R) <- NULL and see what happens. -- O__ ---- Peter Dalgaard Blegdamsvej 3 c/ /'_ --- Dept. of Biostatistics 2200 Cph. N (*) \(*) -- University of Copenhagen Denmark Ph: (+45) 35327918 ~~~~~~~~~~ - (p.dalgaard at biostat.ku.dk) FAX: (+45) 35327907
Peter, thank you for the explanation. This is indeed what is happening.
Might I suggest the following passage for inclusion in the help page for
"function", and possibly also "body", in the DETAILS or
WARNING section:
"Note that the text of the original function definition is saved as an
attribute "source" on the function, and this is printed out when the
function is printed. Hence, if the function body is changed in some way
other than by assigning a value via body() (which removes the "source"
attribute), the printed form of the function may not be the same as the
actual function body."
Something along these lines could also go in the help for "eval",
though if
it were only there it might be very difficult to find.
Here is a transcript that shows what is happening, with another suggestion
following it.
> eval(substitute(this.is.R <- function() X,
list(X=!is.null(options("CRAN")[[1]]))))
> this.is.R
function() X
> body(this.is.R)
[1] TRUE
> attributes(this.is.R)
$source
[1] "function() X"
> attributes(this.is.R) <- NULL
> this.is.R
function ()
TRUE
> # the "source" attribute comes from function definition:
> attributes(function() X)
$source
[1] "function() X"
> # and seems to be added by "eval":
> attr(eval(parse(text="function() TRUE")[[1]]),
"source")
[1] "function() TRUE"
>
> # we can assign bogus "source"
> attr(this.is.R, "source") <- "a totally bogus function
body"
> this.is.R
a totally bogus function body
> # assigning to body() removes "source"
> body(this.is.R) <- list(666)
> this.is.R
function ()
666
> attr(this.is.R, "source")
NULL
>
An even better approach might be something that gave a warning on printing
if the parsed "source" attribute was not identical to the language
object
being printed. This would probably belong in the code for "case
LANGSXP:"
in the function PrintValueRec in main/print.c (if it were written in R, I
could contribute a patch, but right now I don't have time to try to
understand the C there.) R code to do the test could be something like this:
> f <- this.is.R
> identical(f, eval(parse(text=attr(f, "source"))[[1]]))
[1] FALSE
> f <- function() TRUE
> identical(f, eval(parse(text=attr(f, "source"))[[1]]))
[1] TRUE
>
-- Tony Plate
At Thursday 09:12 PM 10/23/2003 +0200, Peter Dalgaard
wrote:>Tony Plate <tplate at blackmesacapital.com> writes:
>
> > Why is the body of the function "X" when I substituted a
different
> > expression for X? Also, given that the body of the function is X, how
> > does the function evaluate to TRUE since X is not defined anywhere
> > (except in a list that should have been discarded.)
> >
> > This happens with both R 1.7.1 and R 1.8.0 (under Windows 2000).
> >
> > (yes, I did discover the function is.R(), but I still want to discover
> > what's going here.)
>
>I think you are defeating the keep.source mechanism. What you're
>seeing is not the actual function but its source attribute. Try
>attributes(this.is.R) <- NULL and see what happens.
>
>--
> O__ ---- Peter Dalgaard Blegdamsvej 3
> c/ /'_ --- Dept. of Biostatistics 2200 Cph. N
> (*) \(*) -- University of Copenhagen Denmark Ph: (+45) 35327918
>~~~~~~~~~~ - (p.dalgaard at biostat.ku.dk) FAX: (+45) 35327907
[resent the next day, after not appearing on the list]
Peter, thank you for the explanation. This is indeed what is happening.
Might I suggest the following passage for inclusion in the help page for
"function", and possibly also "body", in the DETAILS or
WARNING section:
"Note that the text of the original function definition is saved as an
attribute "source" on the function, and this is printed out when the
function is printed. Hence, if the function body is changed in some way
other than by assigning a value via body() (which removes the "source"
attribute), the printed form of the function may not be the same as the
actual function body."
Something along these lines could also go in the help for "eval",
though if
it were only there it might be very difficult to find if one were trying to
look up puzzling behavior of a function.
Here is a transcript that shows what is happening, with another suggestion
following it.
> eval(substitute(this.is.R <- function() X,
list(X=!is.null(options("CRAN")[[1]]))))
> this.is.R
function() X
> body(this.is.R)
[1] TRUE
> attributes(this.is.R)
$source
[1] "function() X"
> attributes(this.is.R) <- NULL
> this.is.R
function ()
TRUE
> # the "source" attribute comes from function definition:
> attributes(function() X)
$source
[1] "function() X"
> # and seems to be added by "eval":
> attr(eval(parse(text="function() TRUE")[[1]]),
"source")
[1] "function() TRUE"
>
> # we can assign bogus "source"
> attr(this.is.R, "source") <- "a totally bogus function
body"
> this.is.R
a totally bogus function body
> # assigning to body() removes "source"
> body(this.is.R) <- list(666)
> this.is.R
function ()
666
> attr(this.is.R, "source")
NULL
>
An even better approach might be something that gave a warning on printing
if the parsed "source" attribute was not identical to the language
object
being printed. This would probably belong in the code for "case
LANGSXP:"
in the function PrintValueRec in main/print.c (if it were written in R, I
could contribute a patch, but right now I don't have time to try to
understand the C there.) R code to do the test could be something like this:
> f <- this.is.R
> identical(f, eval(parse(text=attr(f, "source"))[[1]]))
[1] FALSE
> f <- function() TRUE
> identical(f, eval(parse(text=attr(f, "source"))[[1]]))
[1] TRUE
>
-- Tony Plate
At Thursday 09:12 PM 10/23/2003 +0200, Peter Dalgaard
wrote:>Tony Plate <tplate at blackmesacapital.com> writes:
>
> > Why is the body of the function "X" when I substituted a
different
> > expression for X? Also, given that the body of the function is X, how
> > does the function evaluate to TRUE since X is not defined anywhere
> > (except in a list that should have been discarded.)
> >
> > This happens with both R 1.7.1 and R 1.8.0 (under Windows 2000).
> >
> > (yes, I did discover the function is.R(), but I still want to discover
> > what's going here.)
>
>I think you are defeating the keep.source mechanism. What you're
>seeing is not the actual function but its source attribute. Try
>attributes(this.is.R) <- NULL and see what happens.
>
>--
> O__ ---- Peter Dalgaard Blegdamsvej 3
> c/ /'_ --- Dept. of Biostatistics 2200 Cph. N
> (*) \(*) -- University of Copenhagen Denmark Ph: (+45) 35327918
>~~~~~~~~~~ - (p.dalgaard at biostat.ku.dk) FAX: (+45) 35327907