The following note in ?force may help
Note:
'force' does not force the evaluation of promises.
It is there because people have been confused before.
On Tue, 8 Mar 2005, Duncan Murdoch wrote:
I'm working on a function that does adaptive sampling, and I thought
it would be handy to return the function's environment as part of the
result so that I could re-use local variables in a subsequent run. My
first try didn't work, and it came down to code like this:
f <- function( H, prevEnv = NULL) {
+ if (!is.null(prevEnv)) H <- prevEnv$H
+ cat('Evaluate H to get ', H(1), '\n')
+ return(environment(NULL))
+ }
I thought that evaluating H would force it, so that H would be
available in the environment returned by the function. But this is
not so:
env <- f( function(x) x^2 )
Evaluate H to get 1
env$H
<promise: 012094D8>
env$H(1)
Error: attempt to apply non-function
So I tried to explicitly force it:
g <- function( H, prevEnv = NULL) {
+ if (!is.null(prevEnv)) H <- prevEnv$H
+ force(H)
+ return(environment(NULL))
+ }
but this still doesn't work:
env <- g( function(x) x^2 )
env$H
<promise: 01206FC0>
env$H(1)
Error: attempt to apply non-function
It seems that I need to do an assignment to convert H from a promise
to an evaluated object:
h <- function( H, prevEnv = NULL) {
+ if (!is.null(prevEnv)) H <- prevEnv$H
+ H <- H
+ return(environment(NULL))
+ }
env <- h( function(x) x^2 )
env$H
function(x) x^2
env$H(1)
[1] 1
Is this a bug, or just the way things are?
I get the same results in both R-patched and R-devel.
--
Brian D. Ripley, [EMAIL PROTECTED]
Professor of Applied Statistics, http://www.stats.ox.ac.uk/~ripley/
University of Oxford, Tel: +44 1865 272861 (self)
1 South Parks Road, +44 1865 272866 (PA)
Oxford OX1 3TG, UK Fax: +44 1865 272595
______________________________________________
R-devel@stat.math.ethz.ch mailing list
https://stat.ethz.ch/mailman/listinfo/r-devel
