Thanks Bill. I think my confusion may have been in part due to my
conflating two distinct meanings of the term "evaluate"; the help for
force says it "forces the evaluation of a function argument" whereas the
help for eval says it "evaluates the ... argument ... and returns the
computed value". I found it helpful to compare:
> lapply(list(a=1,b=2,c=3), function(x){ force(substitute(x)) })
$a
X[[i]]
$b
X[[i]]
$c
X[[i]]
versus
> lapply(list(a=1,b=2,c=3), function(x){ eval(substitute(x)) })
Error in eval(substitute(x)) : object 'X' not found
Now for the context my question arose in: given a function
loader <- function(package, quietly = TRUE) {
wrapper <- if (quietly) suppressPackageStartupMessages else `{`
expr <- substitute(wrapper(library(package = package)))
eval(expr)
}
prior to R version 3.2, one could do things like
lapply(c("MASS", "boot"), loader)
but not anymore (which is fine; I agree that one should not depend on
lapply's implementation details).
Regards,
Ben
On 07/28/2017 06:53 PM, William Dunlap wrote:
1: substitute(), when given an argument to a function (which will be a
promise) gives you the unevaluated expression given as the argument:
> L <- list(a=1, b=2, c=3)
> str(lapply(L, function(x) substitute(x)))
List of 3
$ a: language X[[i]]
$ b: language X[[i]]
$ c: language X[[i]]
The 'X' and 'i' are in a frame constructed by lapply and you are not
really supposed to depend on the precise form of those expressions.
2: An evaluated promise is still a promise: it has the 'evaled' field
set to TRUE and the 'value' field set to the result of evaluating
'code' in 'env'.
> f <- function(x, force) {
if (force) force(x)
if (pryr::is_promise(x)) promise_info(x)
else "not a promise"
}
> str(f(log(-1), force=FALSE))
List of 4
$ code : language log(-1)
$ env :<environment: R_GlobalEnv>
$ evaled: logi FALSE
$ value : NULL
> str(f(log(-1), force=TRUE))
List of 4
$ code : language log(-1)
$ env : NULL
$ evaled: logi TRUE
$ value : num NaN
Warning message:
In log(-1) : NaNs produced
Can you give a concrete example of what you are try to accomplish?
Bill Dunlap
TIBCO Software
wdunlap tibco.com <http://tibco.com>
On Fri, Jul 28, 2017 at 3:04 PM, Benjamin Tyner <bty...@gmail.com
<mailto:bty...@gmail.com>> wrote:
Hi,
I thought I understood the change to lapply semantics resulting
from this,
https://bugs.r-project.org/bugzilla/show_bug.cgi?id=16093
<https://bugs.r-project.org/bugzilla/show_bug.cgi?id=16093>
However, would someone care to explain why this does not work?
> L <- list(a=1, b=2, c=3)
> str(lapply(L, function(x){ y <- substitute(x); force(x);
eval(y) }))
Error in eval(y) : object 'X' not found
Basically, my primary goal is to achieve the same result as,
> str(lapply(L, function(x){ eval.parent(substitute(x)) }))
List of 3
$ a: num 1
$ b: num 2
$ c: num 3
but without having to resort to eval.parent as that seems to rely
on an implementation detail of lapply.
My secondary goal is to understand why force(x) does not actually
force the promise here,
> str(lapply(L, function(x){ force(x); pryr::is_promise(x) }))
List of 3
$ a: logi TRUE
$ b: logi TRUE
$ c: logi TRUE
,
Regards
Ben
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