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
On Fri, Jul 28, 2017 at 3:04 PM, Benjamin Tyner <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
>
> 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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