[Rd] force promises inside lapply
William Dunlap
wdunlap at tibco.com
Tue Aug 1 01:28:43 CEST 2017
quote(expr) will make no changes in expr, it just returns its one argument,
unevaluated.
substitute could be used in your lapply(..., library) example to give
library a name instead
of a character string for an input (which might be necessary if the
character.only argument
were not available)
lapply(c("MASS", "splines"), function(pkg)
eval(substitute(library(pkg), list(pkg=as.name(pkg)))))
bquote() could be used as well
lapply(c("MASS", "splines"), function(pkg) eval(bquote(library(.(pkg)),
list(pkg=as.name(pkg)))))
But avoiding such things is much easier.
Bill Dunlap
TIBCO Software
wdunlap tibco.com
On Mon, Jul 31, 2017 at 2:41 PM, Benjamin Tyner <btyner at gmail.com> wrote:
> Thanks again Bill; I agree that substitute is overkill here.
>
> As an aside, for cases where someone may be tempted to use substitute(),
> it seems quote() might be a safer alternative; compare
>
> > lapply(list(1), function(y) c(quote(y), substitute(y)))
> [[1]]
> [[1]][[1]]
> y
>
> [[1]][[2]]
> X[[i]]
>
> versus in R < 3.2,
>
> > lapply(list(1), function(y) c(quote(y), substitute(y)))
> [[1]]
> [[1]][[1]]
> y
>
> [[1]][[2]]
> X[[1L]]
>
> in any case, the lesson seems to be that quote and substitute are not
> interchangeable, even though for example
>
> > (function() identical(quote({a}), substitute({a})))()
> [1] TRUE
>
>
> On 07/29/2017 09:39 AM, William Dunlap wrote:
>
>> Functions, like your loader(), that use substitute to let users confound
>> things and their names, should give the user a way to avoid the use of
>> substitute. E.g., library() has the 'character.only' argument; if TRUE
>> then the package argument is treated as an ordinary argument and not passed
>> through substitute().
>>
>> myLoader <- function(package, quietly = TRUE) {
>> wrapper <- if (quietly) suppressPackageStartupMessages else `{`
>> wrapper(library(package = package, character.only=TRUE))
>> }
>>
>> > lapply(c("MASS","boot"), myLoader, quietly=FALSE)
>> [[1]]
>> [1] "MASS" "splines" "pryr" "stats" "graphics" "grDevices"
>> [7] "utils" "datasets" "methods" "base"
>>
>> [[2]]
>> [1] "boot" "MASS" "splines" "pryr" "stats"
>> "graphics"
>> [7] "grDevices" "utils" "datasets" "methods" "base"
>>
>> "Non-standard" evaluation (using substitute(), formulas, promises, the
>> rlang or lazyeval packages, etc.) has it uses but I wouldn't use it for
>> such a function as your loader().
>>
>>
>> Bill Dunlap
>> TIBCO Software
>> wdunlap tibco.com <http://tibco.com>
>>
>>
>> On Fri, Jul 28, 2017 at 8:20 PM, Benjamin Tyner <btyner at gmail.com
>> <mailto:btyner at gmail.com>> wrote:
>>
>> 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> <http://tibco.com>
>>
>>
>> On Fri, Jul 28, 2017 at 3:04 PM, Benjamin Tyner
>> <btyner at gmail.com <mailto:btyner at gmail.com>
>> <mailto:btyner at gmail.com <mailto:btyner at 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>
>> <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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>
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