[Rd] force promises inside lapply

[William Dunlap]

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

On Fri, Jul 28, 2017 at 8:20 PM, Benjamin Tyner <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>
>>
>>
>> On Fri, Jul 28, 2017 at 3:04 PM, Benjamin Tyner <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>
>>
>>     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
>>
>>     ______________________________________________
>>     R-devel at r-project.org <mailto:R-devel at r-project.org> mailing list
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>>     <https://stat.ethz.ch/mailman/listinfo/r-devel>
>>
>>
>>
>

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