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 <bty...@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 <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 >> >> ______________________________________________ >> R-devel@r-project.org <mailto:R-devel@r-project.org> mailing list >> https://stat.ethz.ch/mailman/listinfo/r-devel >> <https://stat.ethz.ch/mailman/listinfo/r-devel> >> >> >> > [[alternative HTML version deleted]] ______________________________________________ R-devel@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-devel