[Rd] force promises inside lapply

[Benjamin Tyner]

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
>
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