force promises inside lapply

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