[Rd] WISH: eval() to preserve the "visibility" (now value is always visible)

[Henrik Bengtsson]

On Sun, Feb 8, 2015 at 8:44 PM, Suharto Anggono Suharto Anggono via
R-devel <r-devel at r-project.org> wrote:
> Sorry to intervene.

No, I'm very happy you intervened.  You're comment is 100%
valid/correct making my wish moot.

Your explanation is very clear and nails it; one should use
eval(substitute(expr)) or evalq(expr) for what I'm trying to do.

It all came from me trying to prevent

> withOptions({x <- 1}, foo=1)

from printed the value, where (somewhat simplified):

withOptions <- function(expr, ..., envir=parent.frame()) {
  oopts <- options(...)
  on.exit(options(oopts))
  eval(expr, envir=envir)
}

I have a few of these withNnn() functions, but for this particular one
(*) I had forgotten an expr <- substitute(expr) in there, which caused
me to incorrectly blame eval().  <recursive mistake>This is very much
the same problem as you observed with my eval2() example.</recursive
mistake>

Thank you very much

Henrik
(*) Actually withSeeds() which is to messy to use as an example.

>
> Argument passed to 'eval' is evaluated first.
> So,
> eval(x <- 2)
> is effectively like
> { x <- 2; eval(2) } ,
> which is effectively
> { x <- 2; 2 } .
> The result is visible.
>
> eval(expression(x <- 2))
> or
> eval(quote(x <- 2))
> or
> evalq(x <- 2)
> gives the same effect as
> x <- 2 .
> The result is invisible.
>
> In function 'eval2',
> res <- eval(withVisible(expr), envir=envir, ...)
> is effectively
> res <- withVisible(expr) .
>
> -------------------
>
> Would it be possible to have the value of eval() preserve the
> "visibility" of the value of the expression?
>
>
> "PROBLEM":
>
> # Invisible
>> x <- 1
>
> # Visible
>> eval(x <- 2)
> [1] 2
>
> "TROUBLESHOOTING":
>> withVisible(x <- 1)
> $value
> [1] 1
> $visible
> [1] FALSE
>
>> withVisible(eval(x <- 2))
> $value
> [1] 2
> $visible
> [1] TRUE
>
>
> WORKAROUND:
> eval2 <- function(expr, envir=parent.frame(), ...) {
>   res <- eval(withVisible(expr), envir=envir, ...)
>   value <- res$value
>   if (res$visible) value else invisible(value)
> }
>
>> x <- 1
>> eval(x <- 2)
> [1] 2
>> eval2(x <- 3)
>> x
> [1] 3
>
> /Henrik
>
> ______________________________________________
> R-devel at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel