[Rd] Lazy-evaluate elements wrapped with invisible

[Bill Dunlap]

You can play with the idea by returning an environment that contains
delayed assignments.  E.g.,

> f <- function(x) {
+    delayedAssign("eval_date", { cat("Evaluating 'date'\n"); date()})
+    delayedAssign("sum_x", { cat("Evaluating 'sum_x'\n"); sum(x)})
+    environment()
+ }
> fx <- f(1:10)
> date()
[1] "Fri Oct 28 14:22:12 2022"
> Sys.sleep(2)
> fx$eval_date
Evaluating 'date'
[1] "Fri Oct 28 14:22:24 2022"
> Sys.sleep(2)
> fx$eval_date
[1] "Fri Oct 28 14:22:24 2022"
> fx$sum_x
Evaluating 'sum_x'
[1] 55
> fx$sum_x
[1] 55

-Bill

On Fri, Oct 28, 2022 at 2:11 PM Gabriel Becker <gabembecker using gmail.com>
wrote:

> Hi Dipterix,
>
>
> On Fri, Oct 28, 2022 at 1:10 PM Dipterix Wang <dipterix.wang using gmail.com>
> wrote:
>
> > Hi,
> >
> > I was wondering if it is a good idea to delay the evaluation of
> expression
> > within invisible(), just like data()/delayedAssign()?
> >
> > The idea is a function might return an invisible object. This object
> might
> > not be used by the users if the function returns are not assigned nor
> > passed to another function call. For example,
> >
> > f <- function() {
> >   # do something eagerly
> >
> >   return(invisible({
> >     # calculate message that might take long/extra memory, but only
> useful
> > if printed out
> >   }))
> > }
> >
> > If `f()` is not immediately assigned to a variable, then there is no
> > reason to evaluate invisible(…).
> >
>
> This is not quite true. The value, even when invisible, is captured by
> .Last.value, and
>
> > f <- function() invisible(5)
>
> > f()
>
> > .Last.value
>
> [1] 5
>
>
> Now that doesn't actually preclude what you're suggesting (just have to
> wait for .Last.value to be populated by something else), but it does
> complicate it to the extent that I'm not sure the benefit we'd get would be
> worth it.
>
> Also, in the case you're describing, you'd be pushing the computational
> cost into printing, which, imo, is not where it should live. Printing a
> values generally speaking, should just print things, imo.
>
> That said, if you really wanted to do this, you could approach the behavior
> you want, I believe (but again, I think this is a bad idea) by returning a
> custom class that wraps formula (or, I imagine, tidyverse style quosures)
> that reach back into the call frame you return them from, and evaluating
> them only on demand.
>
> Best,
> ~G
>
>
> > This idea is somewhere between `delayedAssign` and eager evaluation.
> Maybe
> > we could call it delayedInvisible()?
> >
> > Best,
> > - Zhengjia
> >
> > ______________________________________________
> > R-devel using r-project.org mailing list
> > https://stat.ethz.ch/mailman/listinfo/r-devel
> >
>
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