Sorry I think I intended to say that
1. the expressions within `delayed` don?t have to be executed if not assigned,
and
2. the enclosing runtime environment that is potentially referenced by the
objects within delayed() can be released immediately either the returned values
are referenced or not (compared to the environment() approach).
For a toy example,
a <- function() {
v <- rnorm(1e8)
return(delayed({
list(m = mean(v), plot_data = 1:3)
}))
}
m1 <- a()
can immediately release the function runtime environment either the results of
`a()` is assigned to an object (`mean(v)` is evaluated) or not (delayed is not
evaluated, and gc?ed), hence the object `v` is freed from the memory.
Compared to the `delayedAssign+environment()` approach
b <- function(){
v <- rnorm(1e8)
delayedAssign(?m?, mean(v))
plot_data <- 1:3
return(environment())
}
m2 <- b()
`v` will be kept in memory until the returned environment itself gets deleted
(rm(m2)). The situation might get tricky if the result of ?b()? is further used
and returned in nested pipes? (might keep parent frame, parent parent frames?)
- D
> On Oct 29, 2022, at 11:54 AM, Bill Dunlap <williamwdunlap at
gmail.com> wrote:
>
> > the `delayed` object is ready to be garbage collected if not assigned
immediately.
> I am not sure what is meant here. Any object (at the R code level) is
ready to be garbage collected if not given a name or is not part of an object
with a name. Do you mean a 'delayed' component of a list should be
considered garbage if not 'immediately' extracted from a list? Could
you show a few usage cases?
>
> -Bill
>
> On Fri, Oct 28, 2022 at 7:41 PM Dipterix Wang <dipterix.wang at
gmail.com <mailto:dipterix.wang at gmail.com>> wrote:
>
>> 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
>
>
> I understand .Last.value will capture the function returns, but that only
happens in the top-level... I guess?
>
> In the followings code, I think .Last.value does not capture the results of
f, h, k, l
>
> g <- function() {
> f(); h(); k(); l()
> return()
> }
> g()
>
>
> Maybe I caused confusion by mentioning `invisible` function. I guess it
should be a new function (let?s call it `delayed`). The function does not have
to be limited to ?printing?. For example, a digest key
>
>
> a <- function(key, value) {
> map$set(key, value)
>
> return(delayed({
> digest(value)
> }))
> }
>
> Or an async evaluation of which the saved result might not be needed if not
assigned (detached), or the result will be ?joined? to the main process
>
> a <- function(path) {
> # async
> f <- future::future({
> # calculate, and then write to path
> saveRDS(?, path)
> })
>
> return(delayed({
> resolve(f) # wait till f to finish
>
> readRDS(path)
> }))
> }
>
> Although I could use wrappers such as formula, quosure, or environment to
achieve similar results, there are two major differences
>
> 1. There is an extra call to get the lazy-evaluated results (if I do want
to resolve it)
> 2. The returned objects have to contain sort of ?environment? component in
it. It can?t just be simple objects like vectors, matrices, lists, ? (also you
can't immediately garbage collect the enclosing environment)
>
> From the implementation perspective, the `delayed` object is ready to be
garbage collected if not assigned immediately.
>
> Best,
> - D
>
>>
>> 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
>>
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