R help - Apr 2022 - Modificatio of function body within a function

Hi,

I'm working on a presentation regarding the modification of the
function body on-the-fly and just discovered something I don't
understand. I would like to modify a body of the function within a
function, but I see that it is possible only when explicitly referring
to the environment where the function is defined:

fun1 <- function() {
  1
  body(fun1)[[2]] <- "one"
}
fun1()
body(fun1)
#> {
#>     1
#>     body(fun1)[[2]] <- "one"
#> }

fun2 <- function() {
  2
  env <- environment(fun2)
  body(env$fun2)[[2]] <- "two"
}
fun2()
body(fun2)
#> {
#>     "two"
#>     env <- environment(fun2)
#>     body(env$fun2)[[2]] <- "two"
#> }

Can I get some explanation or some links / articles about this,
please? I thought it won't be a difference and I should be able to
modify a function body also in the first case, because I'm changing
something in the parent environment being in the child environment.

Best regards,

Grzegorz

Heh ... These self-referential constructs are certainly puzzles...

Does this help:

fun1 <- function() {
   1
   body(fun1)[[2]] <<- 'one'
   'hi'
}
> fun1
function() {
   1
   body(fun1)[[2]] <<- 'one' ## note the double
'<<" assigning to the parent env
   'hi'
}
> fun1()
[1] "hi"
> fun1
function ()
{
    "one" ## the change was made
    body(fun1)[[2]] <<- "one"
    "hi"
}

The point is that assignments within a function occur *within* the
function environment unless otherwise specified (either by <<- or the
use of the 'environment' argument in 'body<-'()  ). This is,
of
course, central to the functional programming paradigm of no side
effects (unless explicitly invoked).

NOTE: Correction to this requested if this is wrong in any way!!

Cheers,
Bert




Bert Gunter

"The trouble with having an open mind is that people keep coming along
and sticking things into it."
-- Opus (aka Berkeley Breathed in his "Bloom County" comic strip )

Bert Gunter

"The trouble with having an open mind is that people keep coming along
and sticking things into it."
-- Opus (aka Berkeley Breathed in his "Bloom County" comic strip )


On Fri, Apr 1, 2022 at 12:50 PM Grzegorz Smoli?ski
<g.smolinski1 at gmail.com> wrote:
>
> Hi,
>
> I'm working on a presentation regarding the modification of the
> function body on-the-fly and just discovered something I don't
> understand. I would like to modify a body of the function within a
> function, but I see that it is possible only when explicitly referring
> to the environment where the function is defined:
>
> fun1 <- function() {
>   1
>   body(fun1)[[2]] <- "one"
> }
> fun1()
> body(fun1)
> #> {
> #>     1
> #>     body(fun1)[[2]] <- "one"
> #> }
>
> fun2 <- function() {
>   2
>   env <- environment(fun2)
>   body(env$fun2)[[2]] <- "two"
> }
> fun2()
> body(fun2)
> #> {
> #>     "two"
> #>     env <- environment(fun2)
> #>     body(env$fun2)[[2]] <- "two"
> #> }
>
> Can I get some explanation or some links / articles about this,
> please? I thought it won't be a difference and I should be able to
> modify a function body also in the first case, because I'm changing
> something in the parent environment being in the child environment.
>
> Best regards,
>
> Grzegorz
>
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