R devel - Feb 2016 - Assignment in environment

Dear all,

I have a function "fn" with its own environment, i.e.

env <- environment(fn)

and env is not .GlobalEnv. And another function

getValue <- function(x) environment(x)$mylist

which returns the list object "mylist" which is in "env". If
I want to
modify "mylist", I could write

'getValue<-' <- function(x, value) { environment(x)$mylist <-
value}

which gives not the desired result, e.g.

getValue(fn)[[1]] <- 3

will set the first list entry of "mylist" to 3. But then
"fn" will also
be 3!

environment(fn)$mylist[[1]] <- 3

does set the list entry correctly while keeping "fn". What's the 
difference?

Cheers,
Daniel

On 07/02/2016 8:08 AM, Daniel Kaschek wrote:
> Dear all,
>
> I have a function "fn" with its own environment, i.e.
>
> env <- environment(fn)
>
> and env is not .GlobalEnv. And another function
>
> getValue <- function(x) environment(x)$mylist
>
> which returns the list object "mylist" which is in
"env". If I want to
> modify "mylist", I could write
>
> 'getValue<-' <- function(x, value) { environment(x)$mylist
<- value}
>
> which gives not the desired result, e.g.
>
> getValue(fn)[[1]] <- 3
>
> will set the first list entry of "mylist" to 3. But then
"fn" will also
> be 3!
>
> environment(fn)$mylist[[1]] <- 3
>
> does set the list entry correctly while keeping "fn". What's
the
> difference?
An assignment function should return the modified value.  So it looks 
like you would need


'getValue<-' <- function(x, value) {
    environment(x)$mylist <- value
    x
  }

but in fact, this doesn't work:

getValue(fn)[[1]] <- 3
Error in getValue(fn)[[1]] <- 3 : could not find function
"getValue"

I suspect this is a parser problem.  However, it does suggest a simple 
workaround for you:

Just define

getValue <- function(x) environment(x)$mylist

and then your expression works as expected.

Duncan Murdoch

> On 07 Feb 2016, at 14:46 , Duncan Murdoch <murdoch.duncan at
gmail.com> wrot8[e:
> 
[snippage]
> 
> but in fact, this doesn't work:
> 
> getValue(fn)[[1]] <- 3
> Error in getValue(fn)[[1]] <- 3 : could not find function
"getValue"
> 
> I suspect this is a parser problem.
Umm, no...

The canonical semantics are that 

foo(x)[[....]] <- bar

is internally converted to

*tmp* <- foo(x)
*tmp*[[....]] <- bar
x <- `foo<-`(x, *tmp*)

so both foo() and `foo<-` are required.

-- 
Peter Dalgaard, Professor,
Center for Statistics, Copenhagen Business School
Solbjerg Plads 3, 2000 Frederiksberg, Denmark
Phone: (+45)38153501
Office: A 4.23
Email: pd.mes at cbs.dk  Priv: PDalgd at gmail.com

On So, Feb 7, 2016 at 2:46 , Duncan Murdoch <murdoch.duncan at gmail.com> 
wrote:
> An assignment function should return the modified value.  So it looks 
> like you would need
> 
> 
> 'getValue<-' <- function(x, value) {
>    environment(x)$mylist <- value
>    x
>  }
Thanks. I have not thought of a return value. That makes the difference.

> 
> but in fact, this doesn't work:
> 
> getValue(fn)[[1]] <- 3
> Error in getValue(fn)[[1]] <- 3 : could not find function
"getValue"

I have a "getValue" function anyway. It was even the other way round.
I
wanted to assign values without a function "getValue<-" and was
rebuked
by R that this function was missing.

Thanks to both of you, Peter and Duncan.

Best regards,
Daniel