R devel - Mar 2021 - Surprising behavior when using the reference class with the dollar symbol

On 27/03/2021 10:16 a.m., Jiefei Wang wrote:
> Hi all,
> 
> I'm trying to get the field value of a reference object by the field
name,
> but the dollar symbol behaves quite unusual. See example below
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> *.foo <- setRefClass(    "foo",    fields = list(        a =
"integer"
> ))x <- .foo$new(a=1L)## This is OKx$a## This is OK`$`(x,
"a")## But this is
> not OKi <- "a"`$`(x, i)*
This is really hard to read.  Please post it again, but don't use HTML.

Duncan Murdoch
> 
> For the last line of code I get this error
> 
> 
> *Error in envRefInferField(x, what, getClass(class(x)), selfEnv) :   ?i? is
> not a valid field or method name for reference class ?foo?*
> 
> It looks like when calling the dollar symbol using the function format, it
> treats the input argument as a character literal and does not evaluate it
> inside the function. I know we have the function `field` to get the slot
> variable, but I wonder if this is designed on purpose as the above example
> works for both list and S4 system.
> 
> Best,
> Jiefei
> 
> 	[[alternative HTML version deleted]]
> 
> ______________________________________________
> R-devel at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel
>

Thanks, Duncan. Below is the repost of my question in plain text mode.


I'm trying to get the field value of a reference object by the field
name, but the dollar symbol behaves quite unusual. See example below

.foo <- setRefClass(
    "foo",
    fields = list(
        a = "integer"
    )
)
x <- .foo$new(a=1L)
## This is OK
x$a
## This is OK
`$`(x, "a")
## But this is not OK
i <- "a"
`$`(x, i)

For the last line of code I get this error

Error in envRefInferField(x, what, getClass(class(x)), selfEnv) :
  ?i? is not a valid field or method name for reference class ?foo?

It looks like when calling the dollar symbol using the function
format, it treats the input argument as a character literal and does
not evaluate it inside the function. I know we have the function
`field` to get the slot variable, but I wonder if this is designed on
purpose as the above example works for both list and S4 system.

Best,
Jiefei


On Sat, Mar 27, 2021 at 10:20 PM Duncan Murdoch
<murdoch.duncan at gmail.com> wrote:
>
> On 27/03/2021 10:16 a.m., Jiefei Wang wrote:
> > Hi all,
> >
> > I'm trying to get the field value of a reference object by the
field name,
> > but the dollar symbol behaves quite unusual. See example below
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> > *.foo <- setRefClass(    "foo",    fields = list(       
a = "integer"
> > ))x <- .foo$new(a=1L)## This is OKx$a## This is OK`$`(x,
"a")## But this is
> > not OKi <- "a"`$`(x, i)*
>
> This is really hard to read.  Please post it again, but don't use HTML.
>
> Duncan Murdoch
>
> >
> > For the last line of code I get this error
> >
> >
> > *Error in envRefInferField(x, what, getClass(class(x)), selfEnv) :  
?i? is
> > not a valid field or method name for reference class ?foo?*
> >
> > It looks like when calling the dollar symbol using the function
format, it
> > treats the input argument as a character literal and does not evaluate
it
> > inside the function. I know we have the function `field` to get the
slot
> > variable, but I wonder if this is designed on purpose as the above
example
> > works for both list and S4 system.
> >
> > Best,
> > Jiefei
> >
> >       [[alternative HTML version deleted]]
> >
> > ______________________________________________
> > R-devel at r-project.org mailing list
> > https://stat.ethz.ch/mailman/listinfo/r-devel
> >
>