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

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)*

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]]

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]]
> 
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