Dplyr: Somewhat unexpected "invalid subscript type 'double'" when using $

Dear all,

I wanted to add a new column to a data frame with a constant value from a list:

someVariable <- list(a = 3, b = 5)
dplyr::mutate(mtcars, newColumn = someVariable$a) %>% head

This works.

But if a variable of the same name ("a") is present, I get an error:

someVariable <- list(a = 3, b = 5)
a <- 10
dplyr::mutate(mtcars, newColumn = someVariable$a) %>% head
Error: invalid subscript type 'double'

A fix consists in encapsulating the offending call with I():

someVariable <- list(a = 3, b = 5)
a <- 10
dplyr::mutate(mtcars, newColumn = I(someVariable$a)) %>% head()

(edit: this fix might not be advisable as newColumn is of class AsIs and not of the expected double. To avoid the issue, simply store the value into a new variable and use this new variable instead of someVariable$a in the call to dplyr::mutate.)

This is reminiscent of bug #421 , which is closed.

Whether it's something to fix or not, I leave it up to you but it was a bit maddening to debug (in my case, the variable 'a' was only sometimes present in the global environment, so my tests were passing when run from a separate environment, but not always when run from the console.).

thanks.