Dollar operator as function argument for sapply not working as expected

Question

I have the following list

test_list=list(list(a=1,b=2),list(a=3,b=4))

and I want to extract all elements with list element name a.

Answer 1

From what I've been able to determine it's a combination of two things.

First, the second element of $ is matched but not evaluated so it cannot be a variable.

Secondly, when arguments are passed to functions they are assigned to the corresponding variables in the function call. When passed to sapply "a" is assigned to a variable and therefore will no longer work with $. We can see this by occurring by running

sapply("a", print)
[1] "a"
  a 
"a"

This can lead to peculiar results like this

sapply(test_list, function(x, a) {`$`(x, a)})
[1] 1 3

Where despite a being a variable (which hasn't even been assigned) $ matches it to the names of the elements in the list.