Given an expression of the form `fn(NULL)` how do I replace the null with `lhs=rhs`?

Question

Consider

ex1 = quote(fn(NULL))

and suppose I wish to make ex1 equals to fn(lhs=rhs) in expression form, how do I do that?

since

ex1[[2]] = quote(lhs=rhs)

gives ex1 = fn((lhs=rhs)) and I can't seem to get rid of the parenthesis.

Answer 1

You can do

ex1 <- quote(fn(NULL))
ex1
#> fn(NULL)

ex1[[2]] <- quote(rhs)
names(ex1)[2] <- "lhs"
ex1
#> fn(lhs = rhs)

^{Created on 2022-01-29 by the reprex package (v2.0.1)}