How to test if an object is a formula in base R?

Question

R has many helpers for testing object type, eg is.character(x)

Is there an equivalent for the (strangely missing) is.formula(x) ?

(PS: I see that at least one package has implemented this outside base R)

Answer 1

From my comment, you could just do:

is.formula <- function(x){
   inherits(x,"formula")
}

Answer 2

You could use rlang::is_formula() or rlang::is_bare_formula() from rlang 's doc :

is_formula() tests if x is a call to ~ . is_bare_formula() tests in addition that x does not inherit from anything else than "formula".

inherits() is a common approach but if we want to be rigorous we should look at the call, not the class attribute, as rlang::is_formula() does, I propose a base version here :

a_formula <- ~foo
not_a_formula <- "foo"
class(not_a_formula) <- "formula"

inherits(a_formula, "formula")
#> [1] TRUE
inherits(not_a_formula, "formula")
#> [1] TRUE

is.formula <- function(x) is.call(x) && x[[1]] == quote(`~`)
is.formula(a_formula)
#> [1] TRUE
is.formula(not_a_formula)
#> [1] FALSE

^{Created on 2019-11-10 by the reprex package (v0.3.0)}