rlang (version 1.1.0)

is_formula: Is object a formula?

Description

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

Note: When we first implemented is_formula(), we thought it best to treat unevaluated formulas as formulas by default (see section below). Now we think this default introduces too many edge cases in normal code. We recommend always supplying scoped = TRUE. Unevaluated formulas can be handled via a is_call(x, "~") branch.

Usage

is_formula(x, scoped = NULL, lhs = NULL)
is_bare_formula(x, scoped = TRUE, lhs = NULL)

Arguments

x

An object to test.

scoped

A boolean indicating whether the quosure is scoped, that is, has a valid environment attribute and inherits from "formula". If NULL, the scope is not inspected.

lhs

A boolean indicating whether the formula has a left-hand side. If NULL, the LHS is not inspected and is_formula() returns TRUE for both one- and two-sided formulas.

Dealing with unevaluated formulas

At parse time, a formula is a simple call to ~ and it does not have a class or an environment. Once evaluated, the ~ call becomes a properly structured formula. Unevaluated formulas arise by quotation, e.g. ~~foo, quote(~foo), or substitute(arg) with arg being supplied a formula. Use the scoped argument to check whether the formula carries an environment.

Examples

is_formula(~10)
is_formula(10)

# If you don't supply `lhs`, both one-sided and two-sided formulas
# will return `TRUE`
is_formula(disp ~ am)
is_formula(~am)

# You can also specify whether you expect a LHS:
is_formula(disp ~ am, lhs = TRUE)
is_formula(disp ~ am, lhs = FALSE)
is_formula(~am, lhs = TRUE)
is_formula(~am, lhs = FALSE)

# Handling of unevaluated formulas is a bit tricky. These formulas
# are special because they don't inherit from `"formula"` and they
# don't carry an environment (they are not scoped):
f <- quote(~foo)
f_env(f)

# By default unevaluated formulas are treated as formulas
is_formula(f)

# Supply `scoped = TRUE` to ensure you have an evaluated formula
is_formula(f, scoped = TRUE)

# By default unevaluated formulas not treated as bare formulas
is_bare_formula(f)

# If you supply `scoped = TRUE`, they will be considered bare
# formulas even though they don't inherit from `"formula"`
is_bare_formula(f, scoped = TRUE)