as_function {rlang}

Convert to function

Description

as_function() transforms a one-sided formula into a function. This powers the lambda syntax in packages like purrr.

Usage

as_function(
  x,
  env = global_env(),
  ...,
  arg = caller_arg(x),
  call = caller_env()
)

is_lambda(x)

Arguments

x

A function or formula.

If a function, it is used as is.

If a formula, e.g. ~ .x + 2, it is converted to a function with up to two arguments: .x (single argument) or .x and .y (two arguments). The . placeholder can be used instead of .x. This allows you to create very compact anonymous functions (lambdas) with up to two inputs. Functions created from formulas have a special class. Use is_lambda() to test for it.

If a string, the function is looked up in env. Note that this interface is strictly for user convenience because of the scoping issues involved. Package developers should avoid supplying functions by name and instead supply them by value.

env

Environment in which to fetch the function in case x is a string.

...

These dots are for future extensions and must be empty.

arg

An argument name as a string. This argument will be mentioned in error messages as the input that is at the origin of a problem.

call

The execution environment of a currently running function, e.g. caller_env(). The function will be mentioned in error messages as the source of the error. See the call argument of abort() for more information.

Examples

f <- as_function(~ .x + 1)
f(10)

g <- as_function(~ -1 * .)
g(4)

h <- as_function(~ .x - .y)
h(6, 3)

# Functions created from a formula have a special class:
is_lambda(f)
is_lambda(as_function(function() "foo"))