fn.md
In rlang: Functions for Base Types and Core R and 'Tidyverse' Features

as_function() has nice errors

Code
  (expect_error(as_function(1)))
Output
  <error/rlang_error>
  Error:
  ! Can't convert `1`, a double vector, to a function.
Code
  (expect_error(as_function(1, arg = "foo")))
Output
  <error/rlang_error>
  Error:
  ! Can't convert `foo`, a double vector, to a function.
Code
  (expect_error(my_function(1 + 2)))
Output
  <error/rlang_error>
  Error in `my_function()`:
  ! Can't convert `my_arg`, a double vector, to a function.
Code
  (expect_error(my_function(1)))
Output
  <error/rlang_error>
  Error in `my_function()`:
  ! Can't convert `my_arg`, a double vector, to a function.
Code
  (expect_error(my_function(a ~ b)))
Output
  <error/rlang_error>
  Error in `my_function()`:
  ! Can't convert `my_arg`, a two-sided formula, to a function.

check inputs in function accessors

Code
  (expect_error(fn_fmls(1)))
Output
  <error/rlang_error>
  Error in `fn_fmls()`:
  ! `fn` must be an R function, not the number 1.
Code
  (expect_error(fn_body(1)))
Output
  <error/rlang_error>
  Error in `fn_body()`:
  ! `fn` must be an R function, not the number 1.
Code
  (expect_error(fn_env(1)))
Output
  <error/rlang_error>
  Error in `fn_env()`:
  ! `fn` must be a function, not the number 1.