on_failure: Custom failure messages for assertions.
In hadley/assertthat: Easy Pre and Post Assertions

Description Usage Arguments Examples

Custom failure messages for assertions.

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on_failure(x)

on_failure(x) <- value

`x`	a assertion function that returns `TRUE` if the assertion is met, `FALSE` otherwise.
`value`	a function with parameters `call` and `env` that returns a custom error message as a string.

is_odd <- function(x) {
  assert_that(is.numeric(x), length(x) == 1)
  x %% 2 == 1
}
see_if(is_odd(2))

on_failure(is_odd) <- function(call, env) {
  paste0(deparse(call$x), " is even")
}
see_if(is_odd(2))