assert_that: Assert that certain conditions are true.
In assertthat: Easy Pre and Post Assertions

Description Usage Arguments Assertions See Also Examples

assert_that is a drop-in replacement for stopifnot but is designed to give informative error messages.

1
2
3

assert_that(..., env = parent.frame(), msg = NULL)

see_if(..., env = parent.frame(), msg = NULL)

`...`	unnamed expressions that describe the conditions to be tested. Rather than combining expressions with `&&`, separate them by commas so that better error messages can be generated.
`env`	(advanced use only) the environment in which to evaluate the assertions.
`msg`	a custom error message to be printed if one of the conditions is false.

Assertion functions should return a single TRUE or FALSE: any other result is an error, and assert_that will complain about it. This will always be the case for the assertions provided by assertthat, but you may need be a more careful for base R functions.

To make your own assertions that work with assert_that, see the help for on_failure. Alternatively, a custom message can be specified for each call.

validate_that, which returns a message (not an error) if the condition is false.

x <- 1
# assert_that() generates errors, so can't be usefully run in
# examples
## Not run: 
assert_that(is.character(x))
assert_that(length(x) == 3)
assert_that(is.dir("asdf"))
y <- tempfile()
writeLines("", y)
assert_that(is.dir(y))
assert_that(FALSE, msg = "Custom error message")

## End(Not run)

# But see_if just returns the values, so you'll see that a lot
# in the examples: but remember to use assert_that in your code.
see_if(is.character(x))
see_if(length(x) == 3)
see_if(is.dir(17))
see_if(is.dir("asdf"))
see_if(5 < 3, msg = "Five is not smaller than three")