hadley/assertthat — pull request #1 by jrnold (2013-05-28 21:19:47+00:00)
Title: added function validate_that

I really like `assert_that`, but I wanted something similar that will work within S4 validate methods. So I added a function `validate_that` which returns a character vector with the error message or TRUE. This could have also been accomplished by adding an option `error` in `assert_that` which determines whether the function returns an error or the message (similar to the argument of the same name in `validObject`)