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\name{assert_that}
\alias{assert_that}
\alias{see_if}
\title{Assert that certain conditions are true.}
\usage{
assert_that(..., env = parent.frame())
see_if(..., env = parent.frame())
}
\arguments{
\item{...}{unnamed expressions that describe the
conditions to be tested. Rather than combining
expressions with \code{&&}, separate them by commas so
that better error messages can be generated.}
\item{env}{(advanced use only) the environment in which
to evaluate the assertions.}
}
\description{
\code{assert_that} is a drop-in replacement for \code{\link{stopifnot}} but
is designed to give informative error messages.
}
\section{Assertions}{
Assertion functions should return a single \code{TRUE} or
\code{FALSE}: any other result is an error, and
\code{assert_that} will complain about it. This will
always be the case for the assertions provided by
\code{assertthat}, but you may need be a more careful for
base R functions.
To make your own assertions that work with
\code{assert_that}, see the help for
\code{\link{on_failure}}.
}
\examples{
x <- 1
# assert_that() generates errors, so can't be usefully run in
# examples
\dontrun{
assert_that(is.character(x))
assert_that(length(x) == 3)
assert_that(is.dir("asdf"))
y <- tempfile()
writeLines("", y)
assert_that(is.dir(y))
}
# 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"))
}
\seealso{
\code{\link{validate_that}}, which returns a message (not an error)
if the condition is false.
}
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