The function assert() was built from stopifnot(). It
emits a message in case of errors, which can be a helpful hint for diagnosing
the errors (stopifnot() only prints the possibly truncated source code
of the expressions).
The infix operator %==% is simply an alias of the
identical() function to make it slightly easier and intuitive
to write test conditions. x %==% y is the same as
identical(x, y).
Usage
assert(fact, ...)
x %==% y
Arguments
fact
a message for the assertions when any of them fails; treated the
same way as expressions in ... if it is not a character string,
which means you do not have to provide a message to this function
...
any number of R expressions, presumably to return vectors of
TRUE's (if FALSE is returned anywhere, an error will show up)
x, y
two R objects to be compared
Value
Invisible NULL if all expressions returned TRUE,
otherwise an error is signalled and the user-provided message is emitted.
Note
The internal implementation of stopifnot() is different with the
function in R base: (1) the custom message fact is emitted if
an error occurs (2) assert() requires the logical values to be
non-empty (3) if ... contains a compound expression in {}
which returns FALSE (e.g., if (TRUE) {1+1; FALSE}), the first
and the last but one line of the source code from deparse()
are printed in the error message, otherwise the first line is printed
Examples
assert("one equals one", 1 == 1)
assert("seq and : produce equal sequences", seq(1L, 10L) == 1L:10L)
assert("seq and : produce identical sequences", identical(seq(1L, 10L), 1L:10L))
# multile tests
T = FALSE
F = TRUE
assert("T is bad for TRUE, and so is F for FALSE", T != TRUE, F != FALSE)
# a mixture of tests
assert("Let's pray all of them will pass", 1 == 1, 1 != 2, letters[4] == "d",
rev(rev(letters)) == letters)
# logical(0) cannot pass assert(), although stopifnot() does not care
try(assert("logical(0) cannot pass", 1 == integer(0)))
stopifnot(1 == integer(0)) # it's OK!
# a compound expression
try(assert("this if statement returns TRUE", if (TRUE) {
x = 1
x == 2
}))
# no message
assert(!FALSE, TRUE, is.na(NA))
providing 
.