an assert() function

hi.  i've looked for (and not found) an assert() function.  needing one,
i created the following (from stop()).  i'm posting it in case 1)
someone sees a problem with this; and 2) someone else has a need.

stopifnot() might do what you want.

The tricky thing in writing this sort of function is handling vector 
inputs.  Sometimes x is a vector, and then you need to decide what to do 
with

assert(x > 0)

Yours will only test the first component and issue a warning; the base 
one will test all(x > 0).  But the base behaviour causes its own 
problems; one came up recently in that

stopifnot( ncol(x) > 1 )

will not stop if x is just a vector, because ncol(x) is NULL, so ncol(x) 
 > 1 is a length 0 vector, and all(ncol(x) > 1) is TRUE.

Duncan Murdoch

cheers, Greg
----
## an assert mechanism...
assert <- function (shouldbe, ...) {
   if (!shouldbe) {
     .Internal(stop(as.logical(TRUE),
                    .makeMessage("assertion failure: ", ..., domain = NULL)))
   }
}

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