diagnose_assertion_failure {precondition}

Implement a custom assertion

Description

diagnose_assertion_failure() displays customized failure message and diagnosis in assertions such as precondition(). This can be used to implement assertion helpers. This function does nothing if invoked outside an assertion (see details). The function forwarded_arg_label() looks up a forwarded argument and formats it as a string (used in custom diagnostic messages).

Usage

diagnose_assertion_failure(message, ..., .details)

forwarded_arg_label(arg)

Arguments

message

diagnostic message to show (see rlang::format_error_bullets())

...

expressions to diagnose (forwarded to diagnose_expressions())

.details

an optional data frame with diagnosis data

arg

a forwarded function argument

Details

If invoked as part of an assertion (e.g. precondition()), diagnose_assertion_failure() provides a custom failure message and diagnosis. If invoked in any other context, the function does nothing. This can be used to implement custom assertions helpers that behave like regular binary predicates (functions) under normal circumstances and generate a customized assertion failure report when used as part of an assertion (see examples).

The first argument to diagnose_assertion_failure() is a character vector with a custom failure message. This vector will be formatted as error bullets viarlang::format_error_bullets(). Any subsequent argument will be forwarded to diagnose_assertion_failure() for diagnosis. For custom diagnosis, the user can supply their own data frame with diagnosis details via optional argument .details. The format of this data frame must be identical to one returned by diagnose_assertion_failure().

The function forwarded_arg_label() looks up a forwarded expression and formats it as a single string suitable for inclusion in diagnostic messages.

Value

diagnose_assertion_failure() always returns FALSE.

Examples

# returns TRUE if x is a positive, integer, FALSE otherwise
# if invoked as part of an assertion displays a custom failure diagnosis
is_positive_int <- function(x) {
  is.integer(x) && length(x) == 1L && (x > 0) || {
    diagnose_assertion_failure(
      sprintf("`%s` must be a positive integer", forwarded_arg_label(x)),
      {{x}}
    )
  }
}

# for all intends and purposes this is just a regular R function that returns
# TRUE or FALSE
is_positive_int(5L)
is_positive_int(-5L)

# guard to avoid throwing errors
if(FALSE) {

# ... but it will provide custom diagnosis if invoked inside an assertion
precondition(is_positive_int(-5L))

}