Declarative Checks with Templates


Declare a template that an object should conform to, and let vetr take care of the rest:

library(vetr) tpl <- numeric(1L) vet(tpl, 1:3) vet(tpl, "hello") vet(tpl, 42)

The template concept is based on vapply, but generalizes to all S3 objects and adds some special features to facilitate comparison. For example, zero length templates match any length:

tpl <- integer() vet(tpl, 1L:3L) vet(tpl, 1L)

And for convenience short (<= 100 length) integer-like numerics are considered integer:

tpl <- integer(1L) vet(tpl, 1) # this is a numeric, not an integer vet(tpl, 1.0001)

vetr can compare recursive objects such as lists, or data.frames:

tpl.iris <- iris[0, ] # 0 row DF matches any number of rows in object iris.fake <- iris levels(iris.fake$Species)[3] <- "sibirica" # tweak levels vet(tpl.iris, iris) vet(tpl.iris, iris.fake)

From our declared template iris[0, ], vetr infers all the required checks. In this case, vet(iris[0, ], iris.fake, stop=TRUE) is equivalent to:

stopifnot_iris <- function(x) { stopifnot( is.data.frame(x), is.list(x), length(x) == length(iris), identical(lapply(x, class), lapply(iris, class)), is.integer(attr(x, 'row.names')), identical(names(x), names(iris)), identical(typeof(x$Species), "integer"), identical(levels(x$Species), levels(iris$Species)) ) } stopifnot_iris(iris.fake)

vetr saved us typing, and the time and thought needed to come up with what needs to be compared.

You could just as easily have created templates for nested lists, or data frames in lists. Templates are compared to objects with the alike function. For a thorough description of templates and how they work see the [alike vignette][2]. For template examples see example(alike).

Auto-Generated Error Messages

Let's revisit the error message:

vet(tpl.iris, iris.fake)

It tells us:

  • The reason for the failure
  • What structure would be acceptable instead
  • The location of failure levels(iris.fake$Species)[3]

vetr does what it can to reduce the time from error to resolution. The location of failure is generated such that you can easily copy it in part or full to the R prompt for further examination.