Get Started with chk

Introduction

chk is an R package for developers to check user-supplied function arguments. It is designed to be simple, customizable and fast.

chk_ Functions

chk_ functions check the properties of individual objects. For example chk_flag(x) checks whether x is a flag, i.e., a non-missing logical vector of length 1.

chk_ functions are called for their side-effects, i.e., they throw an informative error if the object fails the check. Although do return an invisible copy of the first argument so they can be used in pipes.

library(chk)
y <- TRUE
chk_flag(y)
y <- NA
chk_flag(y)
#> Error in err():
#> ! y must be a flag (TRUE or FALSE).

The error messages, which follow the tidyverse style guide, are designed to allow the user to quickly identify the problem with the argument value(s) they are providing. The errors are rlang errors of subclass 'chk_error'.

vld_ Functions

Each chk_ function has a matching vld_ function which returns a flag indicating whether the object passed the check.

vld_flag(TRUE)
#> [1] TRUE
vld_flag(NA)
#> [1] FALSE

The vld_ functions allow developers to provide their own error messages.

if (!vld_flag(NA)) abort_chk("NA is not TRUE or FALSE!!")
#> Error in err():
#> ! NA is not TRUE or FALSE!!

Using chk

The chk_ functions are designed to be used within functions. Consequently, when constructing an error message they use the name of the object that they received as this is expected to be the name of the argument.

fun1 <- function(x) {
chk_whole_number(x)
# use x
}

fun1(1)
y <- 1.3
fun1(x = y)
#> Error in err():
#> ! x must be a whole number (non-missing integer scalar or double equivalent).

If this is not the case, developers can provide a different name using the x_name argument.

x <- NA
chk_flag(x, x_name = "zzz")
#> Error in err():
#> ! zzz must be a flag (TRUE or FALSE).

IMPORTANT NOTE

As the chk_ (and vld_) functions are not expected to be directly exposed to users they don’t check any of their arguments (other than the object of interest of course!) to ensure that they are as fast as possible.

Extending chk

The chk_flag() function illustrates the general structure of a chk_ function.

chk_flag
#> function(x, x_name = NULL){
#>   if(vld_flag(x)) return(invisible(x))
#>   if(is.null(x_name))  x_name <- deparse_backtick_chk(substitute(x))
#>   abort_chk(x_name, " must be a flag (TRUE or FALSE)")
#> }
#> <bytecode: 0x7fe802835670>
#> <environment: namespace:chk>

A chk_ function initially checks the object (using its vld_ partner) and if the object passes the check immediately returns an invisible copy of the object. If, and only if, the object fails the check does the chk_ function construct and then throw an informative error message.

The deparse_backtick_chk() and abort_chk() functions are exported to make it easy for programmers to develop their own chk_ functions. The chk-lgl.R script illustrates the general template to use when developing your own chk_ functions.

abort_chk()

The abort_chk() function converts multiple arguments to a string using paste0(..., collapse = '') and provides number sensitive sprintf-like types. By default it also capitalizes the first character and adds a missing period.

abort_chk("There %r %n problem director%y%s.", n = 1)
#> Error in err():
#> ! There is 1 problem directory.
abort_chk("there %r %n ", "problem director%y%s", n = 2)
#> Error in err():
#> ! There are 2 problem directories.

check_ Functions

The check_ functions are more complex then the chk_ functions which make them slower but makes doing some general tests easier.