How many times have you written R functions that start with a bunch of code that looks like this?

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my_funct <- function(dob, enddate = "2015-07-05"){
if (!inherits(dob, "Date") | !inherits(enddate, "Date")){
    stop("Both dob and enddate must be Date class objects")
  } 
...
}

Because R was designed to be interactive, it is incredibly tolerant to bad user input. Functions are not type safe, meaning function arguments do not have to conform to specified data types. But most of my R code is not run interactively. I have to trust my code to run on servers on schedules or on demand as a part of production systems. So I find myself frequently writing code like the above– manually writing type checks for safety.

There has been some great action in the R community around assertive programming, as you can see in the link. My favorite development, by far, are type-safe functions in the ensurer package. The above function definition can now be written like this:

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my_funct <- function_(dob ~ Date, enddate ~ Date: as.Date("2015-07-05"), {
  ...
})

All the type-checking is done.

I really like the reuse of the formula notation ~ and the use of : to indicate default values.

Along with packages like testthat, R is really growing up and modernizing.