envvar

Environment variables are a powerful tool that enable your code to react to its environment. However, two common design choices are a frequent source of friction. First, unlike most other “getter”-type functions, those functions that retrieve values from environment variable typically fail silently. Second, while programmers often use environment variables to store a wide variety of data types from numbers to timestamps to URLs, values are almost always returned as strings. These choices necessitate additional code that checks whether an environment variable was actually set and to coerce its value into the intended format. For frequent users of environment variables, writing all this extra code is unpleasant and time consuming.

Failing Loudly

envvar takes a slightly opinionated perspective to make working with environment variables easier and more consistent. Unless a default value is explicitly given, envvar_get() raises an error if an environment variable is not defined.

For example, let’s say our code depends on an environment variable called NUM_CPUS. In base R, we have to first get the value using Sys.getenv() and then see whether the result is the empty string (not NA like you might expect):

num_cpus <- Sys.getenv("NUM_CPUS")

if (identical(num_cpus, "")) {
  stop("I need `NUM_CPUS` to be set!")
}
#> Error in eval(expr, envir, enclos): I need `NUM_CPUS` to be set!

envvar’s envvar_get() will just fail if NUM_CPUS isn’t set:

library(envvar)

envvar_get("NUM_CPUS")
#> Error in `envvar_get()`:
#> ! Environment variable `NUM_CPUS` is not set.

If a reasonable default is known, it can be supplied via the default argument. envvar prints a message, though, so you know that it’s using a default rather than a value specified in the environment.

envvar_get("NUM_CPUS", default = 12)
#> ℹ Environment variable `NUM_CPUS` is not set. Using default value 12.
#> [1] 12

Warnings can be disabled with the warn_default argument.

Speaking Native Types

Let’s say our NUM_CPUS environment variable is set to 8. Because Sys.getenv() returns strings, we can’t immediately treat it like the integer that it is.

Sys.getenv("NUM_CPUS") / 2
#> Error in Sys.getenv("NUM_CPUS")/2: non-numeric argument to binary operator

envvar includes several helper functions that return commonly-used data types as their proper type. Here, we’ll use envvar_get_integer() to get NUM_CPUS and return it as an integer.

envvar_get_integer("NUM_CPUS") / 2
#> [1] 4

Returning to the theme of failing loudly, envvar’s type-specific functions will also fail if a value cannot be coerced to the expected type. For example, using Sys.getenv() and as.integer to load what should be an integer value might not produce what you’d expect.

Sys.setenv("NUM_CPUS" = 12.345)

num_cpus <- as.integer(Sys.getenv("NUM_CPUS"))
num_cpus
#> [1] 12

Using envvar_get_integer():

envvar_get_integer("NUM_CPUS")
#> Error in `transform()`:
#> ! "12.345" is not an integer-like value

This extends to default values:

envvar_get_integer("SOME_UNSET_INTEGER", default = 12.345)
#> Error in `envvar_get_integer()`:
#> ! `default` value 12.345 should be integer-like.

envvar can handle numbers, logical values, version numbers, URLs, timestamps, UUIDs, IP addresses, and more. We’ll work with dates in the next example.

Validation

Sometimes being the right type isn’t enough. envvar’s envvar_get functions can also apply validation logic. For this example, let’s set an environment variable called LAUNCH_DATE that stores a date that absolutely, positively must be in the future. Let’s first set it to a date in the past.

envvar_set("LAUNCH_DATE" = "1969-07-16")

To read LAUNCH_DATE and ensure that it is in the future, we can supply a validate function to envvar_get_date() that checks the value. If this function returns FALSE, an error is raised.

envvar_get_date("LAUNCH_DATE", validate = \(x) x > Sys.Date())
#> Error in `envvar_get()`:
#> ! "1969-07-16" is not a valid value for `LAUNCH_DATE`

Let’s try that again:

envvar_set("LAUNCH_DATE" = "2028-08-28")

envvar_get_date("LAUNCH_DATE", validate = \(x) x > Sys.Date())
#> [1] "2028-08-28"

Note that the validate argument supports one function. If you’re in need of complex validation, just use a function that encapsulates all of that fanciness.

Installation

You can install the latest released version of envvar by running:

install.packages("envvar")

If you’d like to try out the development version, you can install directly from GitHub:

# install.packages("remotes")
remotes::install_github("briandconnelly/envvar")

Related Packages

• dotenv package for loading environment variables from .env files
• config package for defining and using multiple environments
• options package for defining and using R package options, another way of adding flexibility to your code