Comment by andrewghull

Here's the final example in TypeScript if you find that easier to read:

  type NonEmpty<T> = [T, ...T[]]

  const head = <T>(list: NonEmpty<T>) => list[0]

  function getConfigurationDirectories(): NonEmpty<string> {
    const configDirsString = process.env["CONFIG_DIRS"]
    const [firstDir, ...restDirs] = configDirsString.split(',')
    if (firstDir === undefined) throw Error("CONFIG_DIRS cannot be empty");
    return [firstDir, ...restDirs];
  }

  function main() {
    const configDirs = getConfigurationDirectories();
    initializeCache(head(configDirs))
  }