Writing Command Line Tools in Swift Using ArgumentParser, Part 2: Validation & Errors

Last week we explored how we can build a simple command line tool. We learned how to use @Argument, @Option, and @Flag as the building blocks for ArgumentParser command line tools. We we saw last week was enough to build many simple tools, but there’s still a lot to explore, and cool things to learn.

This week we will learn about input validation and errors, so we can build better tools that take more constrained parameters when relevant.

ArgumentParser Validation

ArgumentParser has all the facilities you need to validate your input, both before you need them and when exception occurs when you are using it.

ArgumentParser allows you to perform two types of validation: Pre-Running Validation, which lets you check your arguments before your run() function is reached, and Post-Validation errors, which allows you to throw errors when an exception occurs with an otherwise valid input.

Command-Line Input Validation

The framework already does a lot of validation for you for free. It will validate the data types you are passing so they conform to the types specified in each property wrapper.

But you may want to do some additional validations that the framework can’t do. Consider our CharacterCount tool from the last article. Suppose we want to restrict it to strings that are at least 3 character long.

ParsableCommand has a validate() method that you can implement, and you can do any custom validations within it.

To use it, implement custom checks and throw ValidationErrors when the condition fails.

struct CharacterCount: ParsableCommand {
  @Argument(help: "String to count the characters of") var string: String
  
  mutating func validate() throws {
    if string.count < 3 {
      throw ValidationError("'string' must contain at least 3 characters.")
    }
  }
  
  func run() {
    print(string.count)
  }
}

Of course, you could naively do the validation within run itself, but implementing validate lets you do separation of concerns way easier.

Now when you try to run this program with a string with less than 3 characters, you will get an error like this:

./CharacterCount "hi"
Error: 'string' must contain at least 3 characters.
Usage: character-count <string>

For the record, validation is not limited to @Argument. You can use it with @Option and @Flag as well.

Post-Validation Errors

We can use the validate() method to ensure additional constraints are fulfilled before our program runs, but what happens when the conditions are fine, but something else fails?

Consider a program that takes a path to a local file. The framework will take care of validating that the user is indeed passing you a string which is the string to the file, but you can’t check if the file is in a valid format until you try opening it.

To write checks against this cases, you can use a version of the run function that throws errors.

struct CharacterCount: ParsableCommand {
  @Argument(help: "File to count the characters of") var filePath: String
  
  func run() throws {
    let contents = try String(contentsOfFile: filePath, encoding: .utf8)
    print(contents.count)
  }
}

Trying to run the program with an invalid file will produce the following output:

./CharacterCount path_to_file
Error: Error Domain=NSCocoaErrorDomain Code=260 "The file “path_to_file” couldn’t be opened because there is no such file." UserInfo={NSFilePath=path_to_file, NSUnderlyingError=0x7fc2ec40ebe0 {Error Domain=NSPOSIXErrorDomain Code=2 "No such file or directory"}}

Of course, because this will throw Errors to the console, there’s cases when whatever it prints may be cryptic for your users, so you should minimize throwing errors directly and only use them when there’s absolutely no other way to check for them before hand.

In our specific program, there’s two possible errors that can happen when we try to open files:\

  1. They don’t exist.
  2. they cannot be opened as plain text files.

The former case can be dealt with easily, as we can write a check for it, using the FileManager API:

mutating func validate() throws {
	if !FileManager.default.fileExists(atPath: filePath) {
		throw ValidationError("'filePath' does not exist")
  }
}

And now we can at least show a more user-friendly error when the file does not exist.

But the latter isn’t really easy to figure out until you try opening it. You can probably figure out a way to check the first bytes of the file or do anything else crazy before you read the entire file. But it may not be worth it, and in that case, i’d just throw the error to the console directly.

The following example will try opening a PDF file I have in my ~/downloads folder.

./CharacterCount /Users/andyibanez/downloads/Formulario.pdf
Error: Error Domain=NSCocoaErrorDomain Code=261 "The file “Formulario.pdf” couldn’t be opened using text encoding Unicode (UTF-8)." UserInfo={NSFilePath=/Users/andyibanez/downloads/Formulario.pdf, NSStringEncoding=4}

You could, of course, wrap it inside a do-catch block and only print the error when your catch is reached. But then, you have to balance. When do I want to show a friendly message? When do I want to show the entire error for the purpose of diagnostics? The good news is that ArgumentParser allows you to deal with errors easily, so you just need to think about whether showing an entire error makes sense or not.

Conclusion

ArgumentParser lets you validate input and run time exceptions easily. You can write command lines with stricter constrains so your users don’t run your tool with weird input.

If you find any inaccuracies (and that includes typos) or problems in this article please tweet at me (@AndyIbanezK) or send me an e-mail to andy[at]andyibanez[dot]com. Thank you for helping me improve the quality of my blog!

If there’s anything related to Swift, iOS, or another Apple Platform you’d like me to cover, feel free to contact me and I will try to cover it in an upcoming article.