I built Valid8r because I got tired of writing the same input validation code for every CLI tool. You know the pattern: parse a string, check if it's valid, print an error if not, ask again. Repeat for every argument.

The library uses Maybe monads (Success/Failure instead of exceptions) so you can chain parsers and validators:

  # Try it: pip install valid8r
  from valid8r.core import parsers, validators
  
  # Parse and validate in one pipeline
  result = (
      parsers.parse_int(user_input)
      .bind(validators.minimum(1))
      .bind(validators.maximum(65535))
  )
  
  match result:
      case Success(port): print(f"Using port {port}")
      case Failure(error): print(f"Invalid: {error}")

I built integrations for argparse, Click, and Typer so you can drop valid8r parsers directly into your existing CLIs without refactoring everything.

The interesting technical bit: it's 4-300x faster than Pydantic for simple parsing (ints, emails, UUIDs) because it doesn't build schemas or do runtime type checking. It just parses strings and returns Maybe[T]. For complex nested validation, Pydantic is still better. I benchmarked both and documented where each one wins.

I'm not trying to replace Pydantic. If you're building a FastAPI service, use Pydantic. But if you're building CLI tools or parsing network configs, Maybe monads compose really nicely and keep your code functional.

The docs are at https://valid8r.readthedocs.io/ and the benchmarks are in the repo. It's MIT licensed.

Would love feedback on the API design. Is the Maybe monad pattern too weird for Python, or does it make validation code cleaner?

---

Here are a few more examples showing different syntax options for the same port validation:

  from valid8r.core import parsers, validators

  # Option 1: Combine validators with & operator
  validator = validators.minimum(1) & validators.maximum(65535)
  result = parsers.parse_int(user_input).bind(validator)

  # Option 2: Use parse_int_with_validation (built-in)
  result = parsers.parse_int_with_validation(
      user_input,
      validators.minimum(1) & validators.maximum(65535)
  )

  # Option 3: Interactive prompting (keeps asking until valid)
  from valid8r.prompt import ask

  port = ask(
      "Enter port number (1-65535): ",
      parser=lambda s: parsers.parse_int(s).bind(
          validators.minimum(1) & validators.maximum(65535)
      )
  )
  # port is guaranteed valid here, no match needed

  # Option 4: Create a reusable parser function
  def parse_port(text):
      return parsers.parse_int(text).bind(
          validators.minimum(1) & validators.maximum(65535)
      )

  result = parse_port(user_input)

The & operator is probably the cleanest for combining validators. And the interactive prompt is nice because you don't need to match Success/Failure, it just keeps looping until the user gives you valid input.

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