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}")

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)