#moovalid

A lightweight MoonBit validation library that accumulates errors instead of failing fast.

#Why moovalid?

#Installation

moon add ryota0624/moovalid

{ "import": ["ryota0624/moovalid/src/lib"] }

#Quick Start

// User registration: collect all errors at once
fn validate_username(s : String) -> Validated[Nel[String], String] {
  if s.length() == 0 {
    Invalid(Nel::new("username must not be empty"))
  } else if s.length() > 20 {
    Invalid(Nel::new("username must be <= 20 chars"))
  } else {
    Valid(s)
  }
}

fn validate_age(n : Int) -> Validated[Nel[String], Int] {
  if n < 18 {
    Invalid(Nel::new("must be 18 or older"))
  } else {
    Valid(n)
  }
}

struct User { username : String; age : Int }

fn register(username : String, age : Int) -> Validated[Nel[String], User] {
  map2(
    validate_username(username),
    validate_age(age),
    fn(u, a) { { username: u, age: a } },
  )
}

// Both errors reported together:
// register("", 15) ==
//   Invalid(Nel { head: "username must not be empty",
//                 tail: ["must be 18 or older"] })

#Validator API with FieldError

// Build a reusable validator for a username field
let username_validator =
  non_empty()
    .zip(max_length(20))
    .map(fn(pair) { pair.0 })
    |> Validator::new
    |> fn(v) { v.at_field("username") }

// Running it:
// username_validator.run("") ==
//   Invalid(Nel { head: FieldError { path: ["username"],
//                                    message: "must be non-empty" }, ... })

#API Reference

#Core types

#Combinators

#Conversions

#Validated helpers

#Built-in validators

#Validator methods

#Inspiration

• Cats Validated (Scala)
• Haskell Validation

#License