tarski

What is It?

tarski makes it easy to add arbitrary validations to any Scala class, without anything fancy, enabling you to validate user input or web service responses at runtime.

tarski also provides meaningful, application-defined information about what validation rules failed, so your application can act accordingly.

Setup

First, include tarski as a project dependency. The current release is for Scala 2.9.1.

If you're using Maven:

<dependencies>
  <dependency>
    <groupId>org.tnmx</groupId>
    <artifactId>tarski_2.9.1</artifactId>
    <version>0.2.0</version>
  </dependency>
</dependencies>

How to Use

tarski provides a base trait, Validation. In order to use, all you need to do is mix in that trait to your class, and define validations. validations is a Seq of type ValidationRule, explained in more detail below.

Instances of your class will then have a isValid method and a getErrors method.

If all the validation rules "pass", isValid will return true.

If any validation rule "fails", isValid will return false.

If there are errors, you can call getErrors, which will return a set of type ValidationError, with string representations of what tests failed. This type is explained in more detail below.

ValidationRule

In your application, you define validation rules. A validation rule is just a case class that extends ValidationRule and implements two methods: validate and onError.

validate is any Boolean expression that you wish to test. onError is a ValidationError you wish to return in case that test fails.

ValidationError

A ValidationError is a case class with a String parameter (such as, "Name was not Tom"). This message can be directly passed on to a front-end web service, or you can use application logic to handle different sorts of validation error responses.

An Example

import org.tnmx.tarski._

Define some validation rules.

case class NameRule(name: String) extends ValidationRule {
  def validate = (name == "Tom")
  def onError = ValidationError("Name was not Tom.")
}

case class AgeRule(age: Int) extends ValidationRule {
  def validate = (age >= 25)
  def onError = ValidationError("Age was not greater than or equal to 25.")
}

case class FriendsRule(friends: List[String]) extends ValidationRule {
  def validate = (friends.size == 3)
  def onError = ValidationError("Friends list was not equal to 3.")
}

Write a case class that extends Validations and defines validations.

case class Person(name: String, age: Int, friends: List[String]) extends Validations {
  val validations = Seq(NameRule(name), AgeRule(age), FriendsRule(friends))
}

Create some objects. In practice, this data will almost certainly come from runtime input, like web forms.

val tom = Person("Tom", 28, List("Jess", "Ben", "Jessica"))
val jess = Person("Jess", 28, List("Ben"))

And now call some methods:

tom.isValid
  > true

jess.isValid
  > false

jess.getErrors
  > Set[ValidationError](ValidationError("Name was not Tom."),
                         ValidationError("Friends list was not equal to 3."))

A Common Use Case

One nice use case for tarski is for validating user input at runtime, and then acting accordingly. For example, you might be using Jerkson to parse JSON data for a web service, and inserting data from a parsed case class into a database. If you want to provide application-level validation for instances of those case classes, you can simply mix Validation into your case class (like above) and define predicates that must be met. Then just call isValid before doing anything more.

Author & MIT License

Copyright (c) Ted Nyman

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.