Macro derived validation for
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Validation utilities for cats. Licensed Apache 2.

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This project is similar to other excellent projects like and but differs in that it focuses entirely on supporting cats.

The main abstraction in monkeytail is the Validator[T] trait which validates instances of T using cats.Validation.

trait Validator[T] {
  def apply(t: T): Validated[NonEmptyList[Violation], T]

You will notice that the error type used in the Validation is the Violation trait. This simple trait provides a means to build up domain specific strongly typed errors and is used throughout monkeytail.

Monkeytail offers macro based builders to quickly build instances of Validator[T] or you can always roll your own. Validators can be combined using the provided instances of Monoid[Validator[T]].

Getting Started

Let's declare a class we want to validate. Yes I'm a Star Trek fan.

case class Starship(name: String, maxWarp: Double)

We can always implement an instance of Validator[Starship] ourselves, in the boilerplatey manual way.

val validator = new Validator[Starship] { 
  override def apply(t: Starship): Validated[NonEmptyList[Violation], Starship] = {
    val v1 = if ( == null) Invalid(NonEmptyList(SimpleViolation("Name cannot be null"))) else Valid(t)
    val v2 = if (t.maxWarp < 10) Valid(t) else Invalid(NonEmptyList(SimpleViolation("Exceeded max warp"))) 
    // combine validators here ..

That's fine for simple cases, but really we want to remove as much boilerplate as possible, otherwise what's the point of this project? So let's use the provided validator utilties which allows us to build up validation field by field.

Creating an empty validator just involves invoking apply on the Validator object. Then, make sure you have imported monkeytail.ValidatorSyntax._ to enable the magic functions on the validator instance.

The main utility is in the validate method, which accepts an extractor function to extract the value we want to test, and then a test function that validates it and returns a bool if it is valid. The test expression can be as simple or as complicated as you want.

val validator = Validator[Starship]
  .validate( != null)
  .validate(_.maxWarp)(_ < 10)

As you will already know, no ship can exceed warp 10, so we must validate that. And of course, a ship without a name is invalid. We can now invoke this validator, like this:

val enterprise = Starship("Enterprise", 9.6) 
validator(enterprise) == Valid(enterprise)

And if we provide some erroneous input we'll get back the appropriate accumulated Invalid.

validator(Starship(null, 11)) == 
      DefaultViolation("Invalid value: null", Path("name")), 
      DefaultViolation("Invalid value: 11.0", Path("maxWarp"))

What's nice here is that the error messages are automatically generated and the violations include the path to the field name. The macro is taking care of that behind the scenes.

It will work for nested paths as well, if you did something like validate(_.address.postcode)(_.length == 8), then the error would include the path address.postcode

Custom Errors

As mentioned earlier, the type used for errors is Violation and by default all errors are instances of DefaultViolation. However we can provide our own instances of Violation. This allows us to introduce a richer type system for errors as well as provide customized error messages instead of the default "Invalid value: $value".

At a basic level, a custom Violation can include no context at all. Let's create a custom error type for ships that dare to exceed warp 10.

object MaxWarpExceededViolation extends Violation

Now we can include this when we add a validation rule and when we validate our instances, we'll get back the proper type.

val validator = Validator[Starship]
  .validate( != null)
  .validate(_.maxWarp)(_ < 10)(MaxWarpExceededViolation)
validator(Starship("Enterprise", 11)) == Invalid(NonEmptyList.of(MaxWarpExceededViolation))

We can add extra information to the violations if we wish. For this we will need a ViolationBuilder which is just a function that accepts the Path to the value that generated the error, and the value that was under test. What we include in the violation type is up to us.

In the following example we're including a custom message using the value that failed.

case class MaxWarpExceededViolation(message: String) extends Violation

object MaxWarpExceededViolationBuilder extends ViolationBuilder[Double] {
  override def apply(path: Path, value: Double) = MaxWarpExceededViolation(s"Max warp exceeded, was $value")

val validator = Validator[Starship]
  .validate( != null)
  .validate(_.maxWarp)(_ < 10)(MaxWarpExceededViolationBuilder)
validator(Starship("Enterprise", 11)) == Invalid(NonEmptyList.of(MaxWarpExceededViolation("Max warp exceeded, was 11.0")))

Validation Helpers

If you want to validate that all the fields of a case class are not then, then Validator.notnull[T] is a convenience macro. It will add a validation rule for every field in the case class to test that the value is not null. In other words, behind the scenes, the macro is doing this:

  .validate(_.a)(_ != null)
  .validate(_.b)(_ != null)
  ... // and so on

The validator returned by this helper can then be used as the basis for your own rules. For example:

  .validate(_.maxWarp)(_ < 10)

Nested Validators

When we have a type that contains another type, and we already have a validator for the nested type, we can tell monkeytail to just use the existing validator and delegate to that.

For example, if our starship model was like this:

case class Designation(prefix: String, code: String)
case class Starship(name: String, maxWarp: Double, designation: Designation)

We could define a validator for the Designation class separately (perhaps we use it in multiple places).

implicit val designationValidator = Validator[Designation]
  .validate(_.prefix)(prefix => prefix != null && (prefix.startsWith("NCC") || prefix.startsWith("NX"))
  .validate(_.code)(_ != null)

Then we we can define a validator for the Starship class and use the previous validator automatically for the designation field, assuming it is available as an implicit in the current scope. Note, we use the valid method rather than the validate method we've been using so far.

val designationValidator = Validator[Starship]
  .validate( != null)
  .validate(_.maxWarp)(_ < 10)(MaxWarpExceededViolation)
  .validate(_.designation) // this will require the previous implicit.

Sanitize / Normalize values

Before a field is checked we might want to change the value to sanitize the value into our standard. For example you might decide that all email addresses should be lowercase, or you might want to trim excess whitespace from certain fields before it is validated.

To do this we can use the sanitize method on the validator. For example:


The sanitized value is passed through the validation chain and so is present in any validations that occur after the sanitized call, as well as being included in the final result. So in the above case, any starship that passes validation will have it's name lower-cased in the result.

Validating sequences

At the moment support for sequences is very basic and will be improved for 1.0 final. You can use an existing validator and pass that into the forall method to validate each element of the sequence individually, accumulating errors as elements fail.

case class Friend(name: String)
case class User(username: String, friends: Seq[Friend])

val friendValidator = Validator[Friend].validate(...) // etc

// reuse the existing validator
val userValidator = Validator[User]
    .forall(_.friends, friendValidator) 

Or you can validate them manually using a regular test expression, like this:

    .forall(_.friends)( != null)

By using the forall method on the validator builder, you can pass in a test expression for a single element, and then this expression will be evaluated in turn for every element of the sequence. Every failing element will be included in the resulting cats.Validated instance.

Using Monkeytail in your project

For gradle users, add (replace 2.12 with 2.11 for Scala 2.11):

compile 'com.sksamuel.elastic4s:elastic4s-core_2.12:x.x.x'

For SBT users add:

libraryDependencies += "com.sksamuel.monkeytail" %% "monkeytail" % "x.x.x"

For Maven users add (replace 2.12 with 2.11 for Scala 2.11):


You will need to set the version by looking for the latest in maven central. You can click the links at the of the page.

Building and Testing

This project is built with SBT. So to build

sbt compile

And to test

sbt test


This software is licensed under the Apache 2 license, quoted below.

Copyright 2013-2016 Stephen Samuel

Licensed under the Apache License, Version 2.0 (the "License"); you may not
use this file except in compliance with the License. You may obtain a copy of
the License at

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
License for the specific language governing permissions and limitations under
the License.