Hanami::Validations is a mixin that, once included by an object, adds lightweight set of validations to it.

It works with input hashes and lets us to define a set of validation rules for each key/value pair. These rules are wrapped by lambdas (or special DSL) that check the input for a specific key to determine if it’s valid or not. To do that, we translate business requirements into predicates that are chained together with Ruby faux boolean logic operators (eg. & or |).

Think of a signup form. We need to ensure data integrity for the name field with the following rules. It is required, it has to be: filled and a string and its size must be greater than 3 chars, but lesser than 64. Here’s the code, read it aloud and notice how it perfectly expresses our needs for name.

class Signup
  include Hanami::Validations
  validations do
    required(:name) { filled? & str? & size?(3..64) }
  end
end
result = Signup.new(name: "Luca").validate
result.success? # => true

There is more that Hanami::Validations can do: type safety, composition, complex data structures, built-in and custom predicates.