Programming language: Crystal
Tags: Validation    
Latest version: v1.0.0-rc5

validator alternatives and similar shards

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∠(・.-)―〉 →◎ validator is a Crystal micro validations module. Very simple and efficient, all validations return true or false.

Also validator/check (not exposed by default) provides error message handling intended for the end user.

Validator respects the KISS principle and the Unix Philosophy. It's a great basis tool for doing your own validation logic on top of it.


  1. Add the dependency to your shard.yml:
    github: nicolab/crystal-validator
  1. Run shards install


There are 2 main ways to use validator:

  • As a simple validator to check rules (eg: email, url, min, max, presence, in, ...) which return a boolean.
  • As a more advanced validation system which will check a series of rules and returns all validation errors encountered with custom or standard messages.

By default the validator module expose only Validator and Valid (alias) in the scope:

require "validator"

Valid.email? "contact@example.org" # => true
Valid.url? "https://github.com/Nicolab/crystal-validator" # => true
Valid.my_validator? "value to validate", "hello", 42 # => true

An (optional) expressive validation flavor, is available as an alternative. Not exposed by default, it must be imported:

require "validator/is"

is :email?, "contact@example.org" # => true
is :url?, "https://github.com/Nicolab/crystal-validator" # => true
is :my_validator?, "value to validate", "hello", 42 # => true

# raises an error if the email is not valid
is! :email?, "contact@@example..org" # => Validator::Error

is is a macro, no overhead during the runtime 🚀 By the nature of the macros, you can't pass the validator name dynamically with a variable like that is(validator_name, "my value to validate", arg). But of course you can pass arguments with variables is(:validator_name?, arg1, arg2).


To perform a series of validations with error handling, the validator/check module offers this possibility 👍

A Validation instance provides the means to write sequential checks, fine-tune each micro-validation with their own rules and custom error message, the possibility to retrieve all error messages, etc.

require "validator/check"

# Validates the *user* data received in the HTTP controller or other.
def validate_user(user : Hash) : Check::Validation
  v = Check.new_validation

  # -- email

  v.check :email, "The email is required.", is :presence?, :email, user
  v.check :email, "#{user[:email]} is an invalid email.", is :email?, user[:email]

  # -- username

  v.check :username, "The username is required.", is :presence?, :username, user

    "The username must contain at least 2 characters.",
    is :min?, user[:username], 2

    "The username must contain a maximum of 20 characters.",
    is :max?, user[:username], 20

v = validate_user user

pp v.valid? # => true (or false)

errors = v.errors

# Inverse of v.valid?
if errors.empty?
  return "no error"

# Print all the errors (if any)
pp errors

# It's a Hash of Array
puts errors.size
puts errors.first_value
errors.each do |key, messages|
  puts key   # => :username
  puts messages # => ["The username is required.", "etc..."]

3 methods #check:

# check(key : Symbol, valid : Bool)
# Using default error message
  is(:min?, user[:username], 2)

# check(key : Symbol, message : String, valid : Bool)
# Using custom error message
  "The username must contain at least 2 characters.",
  is(:min?, user[:username], 2)

# check(key : Symbol, valid : Bool, message : String)
# Using custom error message
  is(:min?, user[:username], 2),
  "The username must contain at least 2 characters."

Check is a simple and lightweight wrapper. The Check::Validation is agnostic of the checked data, of the context (model, controller, CSV file, HTTP data, socket data, JSON, etc).

Use case example: Before saving to the database, the custom error messages can be used for the end user response.

Let your imagination run wild to add your logic around it.

Custom validator

Just add your own method to register a custom validator or to overload an existing validator.

module Validator
  # My custom validator
  def self.my_validator?(value, arg : String, another_arg : Int32) : Bool
    # TODO: write the logic of your validator
    return true

# Call it
puts Valid.my_validator?("value to validate", "hello", 42) # => true

# or with the `is` flavor
puts is :my_validator?, "value to validate", "hello", 42 # => true


  • The word "validator" is the method to make a "validation" (value validation).
  • A validator returns true if the value (or/and the condition) is valid, false if not.
  • The first argument(s) is (are) the value(s) to be validated.
  • Always add the Bool return type to a validator.
  • Always add the suffix ? to the method name of a validator.
  • If possible, indicates the type of the validator arguments.
  • Spec: Battle tested.
  • KISS and Unix Philosophy.


crystal spec
crystal tool format --check

TODO: add ameba


  1. Fork it (https://github.com/nicolab/crystal-validator/fork)
  2. Create your feature branch (git checkout -b my-new-feature)
  3. Commit your changes (git commit -am 'Add some feature')
  4. Push to the branch (git push origin my-new-feature)
  5. Create a new Pull Request


MIT (c) 2020, Nicolas Talle.


Nicolas Tallefourtane - Nicolab.net
Nicolas Talle
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*Note that all licence references and agreements mentioned in the validator README section above are relevant to that project's source code only.