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Finitio is a language for capturing information structure. Think "JSON/XML schema" but the right way. For more information about Finitio itself, see

finitio-rb is the ruby binding of Finitio. It allows defining data schemas and validating and coercing data against them in an idiomatic ruby way.


require 'finitio'
require 'json'

# Let load a schema
schema = Finitio.system <<-FIO
  @import finitio/data

    name: String( s | s.strip.size > 0 ),
    at: DateTime

# Let load some JSON document
data = JSON.parse <<-JSON
  { "name": "Finitio", "at": "20142-03-01" }

# And try dressing that data
puts schema.dress(data)

ADTs with internal contracts

finitio-rb tries to provide an idiomatic binding for ruby developers. In particular, it uses a simple convention-over-configuration protocol for information contracts. This protocol is easily described through an example. The following ADT definition:

Color = .Color <rgb> {r: Byte, g: Byte, b: Byte}

expects the following ruby class:

class Color

  # Constructor & internal representation
  def initialize(r, g, b)
    @r, @g, @b = r, g, b
  attr_reader :r, :g, :b

  # Public dresser for the RGB information contract on the class
  def self.rgb(tuple)
    new(tuple[:r], tuple[:g], tuple[:b])

  # Public undresser on the instance
  def to_rgb
    { r: @r, g: @g, b: @b }

  # ...


ADTs with external contracts

When the scenario above is not possible or not wanted (would require core extensions for instance), finitio-rb allows defining ADTs with external contracts. The following ADT definition:

Color = .Color <rgb> {r: Byte, g: Byte, b: Byte} .RgbContract

expected the following ruby module:

module RgbContract

  def self.dress(tuple)[:r], tuple[:g], tuple[:b])

  def self.undress(color)
    { r: color.r, g: color.g, b: color.b }


Decompose complex system with imports

It is useful to decompose complex systems in many files using the import feature. The latter works with relative file paths like this:

# child.fio

Posint = .Integer(i | i >= 0)
# parent.fio
@import ./child.fio

# Child's types are available inside the system, but not outside it, that
# is, imported types are not themselves exported
Byte = Posint(i | i <= 255 )
@import ./parent.fio

# This will work
HalfByte = Byte(i | i <= 128)

# But this will not: Posint is not defined
Posint(i | i <= 128)

See the next section about the standard library if you need to share types without relying on relative paths.

Standard library

Usual type definitions are already defined for simple data types, forming Finitio's default system:

  • Most ruby native (data) classes are already aliased to avoid explicit use of builtins. In particular, Integer, String, etc.

  • A Boolean union type also hides the TrueClass and FalseClass distinction.

  • Date, Time and DateTime ADTs are also provided that perform common conversions from JSON strings, through iso8601.

This system is best used through Finitio's so-called "standard library", e.g.

@import finitio/data

# String, Integer, Boolean, etc. are now available in this system

See lib/finitio/stdlib/*.fio for the precise definition of the standard library.

Contributing to the standard library

Ruby gems may contribute to the standard library by specifying resolve paths. We suggest the following system file structure inside your gem source code:


Registering the standard library path can then be done as follows:

# inside myrubygem.rb
Finitio.stdlib_path(File.expand_path('../../finitio', __FILE__))

Then, a Finitio schema will have access to the types defined in your extension:

@import myrubygem/base
@import myrubygem/advanced

About representations

The Rep representation function mapping Finitio types to ruby classes is as follows:

# Any type is represented by Ruby's Object class
Rep(.) = Object

# Builtins are represented by the corresponding ruby class
Rep(.Builtin) = Builtin

# Sub types are represented by the same representation as the super type
Rep(SuperType( s | ... )) = Rep(SuperType)

# Unions are represented by the corresponding classes. The guaranteed result
# class is thus the least common super class (^) of the corresponding
# representations of candidate types
Rep(T1 | ... | Tn) = Rep(T1) ^ ... ^ Rep(Tn)

# Sequences are represented through ::Array.
Rep([ElmType]) = Array<Rep(ElmType)>

# Sets are represented through ::Set.
Rep({ElmType}) = Set<Rep(ElmType)>

# Tuples are represented through ruby ::Hash. Attribute names are always
# symbolized
Rep({Ai => Ti}) = Hash<Symbol => Rep(Ti)>

# Relations are represented through ruby ::Set of ::Hash.
Rep({{Ai => Ti}}) = Set<Hash<Symbol => Rep(Ti)>>

# Abstract data types are represented through the corresponding class when
# specified. ADTs behave as Union types if no class is bound.
Rep(.Builtin <rep> ...) = Builtin