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The missing coercion framework for Ruby

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Myrrha (v1.2.2)

Build Status Dependency Status


Myrrha provides the coercion framework which is missing to Ruby, IMHO. Coercions are simply defined as a set of rules for converting values from source to target domains (in an abstract sense). As a typical and useful example, it comes bundled with a coerce() method providing a unique entry point for converting a string to a numeric, a boolean, a date, a time, an URI, and so on.


% [sudo] gem install myrrha

Bundler & Require

# Bug fixes (tiny) do not even add new default rules to coerce and
# to\_ruby\_literal. Minor version can, which could break your code.
# Therefore, please always use:
gem "myrrha", "~> 1.2.2"


The coerce() feature

Myrrha.coerce(:anything, Domain)
coerce(:anything, Domain)                    # with core extensions

What for?

Having a single entry point for coercing values from one data-type (typically a String) to another one is very useful. Unfortunately, Ruby does not provide such a unique entry point... Thanks to Myrrah, the following scenario is possible and even straightforward:

require 'myrrha/with_core_ext'
require 'myrrha/coerce'
require 'date'

values = ["12", "true", "2011-07-20"]
types  = [Integer, Boolean, Date] do |value,domain|
  coerce(value, domain)
# => [12, true, #<Date: 2011-07-20 (...)>]

Implemented coercions

Implemented coercions are somewhat conservative, and only use a subset of what ruby provides here and there. This is to avoid strangeness ala PHP... The general philosophy is to provide the natural coercions we apply everyday.

The master rules are

  • coerce(value, Domain) returns value if belongs_to?(value, Domain) is true (see last section below)
  • coerce(value, Domain) returns Domain.coerce(value) if the latter method exists.
  • coerce("any string", Domain) returns Domain.parse(value) if the latter method exists.

The specific implemented rules are

require 'myrrha/with_core_ext'
require 'myrrha/coerce'

# NilClass -> _Anything_ returns nil, always
coerce(nil, Integer)              # => nil

# Object -> String, via ruby's String()
coerce("hello", String)           # => "hello"
coerce(:hello, String)            # => "hello"

# String -> Numeric, through ruby's Integer() and Float()
coerce("12", Integer)             # => 12
coerce("12.0", Float)             # => 12.0

# String -> Numeric is smart enough:
coerce("12", Numeric)             # => 12 (Integer)
coerce("12.0", Numeric)           # => 12.0 (Float)

# String -> Regexp, through Regexp.compile
coerce("[a-z]+", Regexp)          # => /[a-z]+/

# String -> Symbol, through to_sym
coerce("hello", Symbol)           # => :hello

# String -> Boolean (hum, sorry Matz!)
coerce("true", Boolean)           # => true
coerce("false", Boolean)          # => false
coerce("true", TrueClass)         # => true
coerce("false", FalseClass)       # => false

# String -> Date, through Date.parse
require 'date'
coerce("2011-07-20", Date)        # => #<Date: 2011-07-20 (4911525/2,0,2299161)>

# String -> Time, through Time.parse (just in time issuing of require('time'))
coerce("2011-07-20 10:57", Time)  # => 2011-07-20 10:57:00 +0200

# String -> URI, through URI.parse
require 'uri'
coerce('', URI)  # => #<URI::HTTP:0x8281ce0 URL:>

# String -> Class and Module through constant lookup
coerce("Integer", Class)          # => Integer
coerce("Myrrha::Version", Module) # => Myrrha::Version

# Symbol -> Class and Module through constant lookup
coerce(:Integer, Class)           # => Integer
coerce(:Enumerable, Module)       # => Enumerable

No core extension? no problem!

require 'myrrha/coerce'

Myrrha.coerce("12", Integer)            # => 12
Myrrha.coerce("12.0", Float)            # => 12.0

Myrrha.coerce("true", Myrrha::Boolean)  # => true
# [... and so on ...]

Adding your own coercions

The easiest way to add additional coercions is to implement a coerce method on you class; it will be used in priority.

class Foo
  def initialize(arg)
    @arg = arg
  def self.coerce(arg)

Myrrha.coerce(:hello, Foo)
# => #<Foo:0x869eee0 @arg=:hello>

If Foo is not your code and you don't want to monkey patch the class by adding a coerce class method, you can simply add new rules to Myrrha itself:

Myrrha::Coerce.append do |r|
  r.coercion(Symbol, Foo) do |value, _|

Myrrha.coerce(:hello, Foo)
# => #<Foo:0x8866f84 @arg=:hello>

Now, doing so, the new coercion rule will be shared with all Myrrha users, which might be intrusive. Why not using your own set of coercion rules?

MyRules = Myrrha::Coerce.dup.append do |r|
  r.coercion(Symbol, Foo) do |value, _|

# Myrrha.coerce is actually a shortcut for:
Myrrha::Coerce.apply(:hello, Foo)
# => Myrrha::Error: Unable to coerce `hello` to Foo

MyRules.apply(:hello, Foo)
# =>  #<Foo:0x8b7d254 @arg=:hello>

The to_ruby_literal() feature

[:anything].to_ruby_literal                  # with core extensions

What for?

Object#to_ruby_literal has a very simple specification. Given an object o that can be considered as a true value, the result of o.to_ruby_literal must be such that the following invariant holds:

Kernel.eval(o.to_ruby_literal) == o

That is, parsing & evaluating the literal yields the same value. When generating (human-readable) ruby code, having a unique entry point that respects the specification is very useful.

For almost all ruby classes, but not all, using o.inspect respects the invariant. For example, the following is true:

Kernel.eval("hello".inspect)           == "hello"            # => true
Kernel.eval([1, 2, 3].inspect)         == [1, 2, 3]          # => true
Kernel.eval({:key => :value}.inspect)  == {:key => :value}   # => true
# => true

Unfortunately, this is not always the case:

Kernel.eval( ==
# => false
# => because yields "#<Date: 2011-07-20 ...", which is a comment


Myrrha implements a very simple set of rules for implementing Object#to_ruby_literal that works:

require 'date'
require 'myrrha/with_core_ext'
require 'myrrha/to_ruby_literal'

1.to_ruby_literal                       # => "1"              # => "Marshal.load('...')"
["hello",].to_ruby_literal   # => "['hello', Marshal.load('...')]"

Myrrha implements a best-effort strategy to return a human readable string. It simply fallbacks to Marshal.load(...) when the strategy fails:

(1..10).to_ruby_literal                 # => "1..10"

today =
(        # => "Marshal.load('...')"

No core extension? no problem!

require 'date'
require 'myrrha/to_ruby_literal'

Myrrha.to_ruby_literal(1)              # => 1
Myrrha.to_ruby_literal(     # => Marshal.load("...")
# [... and so on ...]

Adding your own rules

The easiest way is simply to override to_ruby_literal in your class

class Foo
  attr_reader :arg
  def initialize(arg)
    @arg = arg
  def to_ruby_literal

# => ""

As with coerce, contributing your own rule to Myrrha is possible:

Myrrha::ToRubyLiteral.append do |r|
  r.coercion(Foo) do |foo, _|

# => ""

And building your own set of rules is possible as well:

MyRules = Myrrha::ToRubyLiteral.dup.append do |r|
  r.coercion(Foo) do |foo, _|

# Myrrha.to_ruby_literal is actually a shortcut for:
# => "Marshal.load('...')"

# => ""


As the feature fallbacks to marshaling, everything which is marshalable will work. As usual, to_ruby_literal(Proc) won't work.

The general coercion framework

A set of coercion rules can simply be created from scratch as follows:

Rules = Myrrha.coercions do |r|

  # `upon` rules are tried in priority if PRE holds
  r.upon(SourceDomain) do |value, requested_domain|

    # PRE: - user wants to coerce `value` to a requested_domain
    #      - belongs_to?(value, SourceDomain)

    # implement the coercion or throw(:newrule)
    returned_value = something(value)

    # POST: belongs_to?(returned_value, requested_domain)


  # `coercion` rules are then tried in order if PRE holds
  r.coercion(SourceDomain, TargetDomain) do |value, requested_domain|

    # PRE: - user wants to coerce `value` to a requested_domain
    #      - belongs_to?(value, SourceDomain)
    #      - TargetDomain <=> requested_domain

    # implement the coercion or throw(:newrule)
    returned_value = something(value)

    # POST: returned_value belongs either to TorgetDomain or to
    #       requested_domain (very smart converter)


  # fallback rules are tried if everything else has failed
  r.fallback(SourceDomain) do |value, requested_domain|

    # exactly the same as upon rules



When the user invokes Rules.apply(value, domain) all rules for which PRE holds are executed in order, until one succeed (chain of responsibility design pattern). This means that coercions always execute in O(number of rules).

Specifying converters

A converter is the third (resp. second) element specified in a coercion rules (resp. an upon or fallback rule). A converter is generally a Proc of arity 2, which is passed the source value and requested target domain.

Myrrha.coercions do |r|
  r.coercion String, Numeric, lambda{|value,requested_domain|
    # this is converter code
convert("12", Integer)

A converter may also be specified as an array of domains. In this case, it is assumed that they for a path inside the convertion graph. Consider for example the following coercion rules (contrived example)

rules = Myrrha.coercions do |r|
  r.coercion String,  Symbol, lambda{|s,t| s.to_sym }   # 1
  r.coercion Float,   String, lambda{|s,t| s.to_s   }   # 2
  r.coercion Integer, Float,  lambda{|s,t| Float(s) }   # 3
  r.coercion Integer, Symbol, [Float, String]           # 4

The last rule specifies a convertion path, through intermediate domains. The complete rule specifies that applying the following path will work

Integer -> Float -> String -> Symbol
        #3       #2        #1


rules.coerce(12, Symbol)      # => :"12.0"

Semantics of belongs_to? and subdomain?

The pseudo-code given above relies on two main abstractions. Suppose the user makes a call to coerce(value, requested_domain):

  • belongs_to?(value, SourceDomain) is true iif

    • SourceDomain is a Proc of arity 2, and, requested_domain) yields true
    • SourceDomain is a Proc of arity 1, and yields true
    • SourceDomain === value yields true
  • subdomain?(SourceDomain,TargetDomain) is true iif

    • SourceDomain == TargetDomain yields true
    • TargetDomain respond to :superdomain_of? and answers true on SourceDomain
    • SourceDomain and TargetDomain are both classes and the latter is a super class of the former

Advanced rule examples

Rules = Myrrha.coercions do |r|

  # A 'catch-all' upon rule, always fired
  catch_all = lambda{|v,rd| true}
  r.upon(catch_all) do |value, requested_domain|
    if you_can_coerce?(value)
      # then do it!

  # Delegate every call to the requested domain if it responds to compile
  compilable = lambda{|v,rd| rd.respond_to?(:compile)}
  r.upon(compilable) do |value, requested_domain|

  # A fallback strategy if everything else fails
  r.fallback(Object) do |value, requested_domain|
    # always fired after everything else
    # this is your last change, an Myrrha::Error will be raised if you fail


Factoring domains through specialization by constraint

Specialization by constraint (SByC) is a theory of types for which the following rules hold:

  • A type (aka domain) is a set of values
  • A sub-type is a subset
  • A sub-type can therefore be specified through a predicate on the super domain

For example, "positive integers" is a sub type of "integers" where the predicate is "value > 0".

Myrrha comes with a small feature allowing you to create types 'ala' SByC:

PosInt = Myrrha.domain(Integer){|i| i > 0}       # => "PosInt"
PosInt.class      # => Class
PosInt.superclass # => Integer
PosInt.ancestors  # => [PosInt, Integer, Numeric, Comparable, Object, Kernel, BasicObject]
PosInt === 10     # => true
PosInt === -1     # => false    # => 10   # => ArgumentError, "Invalid value -10 for PosInt"

Note that the feature is very limited, and is not intended to provide a truly coherent typing framework. For example:

10.is_a?(PosInt)    # => false
10.kind_of?(PosInt) # => false

Instead, Myrrha domains are only provided as an helper to build sound coercions rules easily while 1) keeping a Class-based approach to source and target domains and 2) having friendly error messages 3) really supporting true reasoning on types and value:

# Only a rule that converts String to Integer
rules = Myrrha.coercions do |r|
  r.coercion String, Integer, lambda{|s,t| Integer(s)}

# it succeeds on both integers and positive integers
rules.coerce("12", Integer)   # => 12
rules.coerce("12", PosInt)    # => 12

# and correctly fails in each case!
rules.coerce("-12", Integer)  # => -12
rules.coerce("-12", PosInt)   # => ArgumentError, "Invalid value -12 for PosInt"

Note that if you want to provide additional tooling to your factored domain, the following way of creating them also works:

class PosInt < Integer
  extend, [], lambda{|i| i > 0})

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