Assertion to test unifiability of two values
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UnificationAssertion provides powerful and simple way to compare two structures which may be different partialy. The comparison is based on unification algorithm, which is used in Prolog implementations and type inference algorithms.

The assertion will be like the following:

assert_unifiable({ "timestamp" => :_,
                   "person" => {
                     "id" => :_,
                     "name" => "John",
                     "email" => "",
                     "created_at" => :_a,
                     "updated_at" => :_a
                 }, JSON.parse(@response.body))

It compares two hash objects, but it does not care the exact value of "timestamp", "id", "created_at", and "updated_at". The meta variable :_a is not a black hole but test the equality between "created_at" and "updated_at".


I have been writing some tests like the following Rails functional tests.

# Testing a web api which create a person data and send the created person object as JSON
post(:create_person, { :person => { :name => "John", :email => "" } })

# Compare expected hash and actual result parsed by JSON parser
assert_equal({ "timestamp" =>,
               "person" => {
                 "id" => 13,
                 "name" => "John",
                 "email" => "",
                 "created_at" =>,
                 "updated_at" =>
             }, JSON.parse(@response.body))

You may point out some problems on the test.

  • The ID of the result may be different from 13
  • It is not sure to assume result["timestamp"] is equal to
  • It is not sure to assume result["person"]["created_at"] is equal to

The root of the problems is that the comparison is too strict. The properties I would like to test is only its name and email fields. So I should test like the following:

assert_equal "John", result["person"]["name"]
assert_equal "", result["person"]["email"]

It looks too complicated, and we need some way to compare structures. This library, UnificationAssertion, provides the primitive for the comparison called assert_unifiable.

assert_unifiable({ "timestamp" => :_,
                   "person" => {
                     "id" => :_,
                     "name" => "John",
                     "email" => "",
                     "created_at" => :_a,
                     "updated_at" => :_a
                 }, JSON.parse(@response.body))

Symbols :_a for example, where its name starts with _ is interpreted as a meta variable. assert_unifiable does not care their exact value is, but only the existence (can be nil) and equalities for each occurance will be tested. The special symbol :_ is a wildcard. It can appear many times, but it will not be bound with any value.


assert_unifiable(:_a, 1)              # pass, :_a will be 1
assert_unifiable([:_a, 1], [1, 1])    # pass, :_a will be 1
assert_unifiable([:_, :_], [1, 2])    # pass, :_ can not be bound with any value
assert_unifiable([:_a, :_a], [1, 2])  # fail, :_a can not be either 1 and 2
assert_unifiable([:_a], [1,2,3])      # fail, :_a can be a value but can not be a sequence

assert_unifiable({ :x => :_a }, { :x => 1 })     # pass, :_a will be 1
assert_unifiable({ :y => :_a }, { })             # fail, a key :y should be present
assert_unifiable({ :y => :_a }, { :y => nil })   # pass, :_a will be nil
assert_unifiable({ :_a => 1 }, { :x => 1 })      # fail, meta variable can not appear as a key

# assert_unifiable can receive a block, which will be yielded with the result of unification.
assert_unifiable([:_a, :_b], [1, 2]) do |unifier|
  assert unifier[:_a] < unifier[:_b]


Update your Gemfile.

gem "unification_assertion", :git => "git://"

Write your test case.

require "minitest/autorun"
require "unification_assertion"

class GreatTest < MiniTest::Unit::TestCase
  include UnificationAssertion
  def test_something
    assert_unifiable([:_a, :_b], [1, 2])

Known Issues

It skips occur check

The recursive pattern can not be processed well.

assert_unifiable(:_a, { :x => :_a })

Usual unification algorithm rejects such input by occur check. This library is expected to be used for testing, so that I omit the checking. (Who in the world will write such comparison?)


Written by Soutaro Matsumoto. (matsumoto at soutaro dot com)

Released under the MIT License: