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Assay defines assertions in the same way that Ruby defines exceptions. An assertion then is nothing more that an extended Exception class. Assay provides a complete set of these assertion classes for the most common assertion types. It also provides both TestUnit-style assertion methods and RSpec-compatiable matchers built from these assertion classes for use in your preferred test harness. Assay is compatible with TestUnit, MiniTest, RSpec and other test frameworks.


  • Patterned after the Ruby exception system.

  • Allows assertions specialized error messages.

  • Supports any variety of assertion “grammers”.

  • Can be used with almost any test framework.


  • Assay is Ruby 1.9+ only!


To install with RubyGems simply open a console and type:

$ gem install assay

Site installation with the tarball can be done with Ruby Setup (gem install setup). See


Assay Classes

Assay consists of a set of Assertion subclasses known as assays. They are akin to Ruby’s Exception subclasses, indeed the Assertion base class is a subclass of Exception. But assays have special class methods that are used to make assertions.

Consider the EqualityAssay class. It defines methods for asserting equality via the ‘#==` method.


Additionally, we can check the assertion’s test without actually raising the assertion if it fails using the query method.

EqualityAssay.pass?(1,1)    #=> true

Assays also provide the opposite method ‘#refute!` along with `#fail?`.

EqualityAssay.refute!(1,2),2)    #=> true

Assay instances are test matchers, which can be conveniently defined with ‘#[]`.

EqualityAssay[1] =~ 1

Notice in the example we have used ‘#=~` to apply the matcher which makes the `#assert!` call to the Assay object. Likewise `#!~` can be used to call `#refute!` instead. And note that `#===` is also an alias for `#=~`.

EqualityAssay[1] === 1

Which means assay matchers can be used in case statments.

case 10
when InstanceAssay[Fixnum]
when EqualityAssay[10.0]

Pretty neat.

Framework Adapters

Assay follows a standard practice of defining assertion error classes with an ‘#assertion?` method that returns true. This can be used by any test framework to easily detect when a raised error is an assertion rather than an ordinary error. To add support for this to common test frameworks Assay provides adapters.

For example, to use assay with MiniTest framework add to your test helper script:

require 'assay/adapter/minitest'

Likewise for TestUnit.

require 'assay/adapter/testunit'

An RSpec adadpter is in the works, and should be out with the next release.

Note that even without the adapter, you can still use Assay with other test frameworks. They will simply count Assay’s assertions as regular errors.

Customized Grammars

Of course the classes are interesting and clearly make for a sound foundation, but in the end we want to write assertions more easily and concisely. For this we turn to separate “grammar” projects that depend on Assay’s classes. The first of these, created as a spin-off project to demonstrate Assay’s prowess, is Fluidity. Here is a quick taste of that gem’s functionality.

require 'fluidity'

But is you are acustom to MiniTest’s spec methods, you might prefer ‘must`.

And to satisfy all those technical aficionados (like yours truly) there is ‘assert`.


Thre are also compatibility grammar projects available, spun-off from Assay, that provide compatability with legacy test frameworks which can serve as transition to Assay from these other frameworks. Follow the links below for each:

Usage is essentially the same for any one of them. For example,

require 'assay/rspec'

include Assay::Matchers

10.should be_kind_of(Integer)

Note that the compatibility modules are not yet 100% compatable, lacking some of the more esoteric and complex features. But they are very nearly so, and will become more so in time.

These are just a few possible grammars. There is no reason not to build your own grammar on top of Assay’s classes if you have another approach in mind. Indeed, please do! That, after all, is the main purpose of having such a set of reusable assertion classes!

Learning More

There’s more learn about Assay, mainly the variety of assay classes available, but also a few other bits of functionality not comvered here. To learn about these check out the QED documentation which provides an overiew of functionality with working examples, and the the API documentation for a more in depth look under the hood.


Copyright © 2009 Rubyworks

This program is ditributed under the terms of the BSD-2-Cluase license.

See COPYING.rdoc file for details.