A Ruby port of Clojure's clojure.spec. See clojure.spec - Rationale and Overview. The Speculation library is largely a copy-and-paste from clojure.spec. All credit goes to the clojure.spec authors.


Add this line to your application's Gemfile:

gem 'speculation'

And then execute:

$ bundle

Or install it yourself as:

$ gem install speculation


The API is more-or-less the same as clojure.spec. If you're already familiar with clojure.spec then you should be write at home with Speculation. Clojure and Ruby and quite different languages, so naturally there are some differences:

Built in predicates

clojure.spec leans on its macro system and rich standard library of predicate functions when writing specs. Ruby has neither of those, so we must be creative with what we define as a 'predicate' in Speculation. Each of the following are valid Speculation predicates:

S.valid?(->(x) { x > 0 }, 2)
S.valid?(:even?.to_proc, 2)
S.valid?(String, "foo")
S.valid?(Enumerable, [1, 2, 3])
S.valid?(/^\d+$/, "123")
S.valid?(Set[:foo, :bar, :baz], :foo)

Namespaced keywords

Namespaced keywords are at the core of clojure.spec. Since spec utilises a global spec registry, namespaced keywords allow libraries to register specs with the same names but under different namespaces, thus removing accidental collisions. Ruby's equivalent to Clojure's keywords are Symbols. Ruby Symbol's don't have namespaces.

In order keep the global spec registry architecture in Speculation, we utilise refinements achieve similar behaviour:

using Speculation::NamespacedSymbols.refine(MyModule)

p :foo.ns
# => :"MyModule/foo"

p :foo.ns(AnotherModule)
# => :"AnotherModule/foo"



Clojure uses Symbols to refer to functions. To refer to a method in Ruby, we must use the method method.

def self.hello(name)
  "Hello #{name}"

S.fdef(method(:hello), :args => S.cat(:name => String), :ret => String)

Block args

In addition to regular arguments which can easily be described as a list, Ruby methods can take blocks. In Speculation, we spec a method's block separately to its args:

def self.hello(name, &block)
  "Hello #{block.call(name)}"

S.fdef(method(:hello), :args => S.cat(:name => String),
                       :block => S.fspec(:args => S.cat(:s => String), :ret => String),
                       :ret => String)

Generators and quick check

Speculation uses Rantly for random data generation. Generator functions in Speculation are Procs that take one argument (Rantly instance) and return random value. While clojure's test.check generators generate values that start small and continue to grow and get more complex as a property holds true, Rantly always generates random values.

Rantly gives Speculation the ability to shrink a failing test case down to a its smallest failing case, however in Speculation we limit this to Integers and Strings. This is an area where Speculation may currently be significantly weaker than clojure.spec.


After checking out the repo, run bin/setup to install dependencies. Then, run rake to run rubocop and the tests. You can also run bin/console for an interactive prompt that will allow you to experiment.


Bug reports and pull requests are welcome on GitHub at https://github.com/english/speculation.


  • tidy up tests
  • write up comparison with clojure.spec
  • write guide

clojure.spec features


  • Find/build an alternative to Rantly
  • Profile and optimise


The gem is available as open source under the terms of the MIT License.