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Generate mocks and other test doubles using clojure.spec


Testing code with side effects, such as I/O, can be painful. It slows down your tests and can cause spurious failures. Mocking out these interactions is a great way to keep your tests fast and reliable.

Specific can generate mock functions from clojure.spec definitions. It can help you make assertions about how the functions were called, or simply remove the side effect and let your spec declarations do the verification. This means it works on programs with example-based tests, property-based generative tests, or both.


Specific works with Clojure 1.10 or 1.9 (or 1.8 with the clojure.spec backport) and test.check version 0.9.0.

You can find the latest version in the Clojars repository, here:

Clojars Project


To show you how to use Specific, let's assume you have three interdependent functions you'd like to test. One of them, cowsay, executes a shell command which might not be available in all environments.

(ns sample
  (:require [ :as shell]
            [clojure.string :as string]))

(defn greet [pre sufs]
  (string/join ", " (cons pre sufs)))

(defn cowsay [msg]
  (shell/sh "cowsay" msg)) ; Fails in some environments

(defn some-fun [greeting & names]
  (:out (cowsay (greet greeting names))))

Specific works best with functions that have clojure.spec definitions. You can include these definitions with the code under test, or you can add them in the tests themselves, or both.

(clojure.spec/def ::exit (clojure.spec/and integer? #(>= % 0) #(< % 256)))
(clojure.spec/def ::out string?)
(clojure.spec/def ::fun-greeting string?)
(clojure.spec/fdef greet :ret ::fun-greeting)
(clojure.spec/fdef cowsay
                   :args (clojure.spec/cat :fun-greeting ::fun-greeting)
                   :ret (clojure.spec/keys :req-un [::out ::exit]))
(clojure.spec/fdef some-fun
                   :args (clojure.spec/+ string?)
                   :ret string?)

Mock Functions

Mocking a function prevents the original function from being called, which is useful when you want to prevent side effects in a test, but still want to ensure it was invoked properly. Mocked functions validate their arguments against the specs defined for the original function, and return data generated from the spec.

You can replace a list of functions with mock functions using the specific.core/with-mocks macro, like so:

(testing "mock functions"
  (with-mocks [sample/cowsay]

    (testing "return a value generated from the spec"
      (is (<= 0 (:exit (sample/cowsay "hello"))))
      (is string? (:out (sample/cowsay "hello"))))

    (testing "validate against the spec of the original function"
      (sample/cowsay "hello"))

      ; (sample/cowsay 1)
      ; val: 1 fails spec: :specific.sample/fun-greeting at: [:args 0] predicate: string?
      ; expected: string?
      ;   actual: 1

    (testing "record the individual calls"
      (sample/cowsay "hello")
      (sample/cowsay "world")
      (is (= [["hello"] ["world"]] (calls sample/cowsay))))))

Test Conforming Arguments

If you want to make assertions about how a particular mock was invoked, you can use specific.core/calls to get list of arguments for all the invocations of any Specific mock function. While easy to understand and extensible, this approach would require that you use generated values in your tests. Instead, you can assert that the arguments passed to a function conform to a spec, using specific.core/args-conform:

(testing "args-conform matcher"
  (spec/def ::h-word #(string/starts-with? % "h"))
  (with-mocks [sample/cowsay]

    (testing "matches with exact values"
      (sample/some-fun "hello" "world") 
      (is (args-conform sample/cowsay "hello, world")))

    (testing "can use a custom spec to validate an argument"
      (sample/some-fun "hello" "world")
      (sample/some-fun "hello" "larry")
      (is (args-conform sample/cowsay ::h-word)))

    (testing "can ensure all invocations are conforming"
      (doall ; Ironically, exercise is lazy
        (spec/exercise-fn `sample/some-fun))
      (is (args-conform sample/cowsay ::sample/fun-greeting)))))

The args-conform matcher is also handy when you need to verify invocations that include generated data returned from a mock or stub. You can use any spec that you want to verify the arguments. You can also mix specs and exact values in a single call.

Stub Functions

Stub functions are more lenient than mocks, not requiring the function to have a spec. Stub functions always return nil.

(testing "stub functions"
  (with-stubs []

    (testing "return nil"
      (is (nil? (sample/some-fun "hello" "world"))))

    (testing "don't need a spec"
      (sample/some-fun "hello" "world")
      (is (args-conform "cowsay" ::sample/fun-greeting)))))

Just as with mocks, when using the args-conform matcher on a stub, you can use specs, exact values, or a mixture of the two

Spy Functions

Spy functions call through to the original function, but still record the calls and enforce the constraints in the function's spec.

(testing "spy functions"
  (with-spies [sample/greet]

    (testing "calls through to the original function"
      (is (= "Hello, World!" (sample/greet "Hello" ["World!"])))
      (is (= [["Hello" ["World!"]]] (calls sample/greet))))))

In practice, spies in Specific work a lot like the default behavior of clojure.spec/instrument, except that they are scoped only to the forms in the with-spies macro.

Generated data

You can use specs to generate test data, optionally overriding certain specs to produce different combinations of values.

  (testing "generating test data"
    (spec/def ::word (spec/and string? #(re-matches #"\w+" %)))
    (spec/def ::short-string (spec/and ::word #(> (count %) 2) #(< (count %) 5)))

    (testing "Returns a constant, conforming value for a given spec"
      (is (= "koI" (generate ::short-string)))
      (is (spec/valid? ::short-string (generate ::short-string))))

    (testing "can override specs"
      (is (= "word" (generate ::short-string ::word #{"word"}))))

    (testing "uses with-gens overrides too"
      (with-gens [::word #{"word"}]
        (is (= "word" (generate ::short-string))))))

Unlike the regular test.check generator, data generated in Specific test doubles is deterministic. This is true for both the generate function and mocks. This means the values generated will not change unless spec itself changes. Whether or not you depend on this consistency is up to you.

Generator Overrides

Sometimes, within the scope of a test (or a group of tests) it makes sense to override the generator for a spec. For example, you want to test a more specific range of values, or have a function return a single value. To do that with Specific you can use the with-gens macro:

(testing "generator overrides"
  (with-mocks [sample/cowsay sample/greet]

    (testing "can temporarily replace the generator for a spec using a predicate"
      (with-gens [::sample/fun-greeting #{"hello!"}]
        (is (= "hello!" (sample/greet "hello" [])))))

    (testing "can replace the generator for a nested value"
      (with-gens [::sample/exit #{0}]
        (is (= 0 (:exit (sample/cowsay "hello"))))))

    (testing "can use another spec's generator"
      (with-gens [::sample/out ::sample/fun-greeting]
        (is (string? (sample/some-fun "hello"))))))))

Since with-gens redefines the generator for a spec, and not an entire function, you can use it to specify a portion of an otherwise default generated return value (a single nested :phone-number value in an entity map, for example).

Friends and Relations

Specific gets along well with the following tools:



  • No longer enforces that a spec specifies a return value when mocking a function


  • More sensible error messages when you forget to mock a function


  • Renamed conforming to args-conform
  • No longer evaluating forms when a mock cannot be created
  • Better failure messages when missing a :ret spec in a mock


  • Generated values are now deterministic
  • Added core/generate to generate test data


The following commands run the tests against various versions of Clojure.

lein with-profile +1.10 test
lein with-profile +1.9 test
lein with-profile +1.8 test


Copyright (C) 2016 Ben Rady

This program is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License version 2 as published by the Free Software Foundation.

This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

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