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Up-front Unit Testing in Haskell

[To Japanese]

This article is a tutorial about unit testing in Haskell using doctest, hspec and Cabal. Readers are supposed to use Haskell Platform 2014.2.0.0 or later.


Important points are summarized as follows:

  1. The behaviors documenting the use of your library should be written with doctest
  2. The behaviors documenting functionality rather than usage of your library should be written with hspec
  3. Cabal is used to automate testing with the frameworks
  4. For pure code, QuickCheck property test cases (with doctest and/or hspec) should be written as much as possible

In this tutorial, we use Base64 encoding/decoding as an example. It is assumed thay you know what it is. If you don't know, please read the explanation about Base64 in wikipedia beforehand.

The package included the examples of this tutorial is available on github. The package name is unit-test-example. It is assumed henceforth that we are at the "unit-test-example" directory.

% git clone git://
% cd unit-test-example


The behaviors documenting the use of your library should be written with doctest. If you do so, they can be documentation and can also be used for testing. (I hope that you also use doctest for design but this article is not about that.)

The Haskell documentation tool is Haddock. With Haddock, documentation is written in comments with markup. For doctest usage examples, the >>> markup is used.

Let's look at an example of "Codec.Base64" in "unit-test-example":

-- |
-- Base64 encoding.
-- >>> encode "foo bar"
-- "Zm9vIGJhcg=="
encode :: String -> String
encode = ...

-- |
-- Base64 decoding.
-- >>> decode "Zm9vIGJhcg=="
-- "foo bar"
decode :: String -> String
decode = ...

The result value is placed in the line following >>>. You can treat >>> as the GHCi prompt. If a function is already defined, you can copy & paste the interactive session from GHCi and then change the GHCi prompt (e.g. Prelude>) to >>>.

You can use let because it is a GHCi session.

-- >>> let xs="Zm9vIGJhcg=="
-- >>> decode xs
-- "foo bar"

Though all examples above are pure, you can use IO. You can specify whatever GHCi can do.

-- >>> doesFileExist "/foo"
-- False

Since test is done by simply comparing strings, you can describe exceptions:

-- >>> 1 `div` 0
-- *** Exception: divide by zero

For more information, please refer to the doctest manual.

You can create an HTML manual under the "dist" directory with the following command:

% cabal haddock --hyperlink-source

Here is an image of an example manual:

An example of manual

Let's run the tests with the "doctest" command:

% doctest Codec/Base64.hs
Examples: 2  Tried: 2  Errors: 0  Failures: 0

If you haven't installed doctest yet, please execute the following command:

% cabal install --enable-test --only-dependencies

This command will install the necessary libraries and commands without installing the current package.


The behaviors documenting functionality rather than usage of your library should be written with hspec. Basically, one test file should be prepared for each module. For instance, "Base64Spec.hs" is created for "Base64.hs".

It is important to note that test files should be placed in a different directory from that of source files. If you follow this rule, you can specify your library as a dependency of the test suite in a Cabal file, which is described later.

If you don't understand what this means, please remember one rule: Making a "test" directory and place all test files there.

The following is an example of "test/Base64Spec":

spec :: Spec
spec = do
    describe "encode" $ do
        it "encodes multiples of 3" $
            encode "no-padding!!" `shouldBe` "bm8tcGFkZGluZyEh"
        it "encodes multiples of 3 + 1" $
            encode "padding  2" `shouldBe` "cGFkZGluZyAgMg=="
        it "encodes multiples of 3 + 2" $
            encode "padding1" `shouldBe` "cGFkZGluZzE="

    describe "decode" $ do
        it "decodes no padding" $
            decode "bm8tcGFkZGluZyEh" `shouldBe` "no-padding!!"
        it "decodes one padding" $
            decode "cGFkZGluZzE=" `shouldBe` "padding1"
        it "decodes two paddings" $
            decode "cGFkZGluZyAgMg==" `shouldBe` "padding  2"

As you can see, you can write test cases with easy-to-understand words such as shouldBe in a fun manner. The sense of fun is crucial.

Though the examples above are pure, you can write test cases for IO. For more information, please refer to the hspec manual. You should carefully check shouldBe, shouldReturn and shouldThrow (they are called "matcher" in RSpec terminologies).

You can run Spec with the "hspec" function:

% ghci test/Base64Spec.hs
> hspec spec
  - encodes multiples of 3
  - encodes multiples of 3 + 1
  - encodes multiples of 3 + 2

  - decodes no padding
  - decodes one padding
  - decodes two paddings

Finished in 0.0956 seconds
6 examples, 0 failures


QuickCheck properties can be specified in hspec, too. Just use prop instead of it:

spec :: Spec
spec = do
    describe "encode" $ do
        prop "reverses decoded string" $ \(Base64 xs) ->
            encode (decode xs) == xs

    describe "decode" $ do
        prop "reverses encoded string" $ \xs ->
            decode (encode xs) == xs

QuickCheck properties can also be specified in Haddock. Use the prop> keyword.

-- |
-- Base64 encoding.
-- >>> encode "foo bar"
-- "Zm9vIGJhcg=="
-- prop> decode (encode xs) == xs


To automate running test suites, use Cabal. You need to specify information about test suites in the Cabal file:

Test-Suite doctest
  Type:                 exitcode-stdio-1.0
  Default-Language:     Haskell2010
  HS-Source-Dirs:       test
  Ghc-Options:          -threaded -Wall
  Main-Is:              doctests.hs
  Build-Depends:        base
                      , doctest >= 0.9.3

Test-Suite spec
  Type:                 exitcode-stdio-1.0
  Default-Language:     Haskell2010
  Hs-Source-Dirs:       test
  Ghc-Options:          -Wall
  Main-Is:              Spec.hs
  Other-Modules:        Base64Spec
  Build-Depends:        base
                      , hspec >= 1.3
                      , QuickCheck
                      , unit-test-example

Please note that your library itself ("unit-test-example" in this case) can be specified as dependency in the test suite for hspec. If you store test files in the same directory of source code, you have to repeat the dependencies of your library here.

For doctest, "test/doctests.hs" should be created in addition to the Cabal file:

module Main where

import Test.DocTest

main :: IO ()
main = doctest ["Codec/Base64.hs"]

The arguments of the "doctest" function is the same as that of the "doctest" command (which is also identical to that of GHCi). They should be stored in a list literal of Haskell.

For hspec, the following one line should be stored in "test/Spec.hs":

{-# OPTIONS_GHC -F -pgmF hspec-discover #-}

The "test/Spec.hs" file above will trigger a process to pick up all the tests in all the hspec files in the same directory.

The procedure to automatically run test suites is as follows:

% cabal configure --enable-tests
% cabal build
% cabal test
Running 2 test suites...
Test suite spec: RUNNING...
Test suite spec: PASS
Test suite logged to: dist/test/unit-test-example-0.0.0-spec.log
Test suite doctest: RUNNING...
Test suite doctest: PASS
Test suite logged to: dist/test/unit-test-example-0.0.0-doctest.log
2 of 2 test suites (2 of 2 test cases) passed.

If tests fail, please read the indicated log files and search the failure cases and their values.

100 random values are generated for one property by default in hspec. If you want to change it, do so as follows:

% cabal test --test-option=--maximum-generated-tests=1000

Travis CI

Travis CI is a service to automatically run test suites when you push your commits to github. To use Travis CI, you need to set it up as follows:

  • Login into Travis CI with your github account
  • Select "Accounts" by clicking your name on the right top to go to your account page
  • Click the "Sync now" button to retrieve package (repository) information from github
  • Enabling the service for necessary packages

Then you need to create ".travis.yml" which stores the following one line:

language: haskell

Then push this file to github. After that, test suites will be run on Travis CI when you push your commits and a result is delivered to you by e-mail.

Miscellaneous things

The doctest arguments

The doctest arguments (function and command) is exactly the same as that of GHCi. To run doctest well, various arguments are sometime necessary. One common case is "-XOverloadedStrings".

If you write C code by yourself and use it from Haskell with FFI, you may wonder what arguments should be specified. In this case, please refer to unix-time.

Internal modules

You may want to use hidden constructors in test cases. Common practice for this is to prepare an internal module (whose typical name is "Internal.hs"). If a test file includes this internal module, hidden constructors can be used.

To use internal modules from test suites specified in a Cabal file, add the parent directory to HS-Source-Dirs:

  HS-Source-Dirs:       test,..

Now test code can import the internal modules. Unfortunately, test suites cannot depend on your library itself in this case. So, you need to repeat that dependencies of your library in Build-Depends of test suites.


I thank Alan Zimmerman for his proofreading.

Please feel free to point out grammar or contents errors. Pull requests are welcome.

Happy unit testing in Haskell!