How to test in Haskell using comments?

March 12, 2020

If you want to easily test your functions in Haskell, you can do so by writing inputs and their expected responses right above your functions as comments. All you need is doctest.

This is how:

1. Setup Project

Let's setup a basic project first.

  1. Install Haskell Stack if you don't have it already.

  2. Setup project and install required dependencies.

stack new my-project
cd my-project
stack setup
  1. Build and execute project. If you see someFunc in the output you are good to go.
stack build
stack exec my-project-exe

Output after running `stack build` and `stack exec my-project-exe`

2. Write a function

Let's write a simple function so we can test it.

  1. Create a file Math.hs in src with a square function:
module Math where

square :: Int -> Int
square x = x * x
  1. Manually test the function in GHCi. Load up the interactive environment using stack ghci.

  2. Try out the function to see everything is working as expected: Math.square 3 should output 9.

Testing our square function

3. Write tests

Now let's get to the actual testing. All we need to do to is write comments.

  1. Just above your function definition, write your set of inputs and their expected outputs. This is how my Math.hs looks like:
module Math where

-- | Square numbers
-- Examples:
-- >>> square 3
-- 9
-- >>> square (-1)
-- 1
square :: Int -> Int
square x = x * x
  1. If you try running stack test you will notice these tests don't run. That's because we haven't set up doctest yet.

4. Configure Project for doctest

  1. First, let's add doctest to our project. Go to the end of package.yaml and add it as dependency of my-project-test. This is how my package.yaml looks like:
name:                my-project
github:              "githubuser/my-project"
license:             BSD3
author:              "Author name here"
maintainer:          "[email protected]"
copyright:           "2020 Author name here"


# Metadata used when publishing your package
# synopsis:            Short description of your package
# category:            Web

# To avoid duplicated efforts in documentation and dealing with the
# complications of embedding Haddock markup inside cabal files, it is
# common to point users to the file.
description:         Please see the README on GitHub at <>

- base >= 4.7 && < 5

  source-dirs: src

    main:                Main.hs
    source-dirs:         app
    - -threaded
    - -rtsopts
    - -with-rtsopts=-N
    - my-project

    main:                Spec.hs
    source-dirs:         test
    - -threaded
    - -rtsopts
    - -with-rtsopts=-N
    - my-project
    - doctest
  1. To install the dependency, run stack test. If the installation goes well, you should see the following message Test suite not yet implemented.

Test suite not implemented

If you go to the actual test file in test/Spec.hs you can see that there indeed are no tests. We need to somehow tell stack to look for tests in our src files instead.

  1. To do that, clear the contents of test/Spec.hs file and replace it with this:
import Test.DocTest

main = doctest ["-isrc", "src/Math.hs"]
  1. Now, if you run stack test it will run the 2 tests we wrote earlier. That's it, we are done!

Tests pass successfully

When you add more modules, simply add them to this list ["-isrc", "src/Math.hs"] in test/Spec.hs to make doctest look for tests in them.


This was my setup while writing this:

  • MacOS 10.15.3
  • Haskell Stack 2.1.3


Last Updated: 15 July 2021

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