Automated Unit Testing in Haskell

Hspec is a BDD library for writing Rspec-style tests in Haskell. In this post, I’m going to describe setting up a Haskell project using this test framework. What we’ll end up with is a series of tests which can be run individually (at the module level), or all together (as part of packaging). Then I’ll briefly mention Guard (a Ruby tool) and how we can use that to automatically run relevant tests as we change code.

Project Layout

For any of this to work, our implementation and test modules must follow a particular layout:

├── src
│   └── Text
│       ├── Liquid
│       │   ├── Context.hs
│       │   ├── Parse.hs
│       │   └── Render.hs
│       └── Liquid.hs
└── test
    ├── SpecHelper.hs
    ├── Spec.hs
    └── Text
        └── Liquid
            ├── ParseSpec.hs
            └── RenderSpec.hs

Notice that for each implementation module (under ./src) there is a corresponding spec file at the same relative path (under ./test) with a consistent, conventional name (<ModuleName>Spec.hs). For this post, I’m going to outline the first few steps of building the Parse module of the above source tree which happens to be my liquid library, a Haskell implementation of Shopify’s template system.

Hspec Discover

Hspec provides a useful function called hspec-discover. If your project follows the conventional layout above, you can simply create a file like so:


And when that file is executed, all of your specs will be found and run together as a single suite.


I like to create a central helper module which gets imported into all specs. It simply exports our test framework and implementation code:


This file might not seem worth it now, but as you add more modules, it becomes useful quickly.

Baby’s First Spec


With this first spec, I’ve already made some assumptions and design decisions.

The API into our module will be a single parseTemplate function which returns an Either type (commonly used to represent success or failure). The Right value (conventionally used for success) will be a list of template parts. One such part can be constructed with the TString function and is used to represent literal text with no interpolation or logic. This is the simplest template part possible and is therefore a good place to start.

The spec function is what will be found by hspec-discover and rolled up into a project-wide test. I’ve also added a main function which just runs said spec. This allows me to easily run the spec in isolation, which you should do now:

$ runhaskell -isrc -itest test/Text/Liquid/ParseSpec.hs

The first error you should see is an inability to find Test.Hspec. Go ahead and install it:

$ cabal install hspec

You should then get a similar error for Text.Liquid.Parse then some more about functions and types that are not yet defined. Let’s go ahead and implement just enough to get past that:


The test should run now and give you a nice red failure due to the attempted evaluation of undefined.

Since implementing Parse is not the purpose of this post, I won’t be moving forward in that direction. Instead, I’m going to show you how to set this library up as a package which can be cabal installed and/or cabal tested by end-users.

For now, you can pass the test easily like so:


For TDD purists, this is actually the correct thing to do here: write the simplest implementation to pass the test (even if you “know” it’s not going to last), then write another failing test to force you to implement a little more. I don’t typically subscribe to that level of TDD purity, but I can see the appeal.


We’ve already got Spec.hs which, when executed, will run all our specs together:

$ runhaskell -isrc -itest test/Spec.hs

We just need to wire that into the Cabal packaging system:


name:          liquid
version:       0.0.0
license:       MIT
copyright:     (c) 2013 Pat Brisbin
author:        Pat Brisbin <pbrisbin@gmail.com>
maintainer:    Pat Brisbin <pbrisbin@gmail.com>
build-type:    Simple
cabal-version: >= 1.8

  hs-source-dirs: src

  exposed-modules: Text.Liquid.Parse

  build-depends: base == 4.*

test-suite spec
  type: exitcode-stdio-1.0

  hs-source-dirs: test

  main-is: Spec.hs

  build-depends: base  == 4.*
               , hspec >= 1.3
               , liquid

With this in place, testing our package is simple:

$ cabal configure --enable-tests
$ cabal build
$ cabal test
Building liquid-0.0.0...
Preprocessing library liquid-0.0.0...
In-place registering liquid-0.0.0...
Preprocessing test suite 'spec' for liquid-0.0.0...
Linking dist/build/spec/spec ...
Running 1 test suites...
Test suite spec: RUNNING...
Test suite spec: PASS
Test suite logged to: dist/test/liquid-0.0.0-spec.log
1 of 1 test suites (1 of 1 test cases) passed.


Another thing I like to setup is the automatic running of relevant specs as I change code. To do this, we can use a tool from Ruby-land called Guard. Guard is a great example of a simple tool doing one thing well. All it does is watch files and execute actions based on rules defined in a Guardfile. Through plugins and extensions, there are a number of pre-built solutions for all sorts of common needs: restarting servers, regenerating ctags, or running tests.

We’re going to use guard-shell which is a simple extension allowing for running shell commands and spawning notifications.

$ gem install guard-shell

Next, create a Guardfile:


Much of this Guardfile comes from this blog post by Michael Xavier. His version also includes cabal sandbox support, so be sure to check it out if that interests you.

If you like to bundle all your Ruby gems (and you probably should) that can be done easily, just see my main liquid repo as that’s how I do things there.

In one terminal, start guard:

$ guard

Finally, simulate an edit in your module and watch the test automatically run:

$ touch src/Text/Liquid/Parse.hs

And there you go, fully automated unit testing in Haskell.

01 Dec 2013, tagged with haskell