24 Days of Hackage: tasty

Last year, when discussing QuickCheck, I said:

While we try and constrain our types as much as possible, there is always a trade off between exact types and pragmatism, not to mention that there are some invariants that are very difficult to encode in the Haskell type system. As such, without rigorous testing, there is still a risk of exceptions or unexpected behaviors at runtime.

The necessessity of testing is just as essential today. Last year we looked at a specific testing techinque, namely the use of QuickCheck, but we didn’t look at testing from the bigger perspective: how do you formulate entire test suites? Hackage has a lot of options available to us here - including test-framework, hspec, the ellusive detailed-1.0 test-suite setting of Cabal, and the newest entry to field: tasty.

For a long time, I was perfectly content with test-framework. However, as time has gone on, test-framework has failed to stay up to date. The Github repository doesn’t show much activity and is accumulating pull requests, and it’s said that the codebase itself can make it difficult to make modifications. This is not meant to be critiscism of Max - these things happen. For these reasons, and a few others, Roman Cheplyaka created his own testing framework - tasty, which is now my testing framework of choice.

There are two main parts to tasty: test trees and ingredients.

Test trees specify the hierarchy of tests. You can specify groups of tests using testGroup, or you can create individual tests using TestTree builders for specific testing tools. tasty-hunit gives us the testCase builder, while tasty-smallcheck gives us testProperty.

To illustrate the formulation of a test tree, allow me to reproduce a subset of the example in tastys documentation:

main :: IO ()
main = defaultMain $
  testGroup "Tests"
    [ testGroup "(checked by SmallCheck)"
        [ testProperty "sort == sort . reverse" $
            \list -> sort (list :: [Int]) == sort (reverse list)

        , testProperty "Fermat's last theorem" $
            \x y z n -> (n :: Integer) >= 3 ==>
              x^n + y^n /= (z^n :: Integer)
        ]

    , testGroup "Unit tests"
        [ testCase "List comparison (different length)" $
            [1, 2, 3] `compare` [1,2] @?= GT

        , testCase "List comparison (same length)" $
            [1, 2, 3] `compare` [1,2,2] @?= LT
        ]
    ]

As you can see, it’s both easy and consistent to form a test tree using different testing tools. This helps encourage us to use the right testing tool for the job. Above we see the use of both SmallCheck and HUnit.

Once you have a TestTree, you presumably want to do something with it - such as actually running the tests! That’s what ingredients are all about. Ingredients are small units of functionality that have the ability to parse command line options, and can conditionally choose to run a TestTree. tasty itself ships with an ingredient to run the tests and output pretty ANSI-coloured terminal output, and also an ingredient that simply lists the names of all tests.

Ingredients provide an extension point for tasty, which is one area where this framework trumps the competition. As a case in point, I wanted the ability to run tests on our Jenkins continuous integration server - which expects test runs to produce an XML file in a specific schema. test-framework ships with this baked right into the library itself, but for tasty I was able to write my own ingredient that observes a test run and renders XML as it goes. And to top it off, it took me little more than 100 lines of code and a few hours of hacking (the result is tasty-ant-xml).

tasty’s interpretation of test trees is also a massive productivity win when we’re developing, as we can run specific parts of the tree by using test patterns. Test patterns let us run only test cases who’s name match a specific pattern, with the ability to match the test hierarchy too. For example, if you’re tweaking the serialisation format of a library you might run with --pattern 'Serialization/**' to avoid the longer IO tests. Or maybe you have added a new PostgreSQL database backend, and are only interested in tests that refer to that, so you would use --patern **/*PostgreSQL*/**.

Roman has already done a lot of work getting tasty usable with the other big testing tools. Specifically, right now the tasty suite (menu?) consists of:

Code for today’s example can be found in my blog’s Github repository.


You can contact me via email at ollie@ocharles.org.uk or tweet to me @acid2. I share almost all of my work at GitHub. This post is licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License.

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