Announcing tasty-rerun

If you’re a firm believer of writing tests for your work, it shouldn’t take much to convince you that having your tests run fast is of the utmost importance. Every second the tests are running is a second that you drift out of the zone and have to context switch when you return back to coding. Sadly, I don’t have a magic solution to make all your tests run faster, but I do have something to that can help ease the pain, and that is tasty-rerun.

Inspired by prove – Perl’s standard testing tool – tasty-rerun adds the ability to run your tests by filtering the test tree based on what happened in a previous test run. For example, tasty-rerun allows you to only run tests that failed when the test suite was last ran, or you can chose to run only tests that have been added since the last run. I think this is a massive win for productivity, as the majority of the time we have the type system to keep our refactorings in check, and now we can use our tests to focus on the design of specific functionality.

tasty-rerun works by providing an ingredient transformer. If you’re not familiar with tasty, ingredients are used to decide how tests are ran, or how test progress should be observed. For tasty-rerun, we only need to do a little bit of filtering before the test run and save some state after the test run, so we hand off the bulk of the work to another ingredient after transforming the test tree. Let’s see some code to see how this works.

An Example tasty-rerun Session

We begin with our test file:

import Test.Tasty
import Test.Tasty.HUnit
import Test.Tasty.Runners

main :: IO ()
main = defaultMainWithIngredients [ consoleTestReporter ] tests

tests :: TestTree
tests = testGroup "Sums"
  [ testCase "Addition" $ 1 + 1 @?= 3
  , testCase "Multiplication" $ 2 * 2 @?= 4

To add tasty-rerun support to this we simply import Test.Tasty.Ingredients.Rerun and then transform our ingredients ([ consoleTestReporter ]) with rerunningTests:

import Test.Tasty.Ingredients.Rerun


main = defaultMainWithIngredients [ rerunningTests [ consoleTestReporter ] ] tests

Simple! Now, when we first run our test suite we supply the --rerun-update flag, which indicates that tasty-rerun should save the results of this test file for use in future sessions.

> ./tests --rerun-update
  Addition:       FAIL
    expected: 3
     but got: 2
  Multiplication: OK

1 out of 2 tests failed

We’d like to focus on just this failing addition test, so we now run our tests with the --rerun-filter failures flag. This indicates that tasty-rerun should first filter the test tree to only those tests that failed in a previous test run (and still exist in the current test tree). With this, another run of the tests shows:

> ./tests --rerun-update --rerun-filter failures
  Addition: FAIL
    expected: 3
     but got: 2

1 out of 1 tests failed

tasty-rerun noticed that multiplication passed without problems on the previous test run, and has filtered it out of this test run. When we fix our addition test…

testCase "Addition" $ 1 + 1 @?= 3

Then running with rerun-filter failures shows:

> ./tests --rerun-update --rerun-filter failures
  Addition: OK

All 1 tests passed

The addition test previously failed, so we try it again. Now it passes, the test state is updated and a further test run (with --rerun-filter failures) shows that there are no tests to run:

> ./tests --rerun-update --rerun-filter failures
All 0 tests passed

At this point, we’re ready to add another test! We’ll add in one final test as a QuickCheck property:

testProperty "Negation involution" $ \x -> negate (negate x) == (x :: Int)

If we run our tests as we did before, nothing will happen:

> ./tests --rerun-update --rerun-filter failures
All 0 tests passed

This is because we told tasty-rerun that we are only interested in tests that have previously failed. However, tasty-rerun allows the --rerun-filter flag to take multiple filters, so we can change this to --rerun-filter failures,new to run new tests:

> ./tests --rerun-update --rerun-filter failures,new
  Addition:            OK
  Multiplication:      OK
  Negation involution: OK
    +++ OK, passed 100 tests.

All 3 tests passed

In this case all the tests are ran because our last test ran no tests at all - thus all the tests are new.

In practice, you probably want to run once with --rerun-update to build your initial test state, and then subsequent runs with only --rerun-filter failures,new. This will allow you to repeatedly run your tests focusing on only tests that have been newly added or were previously broken. Updating the state on every test run can be slightly confusing (as the above example may demonstrate), though I have some ideas on how I might be able to make this a bit more useful.

I hope you find tasty-rerun useful – it’s on Hackage already, so you can start using it today. As always, please report issues on Github so we can make this project even more awesome. Many thanks to Roman Cheplyaka for explaining to me how this project could be achieved in tasty, providing thorough code/style reviews, and of course for tasty itself.

You can contact me via email at 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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