BSD-3-Clause licensed and maintained by Brandon Chinn
This version can be pinned in stack with:tasty-autocollect-0.4.3@sha256:e725d97101fe374abae967f8bf30000c336e4bed48cd49c1935c2bf3b6fc5534,4500


A preprocessor/compiler plugin that will automatically collect Tasty tests and generate a main file to run all the tests.

Design goals:

  • Don’t use any weird syntax so that syntax highlighters, linters, and formatters still work
  • Avoid universally exporting the whole test module, so that GHC can warn about unused test helpers
  • Support arbitrary test functions (e.g. user-defined test helpers or third-party tasty libraries)
  • Minimal dependencies
    • Only uses boot libraries, with two exceptions: tasty + tasty-expected-failure


  1. Add the following to your package.yaml or .cabal file:

        ghc-options: -F -pgmF=tasty-autocollect
          - tasty-autocollect:tasty-autocollect
          - tasty-autocollect
          - ...
    test-suite my-library-tests
      ghc-options: -F -pgmF=tasty-autocollect
  2. Write your main file to contain just:

  3. Write your tests:

    module MyTest (
      {- AUTOCOLLECT.TEST.export -}
    ) where
    import Test.Tasty.Golden
    import Test.Tasty.HUnit
    import Test.Tasty.QuickCheck
    test =
      testCase "Addition" $ do
        1 + 1 @?= (2 :: Int)
        2 + 2 @?= (4 :: Int)
    -- See the "Integration with QuickCheck/SmallCheck/etc." section
    -- for a more seamless integration
    test =
      testProperty "reverse . reverse === id" $ \xs ->
        (reverse . reverse) xs === id (xs :: [Int])
    test =
      goldenVsString "Example golden test" "test/golden/example.golden" $
        pure "example"
    test =
      testGroup "manually defining a test group"
        [ testCase "some test" $ pure ()
        , testCase "some other test" $ pure ()


In addition to automatically collecting tests, this library also provides some additional functionality out-of-the-box, to make writing + managing tests a seamless experience.

Integration with QuickCheck/SmallCheck/etc.

Property test frameworks like QuickCheck or SmallCheck work better when defining the types of arguments instead of using lambdas. So there’s a special syntax for defining properties:

test_prop :: [Int] -> Property
test_prop "reverse . reverse === id" xs = (reverse . reverse) xs === id xs

This will be rewritten to the equivalent of:

test =
    "reverse . reverse === id"
    ( (\xs -> (reverse . reverse) xs === id xs)
        :: [Int] -> Property

Marking tests as “TODO”

If you’re of the Test Driven Development (TDD) mentality, you might want to specify what tests you want to write before actually writing any code. In this workflow, you might not even know what kind of test you want to write (e.g. HUnit, QuickCheck, etc.).

With tasty-autocollect, you can use test_todo to write down tests you’d like to write. By default, they’ll pass with a “TODO” message, but you can also pass --fail-todos at runtime to make them fail instead.

test_todo = "a test to implement later"

Defining batches of tests

With tasty-autocollect, you can write a set of tests in one definition without needing to nest them within a test group. For example,

test_batch =
  [ testCase ("test #" ++ show x) $ pure ()
  | x <- [1, 5, 10 :: Int]

is equivalent to writing:

test = testCase "test #1" $ pure ()

test = testCase "test #5" $ pure ()

test = testCase "test #10" $ pure ()

Integration with tasty-expected-failures

If you need to mark a test as an expected failure or just unconditionally skip a test, you can add an appropriate suffix to your test. For example:

test_expectFail = testCase "this test should fail" $ ...

test_expectFailBecause "Issue #123" = testCase "this test should fail" $ ...

test_ignoreTest = testCase "this test is skipped" $ ...

test_ignoreTestBecause "Issue #123" = testCase "this test is skipped" $ ...

The last example will be converted into the equivalent of:

test =
  ignoreTestBecause "Issue #123" $
    testCase "this test is skipped" $ ...

It also works in combination with other test types, e.g. with test_batch to skip the entire batch of tests:

test_batch_expectFail =
  [ testCase ("this test should fail: " ++ show x) ...
  | x <- ...

test_prop_expectFailBecause :: Int -> Property
test_prop_expectFailBecause "Issue #123" "some property" x = x === x


tasty-autocollect can be configured by adding k = v lines to the same block comment as AUTOCOLLECT.MAIN; e.g.

suite_name = foo

# comments can start with a hash symbol
group_type = flat
  • import: A comma separated list of files (relative to the Main file) containing configuration to import

    • Recommended file extension: .conf
    • Configuration in files later in the list override configuration in files earlier in the list
    • Configuration in the Main file override imported configuration
  • suite_name: The name to use in the testGroup at the root of the test suite TestTree (defaults to the path of the main file)

  • group_type: How the tests should be grouped (defaults to modules)

    • flat: All the tests are in the same namespace

        test 1: OK
        test 2: OK
        test 3: OK
    • modules: Tests are grouped by their module

          test1: OK
          test2: OK
          test3: OK
    • tree: Tests are grouped by their module, which is broken out into a tree

            test1: OK
            test2: OK
            test3: OK
  • strip_suffix: The suffix to strip from a test module, e.g. strip_suffix = Test will relabel a Foo.BarTest module to Foo.Bar

  • ingredients: A comma-separated list of extra tasty ingredients to include, e.g.

    ingredients = SomeLibrary.ingredient1, SomeLibrary.ingredient2
  • ingredients_override: By default, ingredients will add the ingredients in front of the default tasty ingredients. When true, does not automatically include the default tasty ingredients, for complete control over the ingredient order.

  • custom_main: If you’d like fine-grained control over how the Main module is generated (e.g. if you’re using NoImplicitPrelude or custom preludes), set this to true. When set, it will do the following replacements:

    • {- AUTOCOLLECT.MAIN.imports -}: replaced with the import lines needed for the tests.
    • {- AUTOCOLLECT.MAIN.tests -}: replaced with the [TestTree] list, all on one line. If you’re using a formatter or linter, it might be helpful to do tests = id {- AUTOCOLLECT.MAIN.tests -} so that the code still parses.

    Due to current limitations, the above comments need to be matched exactly (e.g. not with -- comments or extra whitespace).

Comparison with other libraries


  • Avoids hardcoding blessed test frameworks
    • Both tasty-discover and tasty-th prefix tests with hardcoded prefixes like unit_ or prop_. While they both provide a mechanism to specify arbitrary TestTrees with test_, it makes for a less seamless integration with unblessed test frameworks, such as tasty-golden:
      test_do_something :: TestTree
      test_do_something =
        goldenVsString "do something" "goldens/do_something.golden" $ ...
    • With tasty-autocollect, all tests are specified the same way (modulo some syntax sugar niceties like test_prop)
  • Avoids rewriting test label twice in function name
    • Both tasty-discover and tasty-th force you to repeat long test names twice:
      unit_this_is_a_test :: Assertion
      unit_this_is_a_test = do
  • Avoids test name restrictions
    • Because tasty-discover and tasty-th couple the function name with the test label, you can’t do things like use punctuation in the test label. So these frameworks don’t allow writing the equivalent of testProperty "reverse . reverse === id".
  • More features out-of-the-box (see “Features” section)
  • More configurable
    • More configuration options
    • Configuration is more extensible, since configuration is parsed from a comment in the main module instead of as preprocessor arguments (for tasty-discover)
    • tasty-th isn’t configurable at all
  • Automatically generates the Main.hs file that discovers + imports all test modules in the directory
    • tasty-discover does this, but tasty-th does not; tasty-th provides defaultMainGenerator, but it only discovers tests in the same module, so if you have tests in multiple files, you’d still have to manually import all of them into a Main.hs file


  • Uses both a preprocessor and a GHC plugin
    • tasty-discover only uses a preprocessor, while tasty-th uses Template Haskell
    • Haven’t tested performance yet, but I wouldn’t be surprised if compilation takes longer



To inspect the generated Main module, build with -keep-tmp-files, and look in $TMPDIR for a ghc_1.hspp file. (Upstream tickets: GHC, Stack)

To inspect the converted test modules, build with -ddump-rn -ddump-to-file and look for the .dump-rn files in .stack-work/ or dist-newstyle/.

Note for Ormolu/Fourmolu

If you’re using Ormolu < 0.7 or Fourmolu < 0.13, use -- $AUTOCOLLECT.TEST.export$ instead; otherwise, the comment will be moved out of the export list.

This works around the issue by reusing Haddock’s named section syntax, but it shouldn’t be an issue because you shouldn’t be building Haddocks for test modules. If this becomes an issue, upgrade Ormolu or Fourmolu.

How it works

The package.yaml/.cabal snippet registers tasty-autocollect as a preprocessor, which does one of three things at the very beginning of compilation:

  1. If the file contains {- AUTOCOLLECT.MAIN -}, find all test modules and generate a main module.
  2. If the file contains {- AUTOCOLLECT.TEST -}, register the tasty-autocollect GHC plugin to rewrite tests (see below).
  3. Otherwise, do nothing

In a test file, the plugin will search for any functions named test. It will then rename the function to tasty_test_N, where N is an autoincrementing, unique number. Then it will collect all the tests into a tasty_tests :: [TestTree] binding, which is exported at the location of the {- AUTOCOLLECT.TEST.export -} comment.



  • Add support for GHC 9.10
  • Drop support for GHC 9.4


  • Add support for GHC 9.8
  • Drop support for GHC 9.0 and 9.2


  • Add support for GHC 9.6


  • Drop support for GHC 8.10
  • Added support for where clauses in test_prop, where the where clause may reference the generated arguments in test_prop
  • Don’t autocollect invalid module names
  • Enable importing configuration from other files
  • Add support for custom main files


  • Added support for GHC 9.4


  • Fix bug where errors in Main module would show the path of the temporary file instead of the original file
  • Ignore binary files in test directory


  • Fix bug when omitting signature after specifying a signature for a prior test
  • Add support for _expectFail, _expectFailBecause, _ignoreTest, _ignoreTestBecause suffixes


  • Fix build for GHC 8.10.2
  • Greatly simplify framework by replacing test_testCase with just test = testCase ...


Initial release