tasty-autocollect

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

Quickstart

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

   tests:
     my-library-tests:
       ghc-options: -F -pgmF=tasty-autocollect
       build-tools:
         - tasty-autocollect:tasty-autocollect
       ...
       dependencies:
         - tasty-autocollect
         - ...
   

   test-suite my-library-tests
     ghc-options: -F -pgmF=tasty-autocollect
     build-tool-depends:
       tasty-autocollect:tasty-autocollect
     ...
     build-depends:
       tasty-autocollect
       ...
   

2. Write your main file to contain just:

   {- AUTOCOLLECT.MAIN -}
   

3. Write your tests:

   {- AUTOCOLLECT.TEST -}
   
   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 ()
       ]
   

Features

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 =
  testProperty
    "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:

tasty_test_4 :: TestTree
tasty_test_4 =
  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

Configuration

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

{- AUTOCOLLECT.MAIN
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

    Main.hs
      test 1: OK
      test 2: OK
      test 3: OK
    

  • modules: Tests are grouped by their module

    Main.hs
      Test.Module1
        test1: OK
        test2: OK
      Test.Module2
        test3: OK
    

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

    Main.hs
      Test
        Module1
          test1: OK
          test2: OK
        Module2
          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 tasty-discover

Advantages:

• Supports test functions with multiple arguments (e.g. tasty-golden)
• Avoids hardcoding testers like unit_ or prop_
• Avoids rewriting test label twice in function name
• Avoids test name restrictions
  • Because tasty-discover couples the function name with the test label, you can’t do things like use punctuation in the test label. So tasty-discover doesn’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

Disadvantages:

• Uses both a preprocessor and a plugin (tasty-discover only uses a preprocessor)
  • Haven’t tested performance yet, but I wouldn’t be surprised if there’s a non-negligible performance cost

Appendix

Debugging

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 or Fourmolu, 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 a problem for you, please open an issue.

Upstream ticket: https://github.com/tweag/ormolu/issues/906

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.