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GHC 9.10.3 · lts/ghc-9.10.x · 248f8f0 · 2026-10-05

Moduletasty-hunit-0.10.2Haskell2010

Test.Tasty.HUnit

Unit testing support for tasty, inspired by the HUnit package.

Here's an example (a single tasty test case consisting of three assertions):

import Test.Tasty
import Test.Tasty.HUnit

main = defaultMain $
  testCase "Example test case" $ do
    -- assertion no. 1 (passes)
    2 + 2 @?= 4
    -- assertion no. 2 (fails)
    assertBool "the list is not empty" $ null [1]
    -- assertion no. 3 (would have failed, but won't be executed because
    -- the previous assertion has already failed)
    "foo" @?= "bar"
  • 4 types
  • 2 classes
  • 10 values

Constructing test cases

3 declarations
valuetestCaseSteps :: TestName -> ((String -> IO ()) -> Assertion) -> TestTree
#

Create a multi-step unit test.

Example:

main = defaultMain $ testCaseSteps "Multi-step test" $ \step -> do
  step "Preparing..."
  -- do something

  step "Running part 1"
  -- do something

  step "Running part 2"
  -- do something
  assertFailure "BAM!"

  step "Running part 3"
  -- do something

The step calls are mere annotations. They let you see which steps were performed successfully, and which step failed.

You can think of step as putStrLn, except putStrLn would mess up the output with the console reporter and get lost with the others.

For the example above, the output will be

Multi-step test: FAIL
  Preparing...
  Running part 1
  Running part 2
    BAM!

1 out of 1 tests failed (0.00s)

Note that:

  • Tasty still treats this as a single test, even though it consists of multiple steps.

  • The execution stops after the first failure. When we are looking at a failed test, we know that all displayed steps but the last one were successful, and the last one failed. The steps after the failed one are not displayed, since they didn't run.

Constructing assertions

7 declarations
valueassertFailure
  1. :: HasCallStack
  2. => String

    A message that is displayed with the assertion failure

  3. -> IO a
#

Unconditionally signals that a failure has occured. All other assertions can be expressed with the form:

   if conditionIsMet
       then return ()
       else assertFailure msg
valueassertEqual
  1. :: (Eq a, Show a, HasCallStack)
  2. => String

    The message prefix

  3. -> a

    The expected value

  4. -> a

    The actual value

  5. -> Assertion
#

Asserts that the specified actual value is equal to the expected value. The output message will contain the prefix, the expected value, and the actual value.

If the prefix is the empty string (i.e., ""), then the prefix is omitted and only the expected and actual values are output.

value(@=?)
  1. :: (Eq a, Show a, HasCallStack)
  2. => a

    The expected value

  3. -> a

    The actual value

  4. -> Assertion
#

Asserts that the specified actual value is equal to the expected value (with the expected value on the left-hand side).

value(@?=)
  1. :: (Eq a, Show a, HasCallStack)
  2. => a

    The actual value

  3. -> a

    The expected value

  4. -> Assertion
#

Asserts that the specified actual value is equal to the expected value (with the actual value on the left-hand side).

value(@?)
  1. :: (AssertionPredicable t, HasCallStack)
  2. => t

    A value of which the asserted condition is predicated

  3. -> String

    A message that is displayed if the assertion fails

  4. -> Assertion
#

An infix and flipped version of assertBool. E.g. instead of

assertBool "Non-empty list" (null [1])

you can write

null [1] @? "Non-empty list"

@? is also overloaded to accept IO Bool predicates, so instead of

do
  e <- doesFileExist "test"
  e @? "File does not exist"

you can write

doesFileExist "test" @? "File does not exist"

Data types

2 declarations
typetype Assertion = IO ()
#

An assertion is simply an IO action. Assertion failure is indicated by throwing an exception, typically HUnitFailure.

Instead of throwing the exception directly, you should use functions like assertFailure and assertBool.

Test cases are composed of a sequence of one or more assertions.

Accurate location for domain-specific assertion functions

1 declaration

It is common to define domain-specific assertion functions based on the standard ones, e.g.

assertNonEmpty = assertBool "List is empty" . not . null

The problem is that if a test fails, tasty-hunit will point to the definition site of assertNonEmpty as the source of failure, not its use site.

To correct this, add a HasCallStack constraint (re-exported from this module) to your function:

assertNonEmpty :: HasCallStack => [a] -> Assertion
assertNonEmpty = assertBool "List is empty" . not . null
typetype HasCallStack = IP "callStack" CallStack
#

Request a CallStack.

NOTE: The implicit parameter ?callStack :: CallStack is an implementation detail and should not be considered part of the CallStack API, we may decide to change the implementation in the future.

Deprecated functions and types

3 declarations

These definitions come from HUnit, but I don't see why one would need them. If you have a valid use case for them, please contact me or file an issue for tasty. Otherwise, they will eventually be removed.

valueassertString
  1. :: HasCallStack
  2. => String

    The message that is displayed with the assertion failure

  3. -> Assertion
#

Deprecated. Why not use assertBool instead?

Signals an assertion failure if a non-empty message (i.e., a message other than "") is passed.

classclass Assertable t where
#

Deprecated. This class or type seems dubious. If you have a good use case for it, please create an issue for tasty. Otherwise, it may be removed in a future version.

Allows the extension of the assertion mechanism.

Since an Assertion can be a sequence of Assertions and IO actions, there is a fair amount of flexibility of what can be achieved. As a rule, the resulting Assertion should be the body of a TestCase or part of a TestCase; it should not be used to assert multiple, independent conditions.

If more complex arrangements of assertions are needed, Test and Testable should be used.

Methods

Instances4Assertable
typetype AssertionPredicate = IO Bool
#

Deprecated. This class or type seems dubious. If you have a good use case for it, please create an issue for tasty. Otherwise, it may be removed in a future version.

The result of an assertion that hasn't been evaluated yet.

Most test cases follow the following steps:

  1. Do some processing or an action.

  2. Assert certain conditions.

However, this flow is not always suitable. AssertionPredicate allows for additional steps to be inserted without the initial action to be affected by side effects. Additionally, clean-up can be done before the test case has a chance to end. A potential work flow is:

  1. Write data to a file.

  2. Read data from a file, evaluate conditions.

  3. Clean up the file.

  4. Assert that the side effects of the read operation meet certain conditions.

  5. Assert that the conditions evaluated in step 2 are met.