tasty: Modern and extensible testing framework

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Tasty is a modern testing framework for Haskell. It lets you combine your unit tests, golden tests, QuickCheck/SmallCheck properties, and any other types of tests into a single test suite.

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Change log CHANGELOG.md
Dependencies ansi-terminal (>=0.9), base (>=4.9 && <5), containers, optparse-applicative (>=0.14), semigroups, stm (>=2.3), tagged (>=0.5), time (>=1.4), transformers (>=0.5), unbounded-delays (>=0.1), unix [details]
License MIT
Author Roman Cheplyaka <roma@ro-che.info>
Maintainer Roman Cheplyaka <roma@ro-che.info>
Category Testing
Home page https://github.com/UnkindPartition/tasty
Bug tracker https://github.com/UnkindPartition/tasty/issues
Source repo head: git clone git://github.com/UnkindPartition/tasty.git(core)
Uploaded by Bodigrim at 2022-12-18T21:32:26Z
Distributions Arch:1.4.3, Debian:1.2.3, Fedora:1.4.3, LTSHaskell:1.4.3, NixOS:1.4.3, Stackage:1.5, openSUSE:1.5
Reverse Dependencies 109 direct, 170 indirect [details]
Downloads 165851 total (666 in the last 30 days)
Rating 2.5 (votes: 10) [estimated by Bayesian average]
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Readme for tasty-1.4.3

[back to package description]


Tasty is a modern testing framework for Haskell.

It lets you combine your unit tests, golden tests, QuickCheck/SmallCheck properties, and any other types of tests into a single test suite.


  • Run tests in parallel but report results in a deterministic order
  • Filter the tests to be run using patterns specified on the command line
  • Hierarchical, colored display of test results
  • Reporting of test statistics
  • Acquire and release resources (sockets, temporary files etc.) that can be shared among several tests
  • Extensibility: add your own test providers and ingredients (runners) above and beyond those provided

To find out what's new, read the change log.


Here's how your test.hs might look like:

import Test.Tasty
import Test.Tasty.SmallCheck as SC
import Test.Tasty.QuickCheck as QC
import Test.Tasty.HUnit

import Data.List
import Data.Ord

main = defaultMain tests

tests :: TestTree
tests = testGroup "Tests" [properties, unitTests]

properties :: TestTree
properties = testGroup "Properties" [scProps, qcProps]

scProps = testGroup "(checked by SmallCheck)"
  [ SC.testProperty "sort == sort . reverse" $
      \list -> sort (list :: [Int]) == sort (reverse list)
  , SC.testProperty "Fermat's little theorem" $
      \x -> ((x :: Integer)^7 - x) `mod` 7 == 0
  -- the following property does not hold
  , SC.testProperty "Fermat's last theorem" $
      \x y z n ->
        (n :: Integer) >= 3 SC.==> x^n + y^n /= (z^n :: Integer)

qcProps = testGroup "(checked by QuickCheck)"
  [ QC.testProperty "sort == sort . reverse" $
      \list -> sort (list :: [Int]) == sort (reverse list)
  , QC.testProperty "Fermat's little theorem" $
      \x -> ((x :: Integer)^7 - x) `mod` 7 == 0
  -- the following property does not hold
  , QC.testProperty "Fermat's last theorem" $
      \x y z n ->
        (n :: Integer) >= 3 QC.==> x^n + y^n /= (z^n :: Integer)

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

  -- the following test does not hold
  , testCase "List comparison (same length)" $
      [1, 2, 3] `compare` [1,2,2] @?= LT

And here is the output of the above program:

(Note that whether QuickCheck finds a counterexample to the third property is determined by chance.)


tasty is the core package. It contains basic definitions and APIs and a console runner.

In order to create a test suite, you also need to install one or more «providers» (see below).


The following providers exist:

It's easy to create custom providers using the API from Test.Tasty.Providers.


Ingredients represent different actions that you can perform on your test suite. One obvious ingredient that you want to include is one that runs tests and reports the progress and results.

Another standard ingredient is one that simply prints the names of all tests.

It is possible to write custom ingredients using the API from Test.Tasty.Runners.

Some ingredients that can enhance your test suite are:

  • tasty-ant-xml adds a possibility to write the test results in a machine-readable XML format, which is understood by various CI systems and IDEs
  • tasty-rerun adds support for minimal test reruns by recording previous test runs and using this information to filter the test tree. For example, you can use this ingredient to only run failed tests, or only run tests that threw an exception.
  • tasty-html adds the possibility to write the test results as a HTML file
  • tasty-stats adds the possibility to collect statistics of the test suite in a CSV file.

Other packages

  • tasty-th automatically discovers tests based on the function names and generate the boilerplate code for you
  • tasty-hunit-adapter converts existing HUnit test suites into tasty test suites
  • tasty-discover automatically discovers your tests.
  • tasty-expected-failure provides test markers for when you expect failures or wish to ignore tests.
  • tasty-bench covers performance regression testing and extends tasty to a benchmark framework similar to criterion and gauge.


Options allow one to customize the run-time behavior of the test suite, such as:

  • mode of operation (run tests, list tests, run tests quietly etc.)
  • which tests are run (see «Patterns» below)
  • parameters of individual providers (like depth of search for SmallCheck)

Setting options

There are two main ways to set options:


When using the standard console runner, the options can be passed on the command line or via environment variables. To see the available options, run your test suite with the --help flag. The output will look something like this (depending on which ingredients and providers the test suite uses):

% ./test --help
Mmm... tasty test suite

Usage: test [-p|--pattern PATTERN] [-t|--timeout DURATION] [-l|--list-tests]
            [-j|--num-threads NUMBER] [-q|--quiet] [--hide-successes]
            [--color never|always|auto] [--ansi-tricks ARG]
            [--smallcheck-depth NUMBER] [--smallcheck-max-count NUMBER]
            [--quickcheck-tests NUMBER] [--quickcheck-replay SEED]
            [--quickcheck-show-replay] [--quickcheck-max-size NUMBER]
            [--quickcheck-max-ratio NUMBER] [--quickcheck-verbose]
            [--quickcheck-shrinks NUMBER]

Available options:
  -h,--help                Show this help text
  -p,--pattern PATTERN     Select only tests which satisfy a pattern or awk
  -t,--timeout DURATION    Timeout for individual tests (suffixes: ms,s,m,h;
                           default: s)
  -l,--list-tests          Do not run the tests; just print their names
  -j,--num-threads NUMBER  Number of threads to use for tests execution
                           (default: # of cores/capabilities)
  -q,--quiet               Do not produce any output; indicate success only by
                           the exit code
  --hide-successes         Do not print tests that passed successfully
  --color never|always|auto
                           When to use colored output (default: auto)
  --ansi-tricks ARG        Enable various ANSI terminal tricks. Can be set to
                           'true' or 'false'. (default: true)
  --smallcheck-depth NUMBER
                           Depth to use for smallcheck tests
  --smallcheck-max-count NUMBER
                           Maximum smallcheck test count
  --quickcheck-tests NUMBER
                           Number of test cases for QuickCheck to generate.
                           Underscores accepted: e.g. 10_000_000
  --quickcheck-replay SEED Random seed to use for replaying a previous test run
                           (use same --quickcheck-max-size)
  --quickcheck-show-replay Show a replay token for replaying tests
  --quickcheck-max-size NUMBER
                           Size of the biggest test cases quickcheck generates
  --quickcheck-max-ratio NUMBER
                           Maximum number of discared tests per successful test
                           before giving up
  --quickcheck-verbose     Show the generated test cases
  --quickcheck-shrinks NUMBER
                           Number of shrinks allowed before QuickCheck will fail
                           a test

Every option can be passed via environment. To obtain the environment variable name from the option name, replace hyphens - with underscores _, capitalize all letters, and prepend TASTY_. For example, the environment equivalent of --smallcheck-depth is TASTY_SMALLCHECK_DEPTH.

Note on boolean options: by convention, boolean ("on/off") options are specified using a switch on the command line, for example --quickcheck-show-replay instead of --quickcheck-show-replay=true. However, when passed via the environment, the option value needs to be True or False (case-insensitive), e.g. TASTY_QUICKCHECK_SHOW_REPLAY=true.

If you're using a non-console runner, please refer to its documentation to find out how to configure options during the run time.


You can also specify options in the test suite itself, using localOption. It can be applied not only to the whole test tree, but also to individual tests or subgroups, so that different tests can be run with different options.

It is possible to combine run-time and compile-time options, too, by using adjustOption. For example, make the overall testing depth configurable during the run time, but increase or decrease it slightly for individual tests.

This method currently doesn't work for ingredient options, such as --quiet or --num-threads. You can set them by setting the corresponding environment variable before calling defaultMain:

import Test.Tasty
import System.Environment

main = do
  setEnv "TASTY_NUM_THREADS" "1"
  defaultMain _


It is possible to restrict the set of executed tests using the -p/--pattern option.

Tasty patterns are very powerful, but if you just want to quickly run tests containing foo somewhere in their name or in the name of an enclosing test group, you can just pass -p foo. If you need more power, or if that didn't work as expected, read on.

A pattern is an awk expression. When the expression is evaluated, the field $1 is set to the outermost test group name, $2 is set to the next test group name, and so on up to $NF, which is set to the test's own name. The field $0 is set to all other fields concatenated using . as a separator.

As an example, consider a test inside two test groups:

  testGroup "One" [ testGroup "Two" [ testCase "Three" _ ] ]

When a pattern is evaluated for the above test case, the available fields and variables are:

$0 = "One.Two.Three"
$1 = "One"
$2 = "Two"
$3 = "Three"
NF = 3

Here are some examples of awk expressions accepted as patterns:

  • $2 == "Two" — select the subgroup Two
  • $2 == "Two" && $3 == "Three" — select the test or subgroup named Three in the subgroup named Two
  • $2 == "Two" || $2 == "Twenty-two" — select two subgroups
  • $0 !~ /skip/ or ! /skip/ — select tests whose full names (including group names) do not contain the word skip
  • $NF !~ /skip/ — select tests whose own names (but not group names) do not contain the word skip
  • $(NF-1) ~ /QuickCheck/ — select tests whose immediate parent group name contains QuickCheck

As an extension to the awk expression language, if a pattern pat contains only letters, digits, and characters from the set ._ - (period, underscore, space, hyphen), it is treated like /pat/ (and therefore matched against $0). This is so that we can use -p foo as a shortcut for -p /foo/.

The only deviation from awk that you will likely notice is that Tasty does not implement regular expression matching. Instead, $1 ~ /foo/ means that the string foo occurs somewhere in $1, case-sensitively. We want to avoid a heavy dependency of regex-tdfa or similar libraries; however, if there is demand, regular expression support could be added under a cabal flag.

The following operators are supported (in the order of decreasing precedence):



Type of Result




Type of expr



Field reference





Logical not

Unary minus





expr + expr

expr - expr







expr expr

String concatenation



expr < expr

expr <= expr

expr != expr

expr == expr

expr > expr

expr >= expr

Less than

Less than or equal to

Not equal to

Equal to

Greater than

Greater than or equal to













expr ~ pat

expr !~ pat

(pat must be a literal, not an expression, e.g. /foo/)

Substring match

No substring match





expr && expr

Logical AND



expr || expr

Logical OR



expr1 ? expr2 : expr3

Conditional expression

Type of selected
expr2 or expr3


The following built-in functions are supported:

substr(s, m[, n])

Return the at most n-character substring of s that begins at position m, numbering from 1. If n is omitted, or if n specifies more characters than are left in the string, the length of the substring will be limited by the length of the string s.


Convert the string s to lower case.


Convert the string s to upper case.

match(s, pat)

Return the position, in characters, numbering from 1, in string s where the pattern pat occurs, or zero if it does not occur at all. pat must be a literal, not an expression, e.g. /foo/.


Return the length, in characters, of its argument taken as a string, or of the whole record, $0, if there is no argument.

Running tests in parallel

In order to run tests in parallel, you have to do the following:

  • Compile (or, more precisely, link) your test program with the -threaded flag;
  • Launch the program with +RTS -N -RTS.


To apply timeout to individual tests, use the --timeout (or -t) command-line option, or set the option in your test suite using the mkTimeout function.

Timeouts can be fractional, and can be optionally followed by a suffix ms (milliseconds), s (seconds), m (minutes), or h (hours). When there's no suffix, seconds are assumed.


./test --timeout=0.5m

sets a 30 seconds timeout for each individual test.

Options controlling console output

The following options control behavior of the standard console interface:

Run the tests but don't output anything. The result is indicated only by the exit code, which is 1 if at least one test has failed, and 0 if all tests have passed. Execution stops when the first failure is detected, so not all tests are necessarily run. This may be useful for various batch systems, such as commit hooks.
Report only the tests that has failed. Especially useful when the number of tests is large.
Don't run the tests; only list their names, in the format accepted by --pattern.
Whether to produce colorful output. Accepted values: never, always, auto. auto means that colors will only be enabled when output goes to a terminal and is the default value.

Custom options

It is possible to add custom options, too.

To do that,

  1. Define a datatype to represent the option, and make it an instance of IsOption
  2. Register the options with the includingOptions ingredient
  3. To query the option value, use askOption.

See the Custom options in Tasty article for some examples.

Project organization and integration with Cabal

There may be several ways to organize your project. What follows is not Tasty's requirements but my recommendations.

Tests for a library

Place your test suite sources in a dedicated subdirectory (called tests here) instead of putting them among the main library sources.

The directory structure will be as follows:


test.hs is where your main function is defined. The tests may be contained in test.hs or spread across multiple modules (Mod1.hs, Mod2.hs, ...) which are then imported by test.hs.

Add the following section to the cabal file (my-project.cabal):

test-suite test
      base >= 4 && < 5
    , tasty >= 0.7 -- insert the current version here
    , my-project   -- depend on the library we're testing
    , ...

Tests for a program

All the above applies, except you can't depend on the library if there's no library. You have two options:

  • Re-organize the project into a library and a program, so that both the program and the test suite depend on this new library. The library can be declared in the same cabal file.
  • Add your program sources directory to the Hs-source-dirs. Note that this will lead to double compilation (once for the program and once for the test suite).


Tasty executes tests in parallel to make them finish faster. If this parallelism is not desirable, you can declare dependencies between tests, so that one test will not start until certain other tests finish.

Dependencies are declared using the after combinator:

  • after AllFinish "pattern" my_tests will execute the test tree my_tests only after all tests that match the pattern finish.
  • after AllSucceed "pattern" my_tests will execute the test tree my_tests only after all tests that match the pattern finish and only if they all succeed. If at least one dependency fails, then my_tests will be skipped.

The relevant types are:

  :: DependencyType -- ^ whether to run the tests even if some of the dependencies fail
  -> String         -- ^ the pattern
  -> TestTree       -- ^ the subtree that depends on other tests
  -> TestTree       -- ^ the subtree annotated with dependency information

data DependencyType = AllSucceed | AllFinish

The pattern follows the same AWK-like syntax and semantics as described in Patterns. There is also a variant named after_ that accepts the AST of the pattern instead of a textual representation.

Let's consider some typical examples. (A note about terminology: here by "resource" I mean anything stateful and external to the test: it could be a file, a database record, or even a value stored in an IORef that's shared among tests. The resource may or may not be managed by withResource.)

  1. Two tests, Test A and Test B, access the same shared resource and cannot be run concurrently. To achieve this, make Test A a dependency of Test B:

    testGroup "Tests accessing the same resource"
      [ testCase "Test A" $ ...
      , after AllFinish "Test A" $
          testCase "Test B" $ ...
  2. Test A creates a resource and Test B uses that resource. Like above, we make Test A a dependency of Test B, except now we don't want to run Test B if Test A failed because the resource may not have been set up properly. So we use AllSucceed instead of AllFinish

    testGroup "Tests creating and using a resource"
      [ testCase "Test A" $ ...
      , after AllSucceed "Test A" $
          testCase "Test B" $ ...

Here are some caveats to keep in mind regarding dependencies in Tasty:

  1. If Test B depends on Test A, remember that either of them may be filtered out using the --pattern option. Collecting the dependency info happens after filtering. Therefore, if Test A is filtered out, Test B will run unconditionally, and if Test B is filtered out, it simply won't run.

  2. Tasty does not currently check whether the pattern in a dependency matches anything at all, so make sure your patterns are correct and do not contain typos. Fortunately, misspecified dependencies usually lead to test failures and so can be detected that way.

  3. Dependencies shouldn't form a cycle, otherwise Tasty with fail with the message "Test dependencies form a loop." A common cause of this is a test matching its own dependency pattern.

  4. Using dependencies may introduce quadratic complexity. Specifically, resolving dependencies is O(number_of_tests × number_of_dependencies), since each pattern has to be matched against each test name. As a guideline, if you have up to 1000 tests, the overhead will be negligible, but if you have thousands of tests or more, then you probably shouldn't have more than a few dependencies.

    Additionally, it is recommended that the dependencies follow the natural order of tests, i.e. that the later tests in the test tree depend on the earlier ones and not vice versa. If the execution order mandated by the dependencies is sufficiently different from the natural order of tests in the test tree, searching for the next test to execute may also have an overhead quadratic in the number of tests.


  1. Q: When my tests write to stdout/stderr, the output is garbled. Why is that and what do I do?

    A: It is not recommended that you print anything to the console when using the console test reporter (which is the default one). See #103 for the discussion.

    Some ideas on how to work around this:

    • Use testCaseSteps (for tasty-hunit only).
    • Use a test reporter that does not print to the console (like tasty-ant-xml).
    • Write your output to files instead.
  2. Q: Why doesn't the --hide-successes option work properly? The test headings show up and/or the output appears garbled.

    A: This can happen sometimes when the terminal is narrower than the output. A workaround is to disable ANSI tricks: pass --ansi-tricks=false on the command line or set TASTY_ANSI_TRICKS=false in the environment.

    See issue #152.

  3. Q: Patterns with slashes do not work on Windows. How can I fix it?

    A: If you are running Git for Windows terminal, it has a habit of converting slashes to backslashes. Set MSYS_NO_PATHCONV=1 to prevent this behaviour, or follow other suggestions from Known Issues.


Blog posts and other publications related to tasty. If you wrote or just found something not mentioned here, send a pull request!

GHC version support policy

We only support the GHC/base versions from the last 5 years.


Roman Cheplyaka is the primary maintainer.

Oliver Charles is the backup maintainer. Please get in touch with him if the primary maintainer cannot be reached.