HUnit User's Guide
HUnit is a unit testing framework for Haskell, inspired by the JUnit tool for Java. This
guide describes how to use HUnit, assuming you are familiar with Haskell, though not
necessarily with JUnit. You can obtain HUnit, including this guide, at
[https://github.com/hspec/HUnit](https://github.com/hspec/HUnit)
Introduction
A test-centered methodology for software development is most effective when tests are
easy to create, change, and execute. The JUnit tool
pioneered support for test-first development in Java.
HUnit is an adaptation of JUnit to Haskell, a general-purpose, purely functional
programming language. (To learn more about Haskell, see [www.haskell.org](http://www.haskell.org)).
With HUnit, as with JUnit, you can easily create tests, name them, group them into
suites, and execute them, with the framework checking the results automatically. Test
specification in HUnit is even more concise and flexible than in JUnit, thanks to the
nature of the Haskell language. HUnit currently includes only a text-based test
controller, but the framework is designed for easy extension. (Would anyone care to
write a graphical test controller for HUnit?)
The next section helps you get started using HUnit in simple ways. Subsequent sections
give details on writing tests and running tests.
The document concludes with a section describing HUnit's constituent files
and a section giving references to further information.
Getting Started
In the Haskell module where your tests will reside, import module Test.HUnit:
import Test.HUnit
Define test cases as appropriate:
test1 = TestCase (assertEqual "for (foo 3)," (1,2) (foo 3))
test2 = TestCase (do (x,y) <- partA 3
assertEqual "for the first result of partA," 5 x
b <- partB y
assertBool ("(partB " ++ show y ++ ") failed") b)
Name the test cases and group them together:
tests = TestList [TestLabel "test1" test1, TestLabel "test2" test2]
Run the tests as a group. At a Haskell interpreter prompt, apply the
function runTestTT to the collected tests. (The TT suggests
Text orientation with output to the Terminal.)
> runTestTT tests
Cases: 2 Tried: 2 Errors: 0 Failures: 0
>
If the tests are proving their worth, you might see:
> runTestTT tests
### Failure in: 0:test1
for (foo 3),
expected: (1,2)
but got: (1,3)
Cases: 2 Tried: 2 Errors: 0 Failures: 1
>
Isn't that easy?
You can specify tests even more succinctly using operators and
overloaded functions that HUnit provides:
tests = test [ "test1" ~: "(foo 3)" ~: (1,2) ~=? (foo 3),
"test2" ~: do (x, y) <- partA 3
assertEqual "for the first result of partA," 5 x
partB y @? "(partB " ++ show y ++ ") failed" ]
Assuming the same test failures as before, you would see:
> runTestTT tests
### Failure in: 0:test1:(foo 3)
expected: (1,2)
but got: (1,3)
Cases: 2 Tried: 2 Errors: 0 Failures: 1
>
Writing Tests
Tests are specified compositionally. Assertions are
combined to make a test case, and test cases are combined
into tests. HUnit also provides advanced
features for more convenient test specification.
Assertions
The basic building block of a test is an assertion.
type Assertion = IO ()
An assertion is an IO computation that always produces a void result. Why is an assertion an IO computation? So that programs with real-world side effects can be tested. How does an assertion assert anything if it produces no useful result? The answer is that an assertion can signal failure by calling assertFailure.
assertFailure :: String -> Assertion
assertFailure msg = ioError (userError ("HUnit:" ++ msg))
(assertFailure msg) raises an exception. The string argument identifies the
failure. The failure message is prefixed by "HUnit:" to mark it as an HUnit
assertion failure message. The HUnit test framework interprets such an exception as
indicating failure of the test whose execution raised the exception. (Note: The details
concerning the implementation of assertFailure are subject to change and should
not be relied upon.)
assertFailure can be used directly, but it is much more common to use it
indirectly through other assertion functions that conditionally assert failure.
assertBool :: String -> Bool -> Assertion
assertBool msg b = unless b (assertFailure msg)
assertString :: String -> Assertion
assertString s = unless (null s) (assertFailure s)
assertEqual :: (Eq a, Show a) => String -> a -> a -> Assertion
assertEqual preface expected actual =
unless (actual == expected) (assertFailure msg)
where msg = (if null preface then "" else preface ++ "\n") ++
"expected: " ++ show expected ++ "\n but got: " ++ show actual
With assertBool you give the assertion condition and failure message separately.
With assertString the two are combined. With assertEqual you provide a
"preface", an expected value, and an actual value; the failure message shows the two
unequal values and is prefixed by the preface. Additional ways to create assertions are
described later under Advanced Features
Since assertions are IO computations, they may be combined--along with other
IO computations--using (>>=), (>>), and the do
notation. As long as its result is of type (IO ()), such a combination
constitutes a single, collective assertion, incorporating any number of constituent
assertions. The important features of such a collective assertion are that it fails if
any of its constituent assertions is executed and fails, and that the first constituent
assertion to fail terminates execution of the collective assertion. Such behavior is
essential to specifying a test case.
Test Case
A test case is the unit of test execution. That is, distinct test cases are
executed independently. The failure of one is independent of the failure of any other.
A test case consists of a single, possibly collective, assertion. The possibly multiple
constituent assertions in a test case's collective assertion are not independent.
Their interdependence may be crucial to specifying correct operation for a test. A test
case may involve a series of steps, each concluding in an assertion, where each step
must succeed in order for the test case to continue. As another example, a test may
require some "set up" to be performed that must be undone ("torn down" in JUnit
parlance) once the test is complete. In this case, you could use Haskell's
IO.bracket function to achieve the desired effect.
You can make a test case from an assertion by applying the TestCase constructor.
For example, (TestCase (return ())) is a test case that never
fails, and (TestCase (assertEqual "for x," 3 x))
is a test case that checks that the value of x is 3. Additional ways
to create test cases are described later under Advanced Features.
Tests
As soon as you have more than one test, you'll want to name them to tell them apart. As
soon as you have more than several tests, you'll want to group them to process them more
easily. So, naming and grouping are the two keys to managing collections of tests.
In tune with the "composite" design pattern [1], a
test is defined as a package of test cases. Concretely, a test is either a single
test case, a group of tests, or either of the first two identified by a label.
data Test = TestCase Assertion
| TestList [Test]
| TestLabel String Test
There are three important features of this definition to note:
- A
TestList consists of a list of tests rather than a list of test cases.
This means that the structure of a Test is actually a tree. Using a
hierarchy helps organize tests just as it helps organize files in a file system.
- A
TestLabel is attached to a test rather than to a test case. This means
that all nodes in the test tree, not just test case (leaf) nodes, can be labeled.
Hierarchical naming helps organize tests just as it helps organize files in a file
system.
- A
TestLabel is separate from both TestCase and TestList.
This means that labeling is optional everywhere in the tree. Why is this a good
thing? Because of the hierarchical structure of a test, each constituent test case
is uniquely identified by its path in the tree, ignoring all labels. Sometimes a
test case's path (or perhaps its subpath below a certain node) is a perfectly
adequate "name" for the test case (perhaps relative to a certain node). In this
case, creating a label for the test case is both unnecessary and inconvenient.
The number of test cases that a test comprises can be computed with testCaseCount.
testCaseCount :: Test -> Int
As mentioned above, a test is identified by its path in the test hierarchy.
data Node = ListItem Int | Label String
deriving (Eq, Show, Read)
type Path = [Node] -- Node order is from test case to root.
Each occurrence of TestList gives rise to a ListItem and each
occurrence of TestLabel gives rise to a Label. The ListItems
by themselves ensure uniqueness among test case paths, while the Labels allow
you to add mnemonic names for individual test cases and collections of them.
Note that the order of nodes in a path is reversed from what you might expect: The first
node in the list is the one deepest in the tree. This order is a concession to
efficiency: It allows common path prefixes to be shared.
The paths of the test cases that a test comprises can be computed with
testCasePaths. The paths are listed in the order in which the corresponding
test cases would be executed.
testCasePaths :: Test -> [Path]
The three variants of Test can be constructed simply by applying
TestCase, TestList, and TestLabel to appropriate arguments.
Additional ways to create tests are described later under Advanced Features.
The design of the type Test provides great conciseness, flexibility, and
convenience in specifying tests. Moreover, the nature of Haskell significantly augments
these qualities:
- Combining assertions and other code to construct test cases is easy with the
IO monad.
- Using overloaded functions and special operators (see below), specification of
assertions and tests is extremely compact.
- Structuring a test tree by value, rather than by name as in JUnit, provides for more
convenient, flexible, and robust test suite specification. In particular, a test
suite can more easily be computed "on the fly" than in other test frameworks.
- Haskell's powerful abstraction facilities provide unmatched support for test
refactoring.
Advanced Features
HUnit provides additional features for specifying assertions and tests more conveniently
and concisely. These facilities make use of Haskell type classes.
The following operators can be used to construct assertions.
infix 1 @?, @=?, @?=
(@?) :: (AssertionPredicable t) => t -> String -> Assertion
pred @? msg = assertionPredicate pred >>= assertBool msg
(@=?) :: (Eq a, Show a) => a -> a -> Assertion
expected @=? actual = assertEqual "" expected actual
(@?=) :: (Eq a, Show a) => a -> a -> Assertion
actual @?= expected = assertEqual "" expected actual
You provide a boolean condition and failure message separately to (@?), as for
assertBool, but in a different order. The (@=?) and (@?=)
operators provide shorthands for assertEqual when no preface is required. They
differ only in the order in which the expected and actual values are provided. (The
actual value--the uncertain one--goes on the "?" side of the operator.)
The (@?) operator's first argument is something from which an assertion
predicate can be made, that is, its type must be AssertionPredicable.
type AssertionPredicate = IO Bool
class AssertionPredicable t
where assertionPredicate :: t -> AssertionPredicate
instance AssertionPredicable Bool
where assertionPredicate = return
instance (AssertionPredicable t) => AssertionPredicable (IO t)
where assertionPredicate = (>>= assertionPredicate)
The overloaded assert function in the Assertable type class constructs
an assertion.
class Assertable t
where assert :: t -> Assertion
instance Assertable ()
where assert = return
instance Assertable Bool
where assert = assertBool ""
instance (ListAssertable t) => Assertable [t]
where assert = listAssert
instance (Assertable t) => Assertable (IO t)
where assert = (>>= assert)
The ListAssertable class allows assert to be applied to [Char]
(that is, String).
class ListAssertable t
where listAssert :: [t] -> Assertion
instance ListAssertable Char
where listAssert = assertString
With the above declarations, (assert ()),
(assert True), and (assert "") (as well as
IO forms of these values, such as (return ())) are all
assertions that never fail, while (assert False) and
(assert "some failure message") (and their
IO forms) are assertions that always fail. You may define additional
instances for the type classes Assertable, ListAssertable, and
AssertionPredicable if that should be useful in your application.
The overloaded test function in the Testable type class constructs a
test.
class Testable t
where test :: t -> Test
instance Testable Test
where test = id
instance (Assertable t) => Testable (IO t)
where test = TestCase . assert
instance (Testable t) => Testable [t]
where test = TestList . map test
The test function makes a test from either an Assertion (using
TestCase), a list of Testable items (using TestList), or
a Test (making no change).
The following operators can be used to construct tests.
infix 1 ~?, ~=?, ~?=
infixr 0 ~:
(~?) :: (AssertionPredicable t) => t -> String -> Test
pred ~? msg = TestCase (pred @? msg)
(~=?) :: (Eq a, Show a) => a -> a -> Test
expected ~=? actual = TestCase (expected @=? actual)
(~?=) :: (Eq a, Show a) => a -> a -> Test
actual ~?= expected = TestCase (actual @?= expected)
(~:) :: (Testable t) => String -> t -> Test
label ~: t = TestLabel label (test t)
(~?), (~=?), and (~?=) each make an assertion, as for
(@?), (@=?), and (@?=), respectively, and then a test case
from that assertion. (~:) attaches a label to something that is
Testable. You may define additional instances for the type class
Testable should that be useful.
Running Tests
HUnit is structured to support multiple test controllers. The first
subsection below describes the test execution
characteristics common to all test controllers. The second subsection
describes the text-based controller that is included with HUnit.
Test Execution
All test controllers share a common test execution model. They differ only in how the
results of test execution are shown.
The execution of a test (a value of type Test) involves the serial execution (in
the IO monad) of its constituent test cases. The test cases are executed in a
depth-first, left-to-right order. During test execution, four counts of test cases are
maintained:
data Counts = Counts { cases, tried, errors, failures :: Int }
deriving (Eq, Show, Read)
cases is the number of test cases included in the test. This number is a
static property of a test and remains unchanged during test execution.
tried is the number of test cases that have been executed so far during the
test execution.
errors is the number of test cases whose execution ended with an unexpected
exception being raised. Errors indicate problems with test cases, as opposed to the
code under test.
failures is the number of test cases whose execution asserted failure.
Failures indicate problems with the code under test.
Why is there no count for test case successes? The technical reason is that the counts
are maintained such that the number of test case successes is always equal to
(tried - (errors + failures)). The
psychosocial reason is that, with test-centered development and the expectation that
test failures will be few and short-lived, attention should be focused on the failures
rather than the successes.
As test execution proceeds, three kinds of reporting event are communicated to the test
controller. (What the controller does in response to the reporting events depends on the
controller.)
- start -- Just prior to initiation of a test case, the path of the test case
and the current counts (excluding the current test case) are reported.
- error -- When a test case terminates with an error, the error message is
reported, along with the test case path and current counts (including the current
test case).
- failure -- When a test case terminates with a failure, the failure message is
reported, along with the test case path and current counts (including the current
test case).
Typically, a test controller shows error and failure reports immediately
but uses the start report merely to update an indication of overall test
execution progress.
Text-Based Controller
A text-based test controller is included with HUnit.
runTestText :: PutText st -> Test -> IO (Counts, st)
runTestText is generalized on a reporting scheme given as its first
argument. During execution of the test given as its second argument, the controller
creates a string for each reporting event and processes it according to the reporting
scheme. When test execution is complete, the controller returns the final counts along
with the final state for the reporting scheme.
The strings for the three kinds of reporting event are as follows.
- A start report is the result of the function
showCounts applied to
the counts current immediately prior to initiation of the test case being started.
- An error report is of the form
"
Error in: *path*\n*message*",
where path is the path of the test case in error, as shown by
showPath, and message is a message describing the error. If the path
is empty, the report has the form "Error:\n*message*".
- A failure report is of the form
"
Failure in: *path*\n*message*", where
path is the path of the test case in error, as shown by
showPath, and message is the failure message. If the path is empty,
the report has the form "Failure:\n*message*".
The function showCounts shows a set of counts.
showCounts :: Counts -> String
The form of its result is
Cases: *cases* Tried: *tried* Errors: *errors* Failures: *failures*
where cases, tried, errors, and failures are the count values.
The function showPath shows a test case path.
showPath :: Path -> String
The nodes in the path are reversed (so that the path reads from the root down to the test
case), and the representations for the nodes are joined by ':' separators. The
representation for (ListItem *n*) is (show n). The representation
for (Label *label*) is normally label. However, if label
contains a colon or if (show *label*) is different from label
surrounded by quotation marks--that is, if any ambiguity could exist--then (Label *label*) is represented as (show *label*).
HUnit includes two reporting schemes for the text-based test controller. You may define
others if you wish.
putTextToHandle :: Handle -> Bool -> PutText Int
putTextToHandle writes error and failure reports, plus a report of the final
counts, to the given handle. Each of these reports is terminated by a newline. In
addition, if the given flag is True, it writes start reports to the handle as
well. A start report, however, is not terminated by a newline. Before the next report is
written, the start report is "erased" with an appropriate sequence of carriage return
and space characters. Such overwriting realizes its intended effect on terminal devices.
putTextToShowS :: PutText ShowS
putTextToShowS ignores start reports and simply accumulates error and failure
reports, terminating them with newlines. The accumulated reports are returned (as the
second element of the pair returned by runTestText) as a ShowS
function (that is, one with type (String -> String)) whose
first argument is a string to be appended to the accumulated report lines.
HUnit provides a shorthand for the most common use of the text-based test controller.
runTestTT :: Test -> IO Counts
runTestTT invokes runTestText, specifying (putTextToHandle stderr True) for the reporting scheme, and returns the final counts from the
test execution.
References
The HUnit software and this guide were written by Dean Herington heringto@cs.unc.edu
