HTF-0.3: The Haskell Test Framework



This module provides a short tutorial on how to use the HTF. It assumes that you are using GHC for compiling your Haskell code. (It is possible to use the HTF with other Haskell environments, only the steps taken to invoke the custom preprocessor of the HTF may differ in this case.)

Suppose you are writing a function for reversing lists:

myReverse :: [a] -> [a]
myReverse [] = []
myReverse [x] = [x]
myReverse (x:xs) = myReverse xs

To test this function using the HTF, you first create a new source file with a OPTIONS_GHC pragma in the first line.

{-# OPTIONS_GHC -F -pgmF htfpp #-}

This pragma instructs GHC to run the source file through htfpp, the custom preprocessor of the HTF.

The following import statements are also needed:

import System.Environment ( getArgs )
import Test.Framework

The actual unit tests and QuickCheck properties are defined like this:

test_nonEmpty = do assertEqual [1] (myReverse [1])
                   assertEqual [3,2,1] (myReverse [1,2,3])

test_empty = assertEqual ([] :: [Int]) (myReverse [])

prop_reverse :: [Int] -> Bool
prop_reverse xs = xs == (myReverse (myReverse xs))

When htfpp consumes the source file, it replaces the assertEqual tokens (and other assert-like tokens, see Test.Framework.HUnitWrapper) with calls to assertEqual_, passing the current location in the file as the first argument. Moreover, the preprocessor collects all top-level definitions starting with test_ or prop_ in a test suite with name allHTFTests of type TestSuite.

Definitions starting with test_ denote unit tests and must be of type Assertion. Definitions starting with prop_ denote QuickCheck properties and must be of type T such that T is an instance of the type class Testable.

To run the tests, use the runTestWithArgs function, which takes a list of strings and the test.

main =
    do args <- getArgs
       runTestWithArgs args reverseTests

Here is the skeleton of a .cabal file which you may want to use to compile the tests.

Name:          HTF-tutorial
Version:       0.1
Cabal-Version: >= 1.6
Build-type:    Simple

Executable tutorial
  Main-is: Tutorial.hs
  Build-depends: base, HTF

Compiling the program just shown (you must include the code for myReverse as well), and then running the resulting program with no further commandline arguments yields the following output:

 Main:nonEmpty (Tutorial.hs:17)
 *** Failed! assertEqual failed at Tutorial.hs:18
  expected: [3,2,1]
  but got:  [3]

 Main:empty (Tutorial.hs:19)
 +++ OK

 Main:reverse (Tutorial.hs:22)
 *** Failed! Falsifiable (after 3 tests and 1 shrink):
 Replay argument: "Just (847701486 2147483396,2)"

 * Tests:    3
 * Passed:   1
 * Failures: 2
 * Errors:   0

(To check only specific tests, you can pass commandline arguments to the program: the HTF then runs only those tests whose name contain at least one of the commandline arguments as a substring.)

You see that the message for the first failure contains exact location information, which is quite convenient. Moreover, for the QuickCheck property Main.reverse, the HTF also outputs a string represenation of the random generator used to check the property. This string representation can be used to replay the property. (The replay feature may not be useful for this simple example but it helps in more complex scenarios).

To replay a property you simply use the string representation of the generator to define a new QuickCheck property with custom arguments:

prop_reverseReplay =
  withQCArgs (a -> a { replay = read "Just (1060394807 2147483396,2)" })

To finish this tutorial, we now give a correct definition for myReverse:

myReverse :: [a] -> [a]
myReverse [] = []
myReverse (x:xs) = myReverse xs ++ [x]

Running our tests again on the fixed definition then yields the desired result:

 Main:nonEmpty (Tutorial.hs:17)
 +++ OK

 Main:empty (Tutorial.hs:19)
 +++ OK

 Main:reverse (Tutorial.hs:22)
 +++ OK, passed 100 tests.

 Main:reverseReplay (Tutorial.hs:24)
 +++ OK, passed 100 tests.

 * Tests:    4
 * Passed:   4
 * Failures: 0
 * Errors:   0

The HTF also allows the definition of black box tests. See the documentation of the Test.Framework.BlackBoxTest module for further information.