Posts tagged: testing

Two new libraries to extend tasty testing framework

When I recently wrote about porting my haskell-testing-stub project to tasty I mentioned that test-framework still has more libraries than tasty. I decided to contribute to changing that and released two small packages that extend tasty with extra functionality:

  • tasty-hunit-adapter allows to import existing HUnit tests into tasty (hackage, github):
    module Main where
     
    import Test.HUnit               ( (~:), (@=?)            )
    import Test.Tasty               ( defaultMain, testGroup )
    import Test.Tasty.HUnit.Adapter ( hUnitTestToTestTree    )
     
    main :: IO ()
    main = defaultMain $ testGroup "Migrated from HUnit" $
           hUnitTestToTestTree ("HUnit test" ~: 2 + 2 @=? 4)
  • tasty-program allows to run external program and test whether it terminates successfully (hackage, github):
    module Main (
      main
     ) where
     
    import Test.Tasty
    import Test.Tasty.Program
     
    main :: IO ()
    main = defaultMain $ testGroup "Compilation with GHC" $ [
        testProgram "Foo" "ghc" ["-fforce-recomp", "foo.hs"] 
                    Nothing
      ]
  • This package has only this basic functionality at the moment. A missing feature is the possibility of logging stdout and stderr to a file so that it can later be inspected or perhaps used by a golden test (but for the latter tasty needs test dependencies).

Code testing in Haskell revisited (with Tasty)

About 1,5 year ago I wrote a post about code testing in Haskell. Post was accompanied by haskell-testing-stub: a small project showing how to organize tests and benchmarks in Haskell. I used test-framework package to gather tests written in different testing frameworks (QuickCheck and HUnit in my case) in a coherent suite. Test-framework might have been a good choice in 2012 but that might not be the case today. The major issue is that it has been abandoned by and was unmaintained for several months. Recently the project has been taken up by the community and received some updates but for now it looks like there won’t be any major development.

As a response to the test-framework package being unmaintained Roman Cheplyaka has released tasty (original announcement here). Since its release in August 2013 tasty has received packages supporting integration with QuickCheck, HUnit, SmallCheck, hspec as well as support for golden testing and few others. I decided to give tasty a try and use it in my haskell-testing-stub project. Tasty turned out to be almost a drop-in replacement for test-framework. I had to update cabal file (quite obviously), change imports to point to tasty rather than test-framework and replace usage of [Test] type with TestTree. The only problem I encountered was adapting tests from HUnit. It turns out that tasty-hunit package does not have a function that allows to use an existing suite of HUnit tests. That feature was present in test-framework-hunit as hUnitTestToTests function. I mailed Roman about this and his reply was that this was intentional as he does not “believe it adds anything useful to the API (i.e. the way to *write* code).” That’s not a big issue though as it was easy to adapt the missing function (although I think I’ll just put it in a separate package and release it so others don’t have to reinvent the wheel).

I admit that at this point I am not sure whether switching from test-framework to tasty is a good move. The fact that tasty is actively developed is a huge plus although test-framework has reached a mature state so perhaps active development is no longer of key importance. Also, test-framework still has more supporting libraries than tasty. Migrating them should be easy but up till now no one has done it. So I’m not arguing heavily for tasty. This is more like an experiment to see how it works.

How to shoot yourself in the foot with Haskell

Haskell is “advertised” as a safe language that does all type checking upfront, making sure that you don’t experience runtime type errors, null pointers and all that kind of stuff. It also gives you ways to bypass some of the safety mechanisms, so a conscious programmer can use unsafe functions to get a boost in performance (e.g. by not performing bounds checking when indexing a vector).

I’ve written some very ugly Haskell code that creates a vector using destructive updates. It is in fact an imperative algorithm, not a functional one. When the initialization is over the vector is frozen using unsafeFreeze. I wrote my code using read and write functions, tested it using QuickCheck and when the tests passed I switched to unsafeRead and unsafeWrite to make my program faster. Some time later I started getting random segfaults when running my tests. This never happened before in any of my Haskell programs so I almost panicked. At first I didn’t had a slightest idea how to even approach this problem. I suspected that this might even be a bug in GHC. Then I started disabling groups of tests trying to track down the bug and finally managed to locate a single test that was causing the problem. Guess what – it was the test of my vector initialization with unsafe functions. What happened is that after switching to unsafeRead/unsafeWrite I refactored the function and its test. I made a mistake in the testing code and passed incorrect data that resulted with an attempt to write an element at address -1. Finding this bug took me a little over an hour. A factor that made debugging harder was that disabling tests that seemed to cause the segfault resulted in problems appearing in a completely different part of the program – or so I thought by looking at the output of my program. Looks like I completely forgot about lazy evaluation and unspecified evaluation order!

Second bug I encountered was even trickier. I wrote functions that perform cyclic shifts of a signal by any value. For example shifting [1,2,3,4] left by 1 yields [2,3,4,1]. Note that shifting by 5, 9, 13 and so on gives exactly the same result – the shift function is periodic. You might recall that I used shift functions to demonstrate code testing. This time however I written shifts using Repa library. Then I created QuickCheck property stating that shifting any signal left and then right by same value yields the original signal. This is pretty obvious property and the tests passed without any problems. Later, when writing some other tests I ended up with one of the tests failing. Using ghci I checked that this error should not be happening at all, but after careful debugging it turned out that during actual tests some of the values in the results become zeros. After two hours of debugging I realized that the actual bug is in the shifting functions – they worked only for the basic period, that is shift values from 0 to signal length. Why QuickCheck didn’t manage to falsify my property of left/right shift compositions? Repa is a smart (and tricky) library that attempts to fuse many operations on an array into one. And it fused application of left shift followed by right shift into identity transform! Well, this is great. After all this is the kind of optimization we would like to have in our programs. But it turns out that it can also impact tests! After realizing what is going on it was actually a matter of 5 minutes to fix the bug, but finding it was not a trivial task.

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