[Haskell-cafe] Organizaing tests in Haskell

Jan Stolarek jan.stolarek at p.lodz.pl
Sun Sep 23 11:25:48 CEST 2012

Dear list,

during many years of Java programming I've been faithful to TDD methology. Recently I've been 
trying to figure out how to do tests in Haskell. Thanks to RWH and help from great folks at 
#haskell I've managed to get on my feet. There is however one issue I wasn't able to solve.

In Java it is very easy to separate test code from the actual source. A Java project simply 
contains two folders: src and tests. These two are treated as the source directories. Both these 
directories have the same subdirectory structure. For example I have a source file 
src/myPackage/mySubpackage/MyClass.java and test for it are kept in file 
tests/myPackage/mySubpackage/MyClassTest.java. These two files are considered by Java to be in 
the same package. Here's the main trick: fileds and methods that are marked as protected in Java 
are accessible to other classes in the same package. This allows to test internal methods of a 
class by marking them as protected instead of private. This breaks encapsulation but only within 
a package, which is acceptable.

Now I'd like to achieve something similar in Haskell. I'm using cabal's support for testing. I 
created separate src and tests directories, both with the same subdirectory structure. I keep 
tests for each module in a separate file (e.g. I have src/Math/MyModule.hs and 
tests/Math/MyModuleTest.hs) and I have one file that assembles all the tests into a single test 
suite (I use test-framework for that). The only problem is that in order to test some function 
from a module I have to expose that function, which pretty much forces me to give up on 

Is there any better solution to organize tests in Haskell? Should I just give up on module 
encapsulation, or should I only test functions exposed by the module and don't worry about 
internal functions? Perhaps I should use some different approach?


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