Code Coverage

Code coverage is enabled by passing the --coverage flag to stack build.


stack test --coverage is quite streamlined for the following use-case:

  1. You have test-suites which exercise your local packages.

  2. These test-suites link against your library, rather than building the library directly. Coverage information is only given for libraries, ignoring the modules which get compiled directly into your executable. A common case where this doesn't happen is when your test-suite and library both have something like hs-source-dirs: src/. In this case, when building your test-suite you may also be compiling your library, instead of just linking against it.

When your project has these properties, you will get the following:

1) Textual coverage reports in the build output.

2) A unified textual and HTML report, considering the coverage on all local libraries, based on all of the tests that were run.

3) An index of all generated HTML reports, at $(stack path --local-hpc-root)/index.html.

"stack hpc report" command

The stack hpc report command generates a report for a selection of targets and .tix files. For example, if we have 3 different packages with test-suites, packages A, B, and C, the default unified report will have coverage from all 3. If we want a unified report with just two, we can instead run:

$ stack hpc report A B

This will output a textual report for the combined coverage from A and B's test-suites, along with a path to the HTML for the report. To further streamline this process, you can pass the --open option, to open the report in your browser.

This command also supports taking extra .tix files. If you've also built an executable, against exactly the same library versions of A, B, and C, then you could do the following:

$ stack exec -- an-exe
$ stack hpc report A B C an-exe.tix

This report will consider all test results as well as the newly generated an-exe.tix file. Since this is a common use-case, there is a convenient flag to use all stored results - stack hpc report --all an-exe.tix.

"extra-tix-files" directory

During the execution of the build, you can place additional tix files in $(stack path --local-hpc-root)/extra-tix-files/ in order for them to be included in the unified report. A couple caveats:

  • These tix files must be generated by executables that are built against the exact same library versions. Also note that, on subsequent builds with coverage, the $(stack path --local-hpc-root) directory will be recursively deleted. It just stores the most recent coverage data.

  • These tix files will not be considered by stack hpc report unless listed explicitly by file name.

Implementation details

Most users can get away with just understanding the above documentation. However, advanced users may want to understand exactly how --coverage works:

  1. The GHC option -fhpc gets passed to all local packages. This tells GHC to output executables that track coverage information and output them to .tix files. the-exe-name.tix files will get written to the working directory of the executable.

When switching on this flag, it will usually cause all local packages to be rebuilt (see #1940.

  1. Before the build runs with --coverage, the contents of stack path --local-hpc-root gets deleted. This prevents old reports from getting mixed with new reports. If you want to preserve report information from multiple runs, copy the contents of this path to a new folder.

  2. Before a test run, if a test-name.tix file exists in the package directory, it will be deleted.

  3. After a test run, it will expect a test-name.tix file to exist. This file will then get loaded, modified, and outputted to $(stack path --local-hpc-root)/pkg-name/test-name/test-name.tix).

The .tix file gets modified to remove coverage file that isn't associated with a library. So, this means that you won't get coverage information for the modules compiled in the executable or test-suite stanza of your cabal file. This makes it possible to directly union multiple *.tix files from different executables (assuming they are using the exact same versions of the local packages).

If there is enough popular demand, it may be possible in the future to give coverage information for modules that are compiled directly into the executable. See #1359.

  1. Once we have a .tix file for a test, we also generate a textual and HTML report for it. The textual report is sent to the terminal. The index of the test-specific HTML report is available at $(stack path --local-hpc-root)/pkg-name/test-name/index.html

  2. After the build completes, if there are multiple output *.tix files, they get combined into a unified report. The index of this report will be available at $(stack path --local-hpc-root)/combined/all/index.html

  3. Finally, an index of the resulting coverage reports is generated. It links to the individual coverage reports (one for each test-suite), as well as the unified report. This index is available at $(stack path --local-hpc-root)/index.html